Method and Apparatus for an Alternate Home Channel for a Land Mobile Transmission Trunked Communication System

This invention relates generally to land mobile radio communication systems. In particular, the present invention relates to a method and apparatus for an alternate home channel for a land mobile transmission trunked communication system.

Mobile radio communication systems provide for two way radio communication between a central control station and a fleet of cars, trucks, or other vehicles. Typical users of mobile radio communication systems include, for example, police departments, fire departments, taxi services and delivery services. Present mobile radio communication systems can be configured to provide for radio communications between the control station and all vehicles in a fleet, between the control station and selected vehicles in the fleet, or between different individual vehicles or groups of vehicles within a fleet.

Conventional mobile radio communication systems are typically organized with a number of vehicles and a control station assigned to a single common channel for a given coverage zone. A user assigned to the common channel must wait until no one else is transmitting on that channel before sending a message, because only one transmission at a time can be processed by a single channel. Even when a transmission is addressed to only one user in a conventional system (and therefor not heard by other users), the other users in the system must wait until that transmission is completed before they can use the system to communicate.

Mobile radio communication systems using transmission trunking are an improvement over conventional mobile radio communication systems in that trunked systems enable two or more users to communicate on the system at the same time. For instance, the dispatch console in a transmission trunked communication system can be communicating with one of the vehicle in the fleet; while, at the same time, two other vehicles in the fleet can be using the same trunked system to communicate with each other. Transmission trunked communication systems group a number of channels for the same coverage zone together into a single radio communication system, with each channel accessible to each user in the system. Because each user will only need to communicate over the trunked system part of the time, and because it is unlikely that all users will need to communicate at precisely the same time, the number of channels assigned to a trunked system group can always be less than the number of users allocated to that trunked system group.

The United States Federal Communications Commission (FCC) has assigned 600 channels in the 800 MHz band for trunked land mobile use. Each channel is comprised of a pair of assigned frequencies, a mobile transmit frequency and a repeater transmit frequency. Mobile transmit frequencies are 806-821 MHz, with the repeater transmit frequencies exactly 45 MHz above the corresponding mobile transmit frequency, or 851-866 MHz. Channel spacing is 25 KHz, with the maximum allowed deviation between channels being +/- 5 KHz. In September 1987, the FCC also allocated 399 channels in the 900 MHz band for trunked land mobile use. Mobile transmit frequencies are 896-901 MHz, with the repeater transmit frequencies exactly 39 MHz above the corresponding mobile transmit frequency, or 925-940 MHz. Channel spacing is 12.5 KHz, with the maximum allowed deviation between channels being +/- 2.5 KHz.

In a transmission trunked communication systems, a signaling protocol is used to send and receive control signals among users on each channel in the trunked system and a switching protocol is used to establish which channels those users will be communicating over. The preferred conventional transmission trunked communication system uses a signaling protocol that transmits the control signals in the subaudio band simultaneously with the transmission of voice or data information signals. Signaling protocols that can communicate control signals within the constraints of the subaudio band are preferred, because use of the subaudio band precludes the need for using a dedicated channel for transmitting the control signals (thereby reducing the number of available channels for voice and data communications). the switching protocol is used by the trunked system to automatically find and engage an open channel when a user initiates a transmission. To maximize the trunking capabilities of such a system, the switching protocol must efficiently allocate channels in the trunked system and avoid channels that are already in use at the time the transmission is initiated. For further explanation of the preferred conventional transmission trunked communication systems, reference is made to the description of the operation of the ClearChannel LTR.RTM. system contained in the manual entitled "E. F. Johnson ClearChannel LTR Application Note". Part No. 009-0001-020 (Rev. 5, Oct. 1988, available from E. F. Johnson Company, Waseca, Minn.

Transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a dispatch console or control station and a fleet of mobile vehicles in a given coverage zone. However, the capability of such trunked systems to provide radio communications over a wide area serviced by a plurality of preferably adjacent coverage zones has been limited because of the problems involved in linking multiple coverage zones and because of the limitations of the switching and signaling protocols of present transmission trunked communication systems.

Present transmission trunked communication systems of the type described above are generally unable to transfer voice/data communication between coverage zones, because the repeaters in such systems are interconnected only by a single time slot status bus. As a result, the only method of interconnecting repeaters in different coverage areas is to use an external network, i.e. routing the communication as a long-distance telephone call from a repeater interconnect or interface to a telephone exchange and then back through a second repeater interconnect to the remote repeater location. These channels are then "hung" to lock the channels in for the duration of the call. In addition to the increased expense and inconvenience of such an external network, the use of a repeater interconnect prevents usage of the channels in each trunked system over which the communication is occurring for the entire period of the communication.

Even in those instances where inter-coverage zone communications are made through an external network, such communication are private for individuals calls and are limited to a few preselected users on each system. It would be desirable to provide unique ID numbers for each user on a wide area network to allow for direct entry of the unique ID to establish private communication between users on the network. Unfortunately, it is impossible to accommodate unique ID numbers in the preferred present transmission trunked communication system because the number of digital bits required to represent such unique ID numbers exceeds the maximum number of bits that the trunked systems can communicate via the established signaling protocol. The signaling protocol of the preferred transmission trunked communication system is also limited in the number of channels that may be grouped together per trunked system. For example, the signaling protocol of the LTR.RTM. trunked system is limited to 20 channels per system and uses a look-up table to translate the channel information transmitted by the signaling protocol into the actual frequency pair assigned to that channel.

Another problem with the conventional switching and signaling protocols is that such protocols are unable to allow for the implementation of an extended feature set of radio communication capabilities on the preferred conventional transmission trunked communication systems. For example, present trunked systems are generally unable to establish priority access for users in a given coverage zone. Present trunked systems are also unable to interrogate and reprogram or otherwise modify the operation of mobiles without having the mobile transceiver unit physically brought to a service facility to be reprogrammed. A mobile traveling from one coverage zone to another coverage zone, for instance, needs to be reprogrammed for a different set of groups over which communications will be received. Such reprogramming cannot be done "on the fly" in present systems, and mobiles are therefor not able to transit coverage zones at will.

The present invention is particularly concerned with two problems in conventional switching and signaling protocols: (1) the reliance on a single home channel over which the subaudio control signals are transmitted, and (2) the inability to scan the activity within a given coverage zone by monitoring the subaudio control signals on a single channel. In present LTR systems, each group of mobile radios is preprogrammed to monitor a single home channel frequency. If the repeater for the home channel frequency is disabled or otherwise fails, the entire group of radios monitoring that home channel will no longer receive any of the control signals transmitted by the system and all of the mobile radios monitoring that home channel would effectively be out of service. With respect to the second problem, often times it is desirable to allow a transceiver to scan several groups of ID's to monitor communication traffic over several channels. This process is commonly referred to as scan mode. In the present LTR systems, the transceiver must switch through all of the reception frequencies for the group ID's being monitored in scan mode by changing the reception frequency programming of the mobile transceiver. This continual changing of reception frequency significantly degrades the amount of time that the mobile transceiver may be monitoring the system for activity.

Although present transmission trunked communication systems have proven to be an economical and effective means for establishing voice and data communications between a control station and a fleet of mobile vehicles in a given coverage zone, it would be advantageous to provide a method and apparatus for establishing an alternate home channel for a land mobile transmission trunked communication system that includes switching and signaling protocols especially adapted for communication of control signals in the subaudio band over both the home channel and the alternate home channel as the single alternate home channel for all channels within a given coverage zone.

In accordance with the present invention, a method and apparatus for an alternate home channel for a land mobile transmission trunked communication system designates one of a plurality of repeater means for a given site or coverage zone as an alternate home repeater means. The control signals transmitted over the subaudio band for all home channel repeater means in that site are also duplicatively transmitted over the subaudio band of the alternate home repeater means.

Each of the transmission trunked communication systems of the present invention includes a plurality of transceiver means for transmitting and receiving radio communications over one or more channels and a plurality of repeater means for establishing radio communication between the repeater means and one or more of the transceiver means. The radio communications are made up of an information signal (voice or data information) and a control signal (control information transmitted pursuant to a signaling protocol) and are communicated between the transceiver means and the repeater means using transmission trunking with the control signals transmitted simultaneously in the subaudio range of the same channel over which the information signals are being transmitted.

Each transceiver means will be programmed with two channel numbers. One for the home repeater means and the other for the site alternate home repeater means. When the power is turned on, the transceiver means determines which repeater means to monitor by scanning both the home and alternate home channels. When a valid control message is detected on either channel, that channel becomes the monitor channel. Operation of the transceiver means is then similar to prior operation, only with the monitor channel (either home channel or alternate home channel) being used instead of the home channel.

Both the home and alternate home channels have an equal chance of being the monitor channel. The monitor channel will not be changed unless valid control messages are no longer detected on the monitor channel or a system programmed with a different site number is selected. In the event that valid control messages are no longer being detected on the monitor channel, the transceiver means will monetarily switch over to the non-monitor channel to determine if valid control messages are being received on that channel. In the preferred embodiment, the transceiver means checks the monitor channel for activity for approximately two seconds and then switches to check the non-monitor channel for approximately 20 msecs. The performance of the transceiver means is not affected by which channel (home or alternate home) is used as the monitor channel.

Accordingly, it is an objective of the present invention to provide a method and apparatus for an alternate home channel for a land mobile transmission trunked communication system.

It is a further objective of the present invention to provide an alternate home channel in addition to the home channels currently used in land mobile transmission trunked communication systems that will duplicatively transmit control signals for the home channels in the subaudio band of the alternate home channel.

It is an additional objective of the present invention to provide an alternate home channel that will duplicatively transmit control signals for all of the home channels of a given coverage zone in the subaudio band of the alternate home channel so that any mobile transceivers in that given coverage zone may monitor the activity in the coverage zone by monitoring the control signals on the alternate home channel.

These and other objectives of the present invention will become apparent with reference to the drawings, the detailed description of the preferred embodiment and the appended claims.

FIG. 1 is an overall block diagram of the preferred embodiment of a multiple radio network terminal (RNT) switch means implementing the wide area network mobile radio communication system of the present invention.

FIG. 2 is a pictorial representation of a multiple site conventional transmission trunked land mobile communication system.

FIG. 3 is a pictorial representation of the RNT switch means of the present invention.

FIG. 4 is a block diagram of the data word used in a conventional transmission trunked land mobile communication system.

FIG. 5 is a block diagram of the time slot bus used to connect repeaters in a conventional transmission trunked land mobile communication system.

FIG. 6 is a block diagram of the data word in accordance with the present invention.

FIG. 7 is a block diagram of the high speed repeater data bus in accordance with the present invention.

FIGS. 8a-8c are block diagrams showing the three alternate methods of establishing priority access in accordance with the present invention.

FIG. 9 is a block diagram showing various interconnections of the modules for the RNT switch means in accordance with the present invention.

FIG. 10 is a block diagram showing various methods of interconnecting the RNT switch means with the repeaters and the dispatch consoles within a coverage zone.

FIG. 11 is a block diagram showing various methods of interconnecting multiple RNT switch means to provide a wide area network of multiple coverage zones.

FIG. 12 is a block diagram showing the various modules for the RNT switch means.

FIGS. 13a-13c are block diagrams of data, control and voice bus structures of the RNT switch means.

FIG. 14 is a flow diagram broadly depicting the operation of a mobile transceiver within a distributive wide area network transmission trunked communication system in accordance with the present invention.

FIG. 15a-15c are flow diagrams depicting in greater detail step 309 of FIG. 14.

FIG. 16a-16b are flow diagrams depicting in greater detail step 306 of FIG. 14.

FIG. 17 is a flow diagram depicting in greater detail steps 308-312 of FIG. 14.

FIGS. 18a-18c are flow diagrams of three dynamic reconfiguration features.

FIG. 19 is a flow diagram broadly depicting the operation of a mobile transceiver within a distributive wide area network transmission trunked communication system in accordance with the present invention.

FIG. 20 is a flow diagram depicting in greater detail the determine monitor channel step 302 of FIG. 19.

FIG. 21a-20c are flow diagrams depicting in greater detail step 304 of FIG. 19.

FIG. 22a-22d are flow diagrams depicting in greater detail step 306 of FIG. 19.

FIG. 23 is a flow diagram depicting in greater detail steps 308-312 of FIG. 19.

In the preferred embodiment of the present invention, a distributive radio network terminal (RNT) switch means is used to distributively interconnect a plurality of transmission trunked communication systems. Although it is preferred that the alternate home channel of the present invention be implemented on such a RNT switch means as described hereinafter, it will also be recognized that the present invention may be implemented equally as well on a land mobile transmission trunked communication system having only a single site or coverage zone. The various components of the preferred embodiment of the present invention using the RNT switch means are described with reference to FIGS. 1-13. FIGS. 14-18 describe the previous operation of a mobile transceiver means without the alternate home channel feature of the present invention. FIGS. 19-23 are a similar description of the operation of a mobile transceiver means with the alternate home channel of the present invention.

Referring now to FIG. 1, the preferred embodiment of the method and apparatus for distributively interconnecting a plurality of land mobile transmission trunked communication systems into a wide area network will be described. The invention includes a plurality of distributive radio network terminal (RNT) switch means 10 that utilize a unique signaling protocol and a switching protocol especially adapted for communication of control signals in the subaudio band. There is one RNT switch means 10 for each transmission trunked communication system. Each transmission trunked communication system includes a plurality of transceiver means 12 for transmitting and receiving transmission trunked radio communications over one or more channels and a plurality of repeater means 14 for establishing transmission trunked radio communication between the repeater means 14 and one or more of the mobile transceiver means 12. The transmission trunked radio communications comprises an information signal and a control signal using transmission trunking, with the control signals transmitted simultaneously in the subaudio range on the same channel over which the information signals are transmitted. The RNT switch means 10 are used for selectively interconnecting two or more of the repeater means 14 in each trunked system, as well as providing the link between trunked systems in a wide area network.

The RNT switch means 10 may also be connected to a dispatch console means 16 for monitoring the users at the transceivers means 12 and dispatching calls and other communications. Alternatively, a remotely located control station means 18 may also be used to perform the dispatching function on the system. The control station means 18 may includes a selective call console and communicates with the repeater means 14 by way of radio communication to link to the RNT switch means 10, rather than via a direct or hard-wired connection with the RNT switch means 10. In addition, the RNT switch means 10 may be connected with the telephone central office 20 or an external communication network 22 (e.g., a satellite communication system or a private PBX system) to allow users on the system to make and receive communications or calls outside of the network.

Two or more of the RNT switch means 10 may be interconnected by a node switch means 30 that operates as a high speed voice grade communication path between two RNT switch means 10. The node switch means 30 may be any type of matrix switching equipment or PABX that is capable of handling directed voice switching. The RNT switch means 10 may also be interconnected to a registration means 32 for maintaining a database of registration information that can be communicated among the switch means as a mobile transceiver moves from coverage zone to coverage zone as described hereinafter. Finally, one or more of the RNT switch means 10 may be connected to a monitoring means 34 for monitoring activity on the network and performing record keeping, billing and other accounting information functions. The activities of the node switch means 30, the registration means 32 and the monitoring means 34 may be contained within a single computer linked to each of the RNT switch means 10, or may be distributed among several processors linked to one or more of the RNT switch means 10, depending upon the type of configuration and the functions that are required of a particular user's network.

The general operation of an independent Transmission Trunked Communication System associated with each RNT switch means 10 will now be described. Control signaling between the repeater means 14 associated with an RNT switch means 10 and the transceiver means 12 is done with subaudio control signals on each channel designated as a "home channel". Each mobile transceiver means 12 in the trunked system is assigned to a home channel that it monitors when the transceiver is not involved in a call. The transceiver means 12 may be either a mobile transceiver located in a car or a handheld transceiver. (The terms mobile, transceiver and mobile transceiver as used hereinafter refer to any such transceiver means 12). The transceiver means 12 receives control signal instructions over the home channel to either stay on the home channel and open its audio to receive a call, or to change to a different channel and open its audio to receive a call. If the transceiver means 12 was sent to another channel to receive a call, it will return to its home channel as soon as the call is completed.

When a transceiver means 12 wants to initiate a call, all the operator has to do is depress the push-to-talk (PTT) button and hold it. If the home channel assigned to the mobile transceiver means 12 is available, the transmitter for the transceiver means 12 will transmit a control signal on the home channel frequency telling the repeater means 14 that a call is being setup on that channel. At that point, the transceiver means 12 turns its transmitter off and waits for a return message from the repeater means 14. If the repeater means 14 is not busy, the repeater means 14 will broadcast a handshake message that will be received by all mobile transceiver means 12 assigned to the same home channel. The Operator of the initiating transceiver means 12 will hear an audible proceed tone and its transmitter will be enabled to proceed with the call. For this type of fleet calling, all other transceiver means 12 with the correct fleet identification will remain on the channel and open their squelch to receive the call. All transceiver means 12 that did not have the proper fleet identification would remain muted, but the logic unit of the transceiver means 12 would receive and store the control signal indicating that their home channel was now busy and that they would get a free channel assignment. As a result, if a transceiver means 12 not involved in the call described above now wanted to initiate a call, it would know where a free channel was so that it would not have to look for a free channel in the trunked system.

If the home channel for a transceiver means 12 was busy when a call was initiated, the initial data transmission would be made on the free channel that it had stored in memory. The repeater means 14 receiving the call would notify the repeater means 14 for the home channel assigned to the transceiver means 12 initiating the call that a call was being setup on the free channel for a fleet assigned to the home channel repeater means 14. The home channel repeater means 14 would then add this information to the control signal being sent out over the home channel, and would also change the free channel assignment to another free channel. The transceiver means 12 that had a proper fleet assignment would receive the control signal sent out over the home channel that a call was being set up for their fleet on the directed channel and would switch to that channel and pen their squelch to receive the call. All other transceiver means 12 that did not have the proper fleet assignment would store the free channel information and would continue to monitor the home channel.

To understand how the method and apparatus for a mobile transmission trunked communication system wide area network of the present invention operates, it is helpful to divide the discussion of the preferred embodiment into a discussion of the signaling and switching protocol used by the invention and a discussion of the architecture and organization of both the wide area network and the individual transmission trunked communication systems associated with each RNT switch means 10. The overall philosophy behind both the protocols and the architectures that will be explained is a distributive approach to networking, as compared to the centralized approach currently applied to wide area networks for transmission trunked communication systems. This distributive philosophy reduces the overhead involved in managing the network and the trunked systems, while at the same time increases the reliability of the network and each trunked system by eliminating reliance on critical components and providing for built-in backup features to insure that communications over the network and the trunked systems are maintained.

The architecture for the signaling protocol of the present invention is an enhanced and expanded version of the LTR.RTM. signaling protocol that was first introduced into the United States market in 1980 and has since become the de facto transmission trunking standard. Familiarity with the LTR system, including the LTR signaling protocol, the LTR repeaters and the various LTR compatible mobile transceivers can be obtained from a review of the previously incorporated reference attached hereto as appendix A and is presumed for the discussion that follows.

Referring now to FIGS. 2 and 3, the differences between a conventional multiple site LTR repeater configuration and a transmission trunked communication system of the present invention will be explained. In the LTR configuration shown in FIG. 2, the mobile transceiver means 12 (Mobiles A, B and C) and the control station means 18 of System 1 cannot communicate with the mobile transceiver means 12 (Mobiles D and E) of System 2. The repeater means 14 use of a time slot bus 40 that indicates which channels are free and which channels are busy in the System. The time slot bus 40 allows the Mobiles in either System 1 or System 2 only to communicate with each other within their respective system by identifying a free channel and communicating together on the repeater means 14 associated with that channel. The time slot bus 40, combined with the LTR control message format described below limit the communication between repeaters in the LTR system to a method for assigning free channels in a group of channels (repeaters) that comprise a given trunked system. There is no provision to communicate information signals (voice/data communication) among repeaters in a trunked system, let alone communicate information signals between repeaters located in different trunked systems.

In the transmission trunked communication system shown in FIG. 3, each of the repeater means 14 is interconnected with a high speed, data bus 42 that provides for an improved switching protocol that allows data words to be communicated among the repeater means 14. The control message format of the signaling protocol of the present invention is also significantly changed to incorporate a unique identification for each mobile transceiver means 12 and to implement the priority access scheme, as well as other enhancements to the feature set radio communication capabilities. In addition, each of the repeater means 14 is also connected by a network bus means 44 with the RNT switch means 10 for their respective trunked system. As described in greater detail hereinafter, the network bus means 44 allows information signals to be communicated among repeaters in a system, as well as allowing both control and information signals to be communicated with another RNT switch means 10, or with an external communication network, i.e., a telephone exchange.

Referring now to FIG. 4, the control message formats 100 and 102 for the LTR signaling protocol is shown. In the LTR system shown in FIG. 2, system control is accomplished by sending control messages 100 and 102 shown in FIG. 4 between the repeater means 124 and the mobile transceivers 12 and control stations 18. Control message signaling occupies the subaudio frequencies below 150 Hz of a channel and occurs even when voice or data information is present on the channel. Control messages 100 are continuously transmitted to the repeater means 14 by the calling mobile transceiver means 12 while a conversation is in progress. The repeater means 14 also continuously transmits control messages 102 to the called mobile transceiver means 12, as well as to all other mobile transceiver means 12 monitoring that channel. The specific information in the control message 100 and 102 depends on whether it is repeater or mobile transmitted. The width of each bit in the control message is 3.36 msec (a data rate of approximately 300 baud) and a complete control message for the LTR signaling protocol is transmitted in approximately 135 msecs.

The data fields in the control messages 100 and 102 include:

Sync 104--The first two bits are used to initialize the receive data circuitry of the mobile transceiver. The other bits are used to detect the arrival of the control message 100 and provide bit synchronization.

Area 106--With mobile transmitted control messages 100, this is the area programmed in the mobile; with repeater transmitted control messages 102 messages, it is the bit programmed by the repeater programming switch. If the area transmitted in a control message does not agree with the programmed area, the call attempt will be unsuccessful. This bit is usually coded "0" unless there are two LTR systems close enough to interfere with each other. In that case it would then be coded "0" in one trunked system and "1" in the other trunked system.

Go-To 108--In a control message 100 from the mobile, this field contains the number of the repeater (1-20) to which the message is being transmitted. These bits must be the same as the number assigned to the repeater or the message is appearing on the wrong channel because of intermodulation and will be ignored. In control message 102 from the repeater to the mobile, this field contains the repeater number. In control messages to mobiles being trunked out to other repeaters, this field contains the repeater number (channel) to which those mobiles should switch to receive a message. If the repeater is not busy, a control message 102 is transmitted every 10 seconds to keep the mobiles updated. The Go-To 108 field then contains the number of the repeater. When the mobile push-to-talk switch is released, the transmitter remains on for a short time and sends a turn-off code in this field. This code is retransmitted by the repeater and it tells the receiving mobiles to squelch and resume monitoring the home channel. This prevents a "squelch tail" (noise burst) when the transmitting mobile unkeys.

Home Repeater 110--In a mobile-to-repeater control message 100, this field contains the home repeater number of the mobile being called. This number is always the programmed home repeater number in the selected system of the mobile making the call. In a repeater-to-mobile control message 102, this field contains the same repeater number received in the data message from the mobile. If the repeater is not busy, it transmits its own number in this field.

ID Code 112--In a mobile-to-repeater control message 100, this field contains the ID code (1-250) of the fleet or sub-fleet being called. In a repeater-to-mobile control message 102, this is the ID code of the fleet or sub-fleet being called on that channel. If other mobiles assigned to that channel have been trunked to other repeaters, additional messages are transmitted containing the ID code of these mobiles. In this case, the ID code information is from the repeater time-slot bus.

Free Channel 114--In a mobile-to-repeater control message 100, this field contains a pass code. In a repeater-to-mobile control message 102, this field contains the number of a repeater that is not busy and is available for service. The free repeater is chosen in a random manner as described in the next section. This information tells mobiles assigned to that repeater which repeater to use to make a call. If a repeater is not busy, it transmits its number in this field. If all repeaters are busy, a "0" is transmitted in this field.

Error Check Bits 116--These seven bits are used as a checksum to check for data errors in a data message. If an error is detected, the message is rejected.

Referring now to FIG. 5, the LTR switching protocol will be described. In a LTR transmission trunked communication system, a single-line serial bus interconnects the logic units of all the repeaters in the trunked system. The serial bus is time-divided to act as a time slot bus 120. One repeater in the system is programmed to generate a synchronization pulse that is used by all other repeaters in the system to determine their time slot 122 on the time slot bus 120. Time slots 1-20 are used by the respective repeaters and time slot 21 is used by the ID Validator. The time slot 122 used by each repeater is determined by the repeater number assigned to that repeater when the programming switch is set. For example, Repeater 1 uses time slot 1, repeater 5 uses time slot 5, and so on. The data rate on the time slot bus 120 is 18.750 baud.

During its time slot 122, each repeater places information on the time slot bus 120 indicating the status of that repeater. The time slot 122 consists of a first word 124 and a second word 126. If a repeater is not busy, only the start bits 128 appears in the time slot 122. If a repeater is busy, information appears in its time slot 122 identifying the home repeater 130 and ID code 132 of the mobile being called on that channel. If a repeater number is unassigned, nothing appears in the time slot 122. Each repeater monitors all the time slots and if that repeater's number is detected in another time slot, the repeater begins transmitting an additional control message using the signaling protocol described above. This message tells a mobile to go to another channel to receive a message. This additional message is transmitted for as long as the mobile is transmitting on the other channel.

The free repeater indicated in the Free Channel 114 field of control message 102 as shown in FIG. 4 is determined by continuously monitoring information in the twenty time slots 122. The free repeater number changes as these slots are monitored. If a time slot 122 has only start bits 128 indicating an idle repeater, that is the current free repeater. If information in a time slot 122 indicates no repeater of a busy repeater, the free repeater number does not change. For example, assume a five repeater system with repeaters assigned numbers 1, 5, 9, 13, and 17. if repeaters 5 and 13 are busy, the free repeater changes as follows as the twenty time slots are monitored: 1 1 1 1 1 1 1 1 9 9 9 9 9 9 9 9 17 17 17 17. The free repeater transmitted in the Free Channel 114 field of control message 102 is the free repeater detected at the instant the message is composed.

Although the present combination of LTR signaling protocol for control messages and switching protocol for determining free channels is an efficient means of signaling mobile and assigning free channels in a single transmission trunked communication system, these same protocols have prevented the use of the LTR system as a wide area network capable of providing transmission trunked radio service over a number of coverage zones. In addition to the lack of voice/data communication among repeaters in a trunked system and the inability to communicate either control or information signals between repeaters located in multiple systems, there is no capability to provide for the unique identification necessary to properly interconnect multiple trunked systems together into a wide area network.

The signaling and switching protocols of the present invention allow the advantages of the LTR signaling protocol to be retained, while allowing multiple systems of repeaters to be grouped together in a wide area, transmission trunked radio communication system. In addition, an expanded feature set is provided that supports features beyond those features currently available on LTR transmission trunked communication systems. The result is a mobile transmission trunked communication system wide area network that does not need a dedicated control channel because the control signals are transmitted subaudibly, simultaneously with the information signals, that provides a fast access time through the system (approximately 0.5 secs) that will not increase as the number of channels in a system are increased, that provides continuous control signaling to enable mobiles just brought into service or coming out of a fade to begin monitoring a call automatically (this is imperative in public safety systems where the risk of loss of communication in an emergency situation is critical), that has a signaling protocol that is frequency transparent and can be used on different frequency bands, that provides unique identification numbers for each mobile on the entire system, and that encodes the trunked transmissions to prevent unauthorized reception.

Referring now to FIG. 6, the control message format for the signaling protocol of the present invention is shown. The data fields in the control messages 150 and 152 include:

Sync 154--The first two bits are used to initialize the receive data circuitry of the mobile transceiver. The other bits are used to detect the arrival of the control message 150 or 152 and provide bit synchronization.

Reserved 156--With mobile transmitted control messages 150, the Reserved 156 bit is always a "0". With repeater transmitted control messages 152, Reserved bit 156 is used to indicate that the control message 152 will not be decoded by the repeater and sent back out. In other words, the Reserved bit 156 indicates that the control message is from the repeater and it is used on critical messages sent from the RNT switch means 10, for example, commands relating to dynamic reprogramming such as disable mobile, reprogram mobile, etc.

Go-To 158--In a control message 150 from the mobile, this field contains the FCC channel number of the repeater to which the message is being transmitted. These bits must be the same as the number assigned to the repeater or the message is appearing on the wrong channel because of intermodulation and will be ignored. Unlike the 5 bit Go-To 108 used in the LTR signaling protocol, Go-To 158 does not require that the mobile be equipped with a predetermined look-up table in order to determine the proper frequency to transmit and receive. The channel number contained in Go-To 158 can be used to directly calculate the transmit and receive frequencies, thereby simplifying the logic in the mobile transceiver. In control message 152 from the repeater to the mobile using the channel, this field contains the FCC channel number. In control messages to mobiles being trunked out to other repeaters, this field contains the FCC channel number to which those mobiles should switch to receive a message. If the repeater is not busy, a control message 152 is transmitted every 10 seconds to keep the mobiles updated. The Go-To 158 field then contains the FCC channel number of the repeater. When the mobile push-to-talk switch is released, the transmitter remains on for a short time and sends a turn-off code in this field. This code is retransmitted by the repeater and it tells the receiving mobiles to squelch and resume monitoring the home channel. This prevents a "squelch tail" (noise burst) when the transmitting mobile unkeys.

Home Repeater 160--In a mobile-to-repeater control message 150, this field contains the home repeater number of the mobile being called (1-30). This number is the programmed home repeater number in the selected system of the mobile making the call and may form a part of the home/group ID of the particular fleet or sub-fleet using the trunked system. In a repeater-to-mobile control message 152, this field contains the same repeater number received in the control message from the mobile. If the repeater is not busy, it transmits its own number in this field.

Group 162--In a mobile-to-repeater control message 160, this field contains the Group ID codes (1-254) of the fleet or sub-fleet being called. In a repeater-to-mobile control message 152, this is the Group ID code of the fleet or sub-fleet being called on that channel. The portion of the Group ID codes from 222-254 are reserved for special functions as described in greater detail hereinafter. It should be noted that the mobile units can be programmed with a different Group ID encode and decode, depending upon how the particular trunked system is to be configured.

Free Channel 164--In a repeater-to-mobile control message 152, this field contains the FCC channel number of a repeater than is not busy and is available for service. This information tells mobiles assigned to that repeater which repeater to use to make a call. If a repeater is not busy, it transmits its own channel number in this field. If all repeaters are busy, a "0" is transmitted in this field.

Unique 166--In both the mobile-to-repeater control message 150 and repeater-to-mobile control message 152, Unique 166 generally contains the unique ID code of the transmitting mobile or of the transmitting dispatch console. The 13 bits of Unique ID 166 provide up to 8,192 unique identification codes with 0, 8,164-8,191 being reserved for special functions unique 166 contains special function data, such as control information for dynamic reconfiguration.

Priority 168--In the mobile-to-repeater control message 150, this field indicates the 5 level APCO (Association of Public Communications Officers) standard priority level assigned to the mobile transmitting the control message. The mobile priority information is also used by the RNT switch to monitor that priority of all of the currently active calls in case the trunked system is full and a dispatch console has requested the RNT switch to force a user off the trunked system to free up a channel for an emergency call. In the repeater-to-mobile control message 152, this field indicates the priority level currently on the trunked system. If in the last control message 152 that a mobile has received an access priority level for Priority 168 that is higher than the priority level assigned to the mobile, then the mobile will not attempt to use the trunked system.

The various methods of using the priority access scheme of the present invention are best understood by reference to the state diagrams shown in FIGS. 8a-8c. In the first case, FIG. 8a, the priority access system is set-up with no channels in the trunked system reserved for emergency or priority use. In this case at State 200, if all channels are busy. Priority 168 is set to "1" and only those mobiles with a priority of "1" will attempt to access the trunked system. If one channel frees up at State 202, Priority 168 remains at a "1". If two or more channels free up at State 204, then Priority is set of "5" and all mobiles may now attempt to access the system. A timed cascading sequence is entered once State 202 is reached such that after each 0.5 seconds of elapsed time without a change in a channel freeing up or becoming busy, the level of priority in Priority 168 is deceased by one to "3" at State 206, "3" at State 208, "4" at State 210, and finally to "5" at State 212 two seconds after State 202 was originally entered. In FIG. 8b, a priority scheme for a trunked system having one channel reserved and two effective levels of priority is shown. At either State 220 or 222, Priority 168 is "1" to insure that there will be at least one channel open if there is a channel free in the trunked system. At State 224, Priority 168 is set to "5" to allow all users to access the trunked system. In FIG. 8c, a combination of reserved and non-reserved priority schemes is shown where there is one reserved channel and five levels of priority. At States 230, 232 and 234, Priority 168 is "1" to insure that at least one channel will remain free. At State 236, if three or more channels are free, Priority 168 is set to "5" to allow all users to access the trunked system. At State 234, a timed cascading sequence similar to that of State 202 is entered that starts the decrease in priority levels through States 238, 240, 242 and 244, when Priority 168 is set to "5" after two seconds of no new activity on the trunked system.

Chk 170--These eight bits are used as a checksum to check for data errors in a data message. If an error is detected, the message is rejected. The first seven bits of Chk 170 are derived from a truncated BCH (127,120) encoding scheme performed on a selected portion of the control message 150 and 152, as described below. The eighth check bit is a simple parity bit based on the number of bits in the selected portion of the control message 150 and 152 and the 7 BCH check bits. It should be noted that although a known BCH encoding scheme is used, any other type of encoding scheme known in the art could be used as well. In the preferred checksum technique, the eight-bit Chk 170 checksum is used only to detect the presence of errors in the control message and not to correct errors. The eight-bit checksum enables 100% detection of all one-, two-, and three-bit errors and also all odd errors of five bits or greater. In addition, approximately 99.3l% of all even errors of four bits or greater are detected.

Unlike the prior LTR signaling protocol, Chk 170 is also used as part of an encryption coding scheme to increase the security of the signaling protocol. A two-word system key is provided to each repeater means 14 and transceiver means 12 on the trunked system. This two word key is used to encode and decode each of the transmissions on the trunked system. In the preferred embodiment, prior to computing the checksum, a selected number of bits from the control message are bit-operated on by the first word of the two word key. The preferred bit-operations to encode a bit stream are well known and may include transposition, exclusive or, shifting, or any other equivalent technique. After this step, the entire control message, except Sync 154, is used to compute the eight-bit checksum. With the checksum completed, a second bit-operation is performed on the completed checksum with the second word of the system key to encode it prior to placing the 8-bit checksum in Chk 170. This dual encoding scheme significantly increases the difficulties associated with decoding the transmission because the receiver of the control signal is never sure whether the transmission was a good transmission without bit errors unless the second word of the system key is used to decode the checksum information.

RSVD 172 and Stat 174-152. RSVD 172 is reserved for future expansion and Stat 174 is defined to return the current status of the mobile.

Referring now to FIG. 7, the high speed repeater data bus 42 for the repeater means 14 of the present invention will be described. The principal behind the switching protocol for the repeater data bus 42 is to provide a method for a fixed number of repeater means 14 to communicate quickly and efficiently over a common data bus with distributed control and variable time frame, while still maintaining synchronization on the bus. The switching protocol on the repeater data bus 42 is arranged into time slots or messages 180. Each message 180 is made up of a fixed number of words 182 that are in turn made up of a fixed number of bits. An entire group of messages for a particular trunked system is a frame. All of the repeater means 14 monitor the repeater data bus 42 constantly, but transmit on the repeater data bus 42 only during their respective timeslot. In the preferred embodiment, the maximum period for a timeslot is the length of one message 180 (approximately 2.4 msecs), with each message 180 comprised of 6 words 182, each word consisting of a start bit, 8 data bits, a sync bit and a stop bit.

Unlike the switching protocol of the present LTR trunked system, the switching protocol of the present invention does not rely on a fixed number of messages in a frame to determine the position of the time slots in that frame. As explained below, the length of a time slot may be effectively shortened if the repeater means 14 designated to transmit during that time interval is missing from the trunked system by skipping the message 180 associated with the missing repeater. The ability to skip messages 180 in the format of the frames for the signaling protocol of the present invention minimizes the time period of the frame for trunked systems having less than the maximum number of 30 repeaters. It also provides the capability for communicating control signal information among the repeaters without increasing the time period of the frame beyond the point where performance of the entire trunked system is degraded.

The first word 182 of each message 180 is uniquely identified as the first word by the sync bit 184. When the sync bit 184 is a "1", the word 182 is recognized by all of the repeaters means 14 on the repeater data bus 42 as the beginning of a new message 180. For all other words 182 in a message 180, the sync bit 184 is set to "0". The contents of the first word 182 are From 186, the FCC channel number of the repeater means transmitting this message 180. When the sync bit 184 is received by each repeater on the repeater data bus 42, the repeater updates the time slot count of the repeater number to the repeater number in From 186. When all of the words 182 of the message 180 have been received, the time slot count is incremented to look for the repeater number of the next repeater. Thus, if all of the repeaters in a five repeater trunked system are present, the protocol of the repeater data bus 42 would look like that shown in the first frame shown in FIG. 7. If a repeater is not present for a time slot, or if the repeater does not communicate a complete message 180, all of the other repeaters on the trunked system wait for a delay period of a little more than one word and then increment the time slot count to the next repeater. In this way, the time slots associated with missing or defective repeaters can be sequenced through much faster than the normal time period. When one or more of the repeaters are missing, the protocol of the repeater data bus 42 would look like the second or third frame shown in FIG. 7.

The second word 182 of the switching protocol is To 188--the channel number of the repeater to whom the following four Data Words 190 are directed. The remaining four words 182 of the switching protocol are Data Words 190 consisting of R 156, GoTo 158, Group 162, and Unique 166 from the switching protocol described above. The ability to transmit the same information being sent out by one repeater to another repeater in the trunked system allows the trunked system to more efficiently monitor and account for the mobile transceiver in the trunked system.

Also, unlike the switching protocol of the present LTR trunked system, the switching protocol of the present invention is not dependent upon a specified repeater in the trunked system to send out the sync word of the protocol format. Because each repeater on the repeater data bus 42 is distributively responsible for establishing synchronization with the other repeaters, a repeater may be added to or removed from the trunked system without affecting the operation of the trunked system and without the need to reprogram the repeaters on the trunked system. Moreover, the trunked system is not dependent upon any single repeater to establish the synchronization for the repeater data bus 42. If one repeater was to fail, the rest of the trunked system would continue to operate and, because of the backup home channel feature, the users would notice only a possible small degradation in service because of one fewer trunked channels on the trunked system.

It will be apparent that the signaling and switching protocols of the present invention will allow a variety of features and options to be implemented to establish an expanded feature set for the mobile communication system wide area network of the present invention, including: variable operation, traffic logging, diagnostics, group validation, selective unit disable, dynamic reconfiguration, priority access, redundant repeater control, alternate home channel backup, dynamic channel assignment, and automatic or manual registration.

The alternate home channel feature of the present invention assigns an alternate home channel for each transceiver means 12 on the trunked system. Generally, for each site or coverage zone of a trunked system, one non-home channel will be assigned as the alternate home channel for all of the home channels in that site or coverage zone. In the event that a mobile transceiver means 12 does not detect any communication on its home channel for a predefined period of time, the mobile switches to the preassigned alternate home channel. After switching to the alternate home channel, if the mobile transceiver does not detect any communication on the backup channel for a predefined time period, the mobile will presume that it has gone out of the coverage zone of the trunked system and, if it has been programmed to do so, will attempt to automatically register on any of the other available trunked systems in the wide area network that the mobile is a user on. If, after switching to the backup channel, the mobile does detect communication, the mobile will use the alternate home channel as its home channel. To implement a distributive approach to the alternate home, the designated alternate home channel retransmits the control signals of all of the home channels for a given coverage zone or site of the trunked system at selected intervals on the alternate home channel. A more detailed description of the operations of the alternate home channel of the present invention is set forth hereinafter with reference to FIGS. 19-23.

Dynamic reconfiguration of the mobile transceivers in any of the trunked systems of the wide area network of the present invention is accomplished without the need for an exchange of overhead information between the repeater and the mobile by using a select portion of the Group ID's in Group 162 and the unique identification number of the mobile or a data field in Unique 166 to transmit the necessary commands in the control message 152 sent from the repeater to the mobile. In the preferred embodiment, the Group ID's between 230 and 254 are reserved for these special communication functions. The information sent and received in the Unique 152 field will vary depending upon the particular dynamic reconfiguration function being performed.

It will be understood that the operation of dynamic reconfiguration on the trunked system requires compatible software programming in both the RNT Switch and the mobile, along with reprogrammable memory in the mobile for those functions that will change or reprogram initial or default values that have been stored in the mobile. For example, a given transceiver means 12 is originally programmed with a specific set of authorized groups for the trunked system it will be operating in. These groups may be stored in a table in memory (i.e. an EEPROM) in the mobile logic unit. If the mobile were to travel out of the coverage zone associated with the trunked system it is assigned to and into another coverage zone, the mobile could also have stored in its memory registration channels associated with adjacent coverage zones that it would allow the mobile to contact those trunked systems. When it is determined that the mobile has left the original zone and entered a new coverage zone, the mobile would contact the trunked system in the new coverage zone and request automatic registration in that zone. The new RNT switch means 10 for that zone would contact the home RNT switch means 10 through the registration means 32 to verify that the mobile was allowed to register in the new coverage zone and inform the home RNT switch means 10 that the mobile had left its coverage zone. Once the mobile is registered in the new coverage zone, the new RNT switch means 10 might reprogram the group that the mobile was associated with might reprogram.

The signaling protocol for any of the function includes the same basic steps of: (1) transmitting a NOP special code with the Unique ID of the mobile being reconfigured (this step is repeated four times on the channel of the mobile being reconfigured to insure that if the mobile is in the coverage zone it will receive the command); (2) transmitting one or more of the special Group ID's for the particular command or function being performed and transmitting the appropriate data for that function in the Unique 166 field; (3) transmitting a DONUT (do nothing) special code with the Unique ID of the mobile being reconfigured to indicate that the particular command or function is complete; (4) waiting for a command acknowledgement (if the command was accepted) or a command no-acknowledgement (if the command was rejected) special Group ID to be transmitted by the mobile in response to the particular command or function; and (5) waiting for a timeout condition if no command acknowledgement or no-acknowledgement is received. FIGS. 18a-18c are sample flow diagrams of three types of a dynamic reconfiguration, a Selective Unit Disable, a Reassign Group ID, and a Reassign Select Data.

The Radio Network Terminal (RNT) switch means 10 of the present invention connects several forms of communication systems together in a network system that allows communication between users of the network system and also allows a plurality of network systems to be linked together into a single wide area network. The RNT switch means 10 provides the interface to standard communication consoles (dispatch consoles 16 or control stations 18) for central dispatching and control. The RNT switch means 10, in conjunction with the dispatch consoles 16 or control stations 18, provides the mechanism to utilize the expanded feature set of the signaling protocol previously described, including Unit Interrogate, Unit Status, Priority, Dynamic Reassignment, Selective Unit Disable, etc.

Referring now to FIG. 9, the various forms of communication systems that are interfaced with the RNT switch means 10 are shown. The RNT switch means 10 is capable of controlling up to 30 channels of radio communications. The channels can be a combination of conventional channel repeaters 24, LTR repeaters 26 or the repeater means 14 of the present invention. The maximum number of each of these types of repeaters is 30 for the conventional channel repeaters 24, 20 for the LTR repeaters 26, and 30 for the repeater means 14 of the present invention. The node switch means 30 allows communication with another RNT switch means 10.

The RNT switch means 10 connects the various communication paths among these elements and gives priority access to the proper modules. System trunking during calls is preserved through the RNT switch means 10 unless fixed by full-duplex or the use of hangtime. Access time for calls to and from any one of the trunked systems is less than 0.5 seconds, excluding signaling time for external devices such as Tone Remote detection time and the passing of DTMF information. The RNT switch means 10 automatically routes outgoing telephone calls to the telephone central office 20 on the least cost line and produces a traffic accounting record of calls made by Group and/or Unique ID. The record may be accessed by the monitoring means 34 and includes: call time, call duration, telephone number dialed, and if a transceiver means 12 terminated or originated the call. In the preferred embodiment, these records are preserved in non-volatile disk memory.

Wide area coverage between multiple RNT switch means 10 is directed by DTMF entries from either the requesting transceiver means 12 or the telephone control office means 20 or the control stations means 18. Wide area coverage on a group basis within a given RNT switch means is performed by a crosspatch of the dispatch console means 16 to the desired repeater means 14. Wide area coverage on either a group or individual basis outside the RNT switch means 10 is determined by the requesting mobile transceiver means 12 and is established by the node switch means 30.

Referring now to FIGS. 10 and 11, various methods for interconnecting the elements associated with the RNT switch means 10 are shown. In FIG. 10, the RNT switch means 10 is shown as interconnecting by RF communication with conventional channel repeaters 24 (either VHF or UHF), by microwave link with an LTR repeater 26, and by microwave, leased line, or direct connection with the repeater means 14 of the present invention. The RNT switch means 10 may be connected with the dispatch console means 16 by RF communication, by microwave link, by direct connection or by leased line through a telephone interconnect. In FIG. 11, the various methods of linking multiple RNT switch means 10 in a wide area network are shown. The RNT switch means 10 may be connected with the node switch means 30 via microwave link, leased lines or by direct connection. The registration means 32 may also be connected with the RNT switch means by each of these methods. Again, the RNT switch means 10 may be connected to the repeater means 14 by any variety of methods, depending upon whether the repeater means 14 is physically located proximate or distant to the RNT switch means 10.

Referring now to FIG. 12, the internal architecture of the RNT switch means 10 will be described. The main interfaces to the RNT switch means 10 include the Channel Interface to the various channels, the interface with Dispatch Consoles 16, the interface with the Public Switched Telephone Network 20, the interface to other RNT's via the Node Switch means 30, and the interface to the System Manager. The RNT switch means 10 is constructed of different modules that interface from the RNT switch means 10 to external devices.

The modules as shown in FIG. 12 and described in the following sections are: the Channel Interface Module (CIM), connecting the RNT switch means 10 to a LTR repeater or to the repeater means 14 of the present invention; the Conventional Channel Module (CCM), connecting the RNT switch means 10 to a Conventional Channel; the Dispatch Interface Module (DIM), connecting the RNT switch means 10 to the Dispatch Consoles 16; the Telephone Interface Module (TIM), connecting the RNT switch means 10 to the PSTN 20 or PABX 22; the System Network Module (SNM), connecting the RNT switch means 10 to other SNM's, and, via the node switch means 30, to other RNT switch means 20; the Voice Tone Module (VTM), providing voice and tone messages to the RNT switch means 10; the Network Interface Module (NIM), connecting the RNT switch means 10 to the SMM; and; the Switch Manager Module (SMM), providing setup and monitoring capabilities for the RNT switch means 10.

The system Manager Module controls the portion of the RNT switch means 10 that connects various communication paths by programming various options on the different connections. Programming is done on a computer equipped with a color monitor and sufficient serial ports for communication with the necessary system elements and an option printer. The system manager functions are protected against access by a minimum of one security level. In the preferred embodiment, the SMM is an 80286 based computer, including a keyboard, color monitor; 640K RAM, 1 floppy disk, and a 40 Megabyte hard disk. The functions performed by the SMM include: monitor traffic and produces a usage record; determine channel usage and amount of time all channels in a site are busy in time periods of 6,10, 15, 30 or 60 minutes; receive notification as soon as a channel or system element failure is detected; program hangtime, initiated on a mobile channel from the switch, to hold a connection on the Trunked System; program a Dispatch Console 16 (equipped with connection control) with the ability to participate in a call; program a Dispatch Console 16 for full duplex communication (the dispatch console 16 informs the system when full duplex communication takes place); program a Dispatch Console 16 for "All Call" transmissions and priority levels (A request for "All Call" transmission acquires all channels in a trunked system for transmission from the dispatch console); program a Dispatch Console with priority levels associated with the Group IDs; initiate ID Validation that determines if a Group or Unique ID is allowed to use the system for voice communication or any operation and maintain a list of which IDs are enabled or mobile units disabled for operation by the switch; performs dynamic reconfiguration of a mobile unit in the Multi-net Trunked System; perform least cost routing for outgoing telephone calls.

The Network Interface Module (NIM) interfaces the Switch Management Module (SMM) to the internal communication of the RNT switch means 10. The NIM buffers data communication via a full duplex RS-232 channel on the Intra-terminal Data Bus (IDB) to the System Management Bus (SMB). The NIM is also a buffer between the Channel Status Bus (CSB) to the System Status Bus (SSB). This data is a "snap shot" of the channel activity sent once every two seconds on a RS-232 channel in one direction only, from the NIM to the SMM. The NIM provides the Master Clock and Master Sync for the signaling control of the pulse code modulation (PCM) voice busses. The Master Clock and Master Sync for the PCM busses are redundant in the NIM. A detection circuit monitors the two different sets and if master clock A is off frequency, B is activated. If both A and B are off frequency, an alarm is sent to the SMM and the NIM removes both sets from distribution in the RNT switch. The RNT switch is allowed Master and Slave NIM, enabled or disabled from operation on the data communication path. The master tells the slave when both its clocks are off frequency and the slave clock is distributed, if the slave clocks are on frequency.

The Channel Interface Module (CIM) connects the RNT switch means 10 to the repeater means 14 and to conventional LTR Repeaters 26. Each repeater means 14 has a CIM that monitors and controls the repeater through a signaling protocol. The CIM controls a LTR repeater means 14 through the Multi-Net Logic Module (MLM). A MLM is the logic drawer in the repeater that interfaces the CIM in the RNT with the receiver and transmitter in the repeater. The MLM is connected to the CIM by 4-wire 600 ohm balanced audio. The MLM is controlled by blank and burst mode, 1200 baud, Audio Frequency Shift Keying (AFSK) data or by digital RS-232 data for both transmit and receive data. The CIM passes information to and from the MLM that controls the repeater and its operation.

The Conventional Channel Module (CCM) connects the RNT switch means 10 to a conventional repeater 24. Each conventional repeater 24 has a CCM that controls the repeater through logic signaling. The CCM controls a conventional repeater through the Conventional Repeater Module (CRM). A CRM is the logic drawer in the repeater that interfaces the CCM in the RNT with the transmitter and receiver in the repeater. The CRM is connected to the CCM by 4-wire 600 ohm balanced audio. The CRM is controlled by blank and burst mode, 12200 baud, Audio Frequency Shift Keying (AFSK) data for both transmit and receive data. The CRM connects to the repeater via 4-wire 600 ohm balanced or unbalanced audio. The CCM reflects the activity of the repeater and acts as a gateway for transmissions between the RNT switch and the conventional repeater, thus allowing the conventional repeater to look like an LTR repeater or the repeater means of the present invention for a given site, home, and group.

The Dispatch Interface Module (DIM) uses one of three methods of interface, the RNT switch to the dispatch equipment, Direct Connection, Tone Remote, and Digital Data. A DIM with direct connection is associated with only one group code and uses a form of Type II E & M lead input similar to the mobile PTT. A DIM with tone remote connection decodes a guard tone for the PTT indication and decodes DTMF to select from one of ten group codes. A control sequence uses the DTMF to place the DIM in scan mode or to stope and use a specific group code. The tone remote feature requires the use of an add on personality card. The most versatile DIM has a digital data connection that requires a separate full duplex 9600 baud, RS-232 channel. The separate data path is required because the intelligent dispatch console can monitor and display the information of the system. The information exchanged between the DIM and an intelligent dispatch console contains what the DIM receives and transmits. The DIM connects the 4-wire 600 ohm balanced audio with the Pulse Code Modulation (PCM) data paths and communicates to other modules via the Intraterminal Data Bus (IDB). The DIM also monitors the Channel interface Module (CIM) Channel Status Bus (CSB) to determine if a CIM is active with a DIM group. Some of the functions performed by the DIM include: unique ID display; status display; unique ID calling; emergency display; emergency answer; site select calling; interrogate--static and mobile.

The Trunk Interface Module (TIM) connects the RNT switch to the telephone lines and handles telephone line protocols. The TIM logic unit is microprocessor controlled and communications to the Switch Management Module (SMM) via the intra-terminal data communications. A RNT may have multiple TIMs with varying styles of telephone lines attached, for example, connection to Central Office, or connection to PABX system. The telephone line audio is processed to interface with the Pulse Code Modulation (PCM) voice path. The basic TIM operation functions are: Signaling, Call Supervision and Intra-terminal Communications.

The System Network Module (SNM) is the interface that allows Unique ID calling from mobile units and multiple RNT switches to be connected in a total wide area network. The SNM uses a 4-wire 600 ohm balanced audio on the interface and connects audio to the PCM data paths. The SNM also uses a Type II E&M lead connection set for connection initiation and supervision. The SNM communicates with other modules via the Intra-terminal Data Bus (IDB) and monitors the Channel Status Bus (CSB) for the group set up to use the SNM. The RNT switch may be one terminal in a multiple terminal system created by connecting the RNT switch via the SNM, through a 4-wire E&M lead interface, to a system switching node or Private Automatic Branch Exchange (PABX). The SNM can dial pulse or DTMF the desired site, another SNM interface in the same RNT switch or in a different RNT switch. SNM's pass information on the interface relative to the type of call desired and connection status of the end device using Audio Frequency Shift Keying (AFSK) in a blank and burst mode. This form of data passing allows a standard 4-wire interface connection and does not require a specialized system switching node.

The Voice Tone Module (VTM) provides tone and voice messages to modules that indicate call progress to the users. The VTM contains 8-fixed slots of digitized PCM audio messages stored in PROM that are 4-seconds in length and repeat continuously on the VTM PCM bus of the RNT switch. The start of the message is indicated in the transmission so modules can present the audio message to the user from the beginning of a message. There may be up to four VTMs within a system, depending upon the messages required.

The various bus means that interconnect the modules of the RNT switch means 10 are shown in FIGS. 13a-13c. Together, these busses comprise the network bus 44 that links the various modules in the RNT switch means 10 to one another and to the repeater means 14. FIG. 13a shows the interrelationship of the various modules and the data busses. The Message Acounting Bus (MAB) is a 9600 baud bus with a sync-escape protocol. The MAB passes information of the system call status once a second, telephone accounting data, and error records. The Channel interface Bus (CIB) is a 1200 baud sync-escape protocol bus that passes system channel control and status to and from the CIM. The Channel Status Bus (CSB) is a 19200 baud sync-escept protocol bus that passes channel status from channel modules to other system modules. The Dispatch interface Bus (DIB) is a 1200 baud sync-escape protocol bus that passes DIM control and status to and from the Dispatch Consoles. The Dispatch Tone Protocol (DTP) passes industry standard tone remote push-to-talk tone and DTMF for DIM control and status to and from the dispatch consoles. The Intra-Terminal Data Bus (IDB) is a 19200 baud sync-escape protocol bus that modules communicate on to pass control information to each other. The System Network Bus (SNB) is a 1200 baud sync-escape protocol bus that passes control and status from one SNM to another. The System Management Bus (SMB) is a 9600 baud sync-escape protocol bus that passes information from the SMM to and from the other modules via the NIM. The System Status Bus (SSB) is a 9600 baud sync-escape protocol bus that passes the CSB data to the SMM or external peripheral equipment once every 2 seconds via the NIM. The Registration Bus (RGB) is a 9600 baud sync-escape protocol bus that passes the mobile unit registration information to and from the registration system.

Referring now to FIG. 13b, the control busses used to control the flow of data and voice information in the RNT switch means 10 are shown. The IDB Idle/Collision line is a three level line that indicates if the line is idle, busy or collision occurred for the IDB communication process. The CSB Idle/Collision line is a three level line that indicates if the line is idle, busy or collision occurred from the CSB communication process.

In FIG. 13c, the voice busses that pass information signals (either voice/data). The VTM Bus is a 2.048 MHz Pulse Code Modulated (PCM) bus containing 32 slots of tones or voice the modules use to indicate call progress to an end user by means of audio information. The Tx PCM bus is a 2.048 MHz Pulse Code Modulated (PCM) bus containing 32 slots. Audio received from the mobile channel is transmitted on these slots by the channel modules and received by the other system modules. The Rx PCM bus is a 2.048 MHz Pulse Code Modulated (PCM) bus containing 32 slots. Audio is transmitted from the other system modules to these slots and received by the channel modules to be transmitted on the mobile channel. The Master Sync provides the synchronization signal for the PCM bus communication to begin the frame of 32 PCM slots. The Master Clock provides the 2.048 MHz clocking signal for the PCM busses.

For a more detailed technical description of the circuitry associated with the RNT switch means 10, as well as a more detailed description of the operation and setup of the RNT switch means 10, reference is made to the description of the operation of the E. F. Johnson Multi-Net.TM. system contained in the manual entitled "Multi-Net.TM. Radio Network Terminal Service manual", Part No. 001-3039-001 (Rev. 1, Mar. 1989), which will be available from E. F. Johnson Company, Waseca, Minn.

The operation of a mobile transceiver 12 within the distributive wide area network transmission trunked communication system of the present invention will now be described with reference to the flow diagrams in FIGS. 14-17.

FIG. 14, broadly depicts in flow diagram format, how a mobile transceiver 10 communicates in the communications system of the present invention. The transceiver is powered up on the initialized radio step 300. Operational flow is then immediately directed to the step 302 wherein the transceiver 10 goes to its preassigned home channel. Operational flow then proceeds to step 304, where the transceiver 10 listens for incoming signals transmitted on its home channel. As described in detail above, the transceiver will be monitoring its home channel, or other temporarily assigned channel, for signaling information relating to system priority, channel availability, etc., that is broadcast by the system.

Operational flow proceeds to step 306, to initiate a system transmission request, when the operator of the transceiver desires to transmit. The request to communicate is acknowledge in step 308 through a return control signal transmitted by the assigned repeater, and operational flow proceeds to step 310 where the transmission takes place. Once the transmission has been completed the transmitter is released in step 312, and operational flow returns to step 304 where the system continues to monitor the system for activity.

The monitor system for activity step 304 of FIG. 14 is described in greater detail in FIGS. 15a through 15c. Operational flow first proceeds to decision block 320 to determine whether the transceiver squelch has been opened for the channel currently being monitored. If the squelch is not open (i.e. the transceiver is not receiving any signals), operational flow is directed to decision block 322 to determine whether the squelch has been closed for a predetermined time frame (such as 20 milliseconds). Operational flow is directed to step 324, where the channel currently being monitored is marked as being free, if the squelch has been closed for the predetermined time period. Operational flow is then directed back to decision block 320 to again determine whether the squelch has become open. If the squelch has not been closed for the appropriate predetermined time, operational flow is directed from decision block 322 directly to block 320, without marking the channel as free. Requiring the squelch to be closed for a predetermined time before marking a channel as free prevents false indications of channel status caused by spurious noise that may be received by the transceiver.

Operational flow is directed from decision block 320 to step 326 if the squelch is determined as being open (i.e. it is determined that the transceiver is receiving signals). At step 326, the transceiver applies the system key, described herein above, and attempts to decode the control message received by the transceiver. The transceiver will attempt to decode the control message for a predetermined time of approximately 250 milliseconds. Operational flow is then directed to decision block 328 to determine whether the control message 152 was decoded. If nothing was decoded, operational flow proceeds to step 329. At step 329 the transceiver determines if three consecutive attempts to decode the control message 152 have been attempted. If there has not been three consecutive attempts, then operational flow returns to step 320 where the transceiver continues to monitor its assigned channel. If there has been three consecutive attempts, then operational flow continues to step 324 and the channel is marked as free.

Operational flow is directed from decision block 328 to decision block 330, if it is determined that a proper control message has been decoded, where the transceiver will determine whether the incoming transmission is addressed to it. If the incoming transmission is addressed to the transceiver, operational flow proceeds to step 340 where the incoming signal is received and processed.

Operational flow is directed from decision block 330 to step 332, if the received transmission is not addressed to the transceiver. The free channel and system access priority information included in the received the control message is at that point saved by the transceiver. Operational flow is then directed to decision block 334 where it is determined whether the push to talk switch of the transceiver is depressed (indicating the operator of the transceiver desires to make a transmission) or not. If the push to talk switch is depressed, operational flow is directed to step 306 where the transceiver initiates a system communication request. It will be appreciated that the free channel and system access priority data saved in step 332 will be used in the system communication request procedure.

Operational flow is returned from decision block 334 to step 320, if the push to talk switch is not depressed, where the transceiver continues to monitor for incoming transmission.

FIG. 15b depicts in detail the active receive information step 340 of FIG. 15a. At step 342, in FIG. 15b, the control message received by the transceiver is analyzed to determine which channel the transceiver needs to change frequencies to receive the transmission intended for it, and the transceiver is switched to that channel. Operational flow is next directed to decision block 344 where it is determined whether the transceiver is being addressed as part of a "normal" group of transceivers (i.e. a predetermined group of listeners such as a squad of police cars, fleet of taxi cabs, etc.), or whether the transmission being received by the transceiver is for special group functions. If the transmission being received by the transceiver is for a special group function, operational flow proceeds to step 400 (detailed in FIG. 15c).

Operational flow is directed from, decision block 344 to decision block 346 if the control message is addressed to a normal group of users. Decision block 346 determines whether the transceiver has been disabled or not. As described hereinabove, the transceiver can be remotely disabled from either receiving or transmitting. For instance, if a particular user is no longer an authorized user, the transceiver can be automatically disabled. The position of decision block 346 in relation to the determination of whether the group ID is a "normal" group is important in that a special group function as detailed in FIG. 15c, can be processed by the transceiver whether or not the transceiver has been disabled for normal voice communications or not. If the transceiver has been disabled, operational flow proceeds to step 304 for further monitoring of the system.

Operational flow proceeds from decision block 346 to step 348 if the transmission received by the transceiver is for a normal group ID and the transceiver has not been disabled. At step 348, the receive audio on the transceiver is opened, portions of the control message are saved by the transceiver for comparison in later steps, and a record entitled "data words try" (for reference later in the program at Step 364) is set to zero.

Operational flow proceeds from step 348 to decision block 350 where the transceiver determines if the squelch continues to be open. If the squelch has been closed, operational flow is directed to decision block 352 to determine whether the squelch has been closed for a predetermined time (such as 250 milliseconds). If the squelch has not been closed for the predetermined amount of time, operational flow returns from decision block 352 to decision block 350 to determine whether the squelch has been opened in the interim. The 250 millisecond delay in operational flow is used to prevent disconnection of the transceiver due to momentary fades in the incoming signal, or similar short interruptions of the signal. If the squelch has been closed for the predetermined amount of time, operational flow proceeds to step 364.

Operational flow proceeds from decision block 350 to step 360 if the transceiver squelch remains open. At step 360, the transceiver tries to again decode the control message. This is a repeat of the decode function performed at step 326. The second decode operation is required to ensure that the transceiver remains on the assigned channel through the transmission. Operational flow proceeds from step 360 to decision block 362 to determine whether successful decoding of the control message was accomplished. If there was not a successful decoding, operational flow is directed from decision block 362 to step 364 where the "data words try" record referred to in step 348 is incremented by one. Operational flow then proceeds to decision block 366 where it is determined whether there have been ten attempts to decode the control message. If there have not been ten attempts to decode the control message, operational flow is returned from decision block 366 to decision block 350 where, if the squelch still remains open, another attempt at decoding the control message is made. If ten unsuccessful tries at decoding the control message are made, operational flow is directed from decision block 366 to steps 354 and 356 where the receive audio is closed, the transceiver is directed back to its home channel, and operational flow again returns to the monitor system activity step 304.

Once the control message has been successfully decoded, operational flow proceeds from decision block 362 to decision block 370, where the control message decoded in step 360 is compared to the control message saved in step 344. The comparison is required because control messages are continuously transmitted by the system, and it is possible that the repeater could interleave different control signals on the same channel during the time the transceiver is receiving a transmission. Comparing the control signals received during a transmission to the control signal that initiated reception of the transmission is required so that the transceiver will not shift to receive a second transmission having a different control signal. If the control message is the same as the save control message, and if the "go to" field of the control message is the turn off code (indicating the end of the received transmission), operational flow is directed to steps 354 and 356 where the receive audio is closed, the transceiver is set to the home channel, and is cleared of any auxiliary and telephone settings. Operational flow then proceeds to step 304 for continued monitoring of system activity.

Operation flow is directed to decision block 372 if the tests of block 370 are not both met. Operational flow is directed form decision block 372 to step 374 if the control message decoded at step 360 is the same as the control message saved in step 348, and the "go to" field is a hang code. The receive audio is closed in step 374 and operational flow is returned to step 304 for continued monitoring of system activity. It will be noted that the operation flow does not reset the transceiver to the home channel when the long code is detected. The option to "hand" the transceiver on a channel, while closing the receive audio, can be used to hold the channel to maintain the priority of an individual call.

Operational flow is directed to decision block 376 if the tests of block 372 are not met. If the control message decoded in step 360 is the same as the message saved in step 348, operational flow is redirected to step 342. It will be appreciated that operational flow will stay in a loop to step 392 throughout a received transmission as long as the received control message remains unchanged.

Operational flow is directed to decision block 378 if the test of block 376 is not met. The transceiver can be preprogrammed with two separate group IDs. If the group ID of the control message decoded in step 360 is one of the two preprogrammed group IDs, operational flow is directed from block 378 to step 342. If the test of block 378 is not met, operational flow is directed to step 364.

The processing of special group IDs, step 400, is depicted in detail in FIG. 15c. Operational flow is first directed to decision block 402 where it is determined whether the special group ID is an "all call" ID (i.e., all transceivers are to receive the communication). If the special group ID is an "all call" ID, operational flow is directed back to decision block 346 of FIG. 15b. If the transceiver has not been disabled (decision block 346), the incoming transmission is processed for reception by the transceiver as described above.

Operational flow is directed from decision block 402 to decision block 404 if a special group ID is not an "all call" ID. Block 404 determines whether or not there is an "interrogate" special group ID presented by the control message. The interrogate function instructs the transceiver to immediately respond to the transmitting repeater with the present status of the transceiver. In particular, the operator of the transceiver can selectively input a message to be automatically transmitted to the repeater as to what the operator is presently doing (for example, the operator is not in the vehicle, the operator has completed an assigned job, or the operator is waiting for a new assignment). Operational flow is directed to the decision block 406 if the test of block 404 is met. At block 406, it is determined whether the transceiver has been disabled from the interrogate function. For instance, if the transceiver had been stolen, and the system no longer wishes to track the location of the mobile, the interrogate function of the transceiver could be disabled. If the transceiver has not been disabled from the interrogate function, operation flow proceeds from decision block 406 to step 408 where the preprogrammed response to the interrogate special group ID is transmitted.

Operational flow is directed from decision block 404 to decision block 412 if the group ID is not an "interrogate" signal for the transceiver. Operational flow is then directed from decision block 412 to step 410 if the transceiver has been disabled. The transceiver is set to its "home" channel at step 410 and is directed back to step 304 for continued monitoring of system activity.

Operational flow is directed from decision block 412 to decision block 414, where it is determined whether the special group ID is an auxiliary call for the transceiver, if the transceiver has not been disabled. If the incoming transmission is an auxiliary call, (i.e., an individual as opposed to a fleet call to a particular transceiver), operational flow is directed from decision block 414 to step 416 where the incoming call is marked for full duplex communications to handle the auxiliary call. Operational flow is then directed from step 416 to decision block 346 of FIG. 15b.

Operational flow is directed from decision block 414 to decision block 418 if it is determined that the incoming transmission is not an auxiliary call for the transceiver. At decision block 418 it is determined whether the incoming transmission is a telephone call for the transceiver. If the incoming transmission is a telephone call for the transceiver, operational flow is directed from decision block 418 to step 420 where the incoming transmission is marked for full duplex communications to handle the telephone call. Operational flow is then directed from step 420 to decision block 346 of FIG. 15b.

Operational flow is directed to decision block 422 if the test of block 418 is not met. At block 422 it is determined whether the incoming transmission is a dynamic reassignment for the transceiver. If the incoming transmission is a dynamic reassignment for the transceiver, operational flow is directed to step 424 where the dynamic reassignment occurs, as described hereinabove. Operational flow is then directed from step 424 to step 410 and is then returned to step 304 for continued monitoring of the system activity.

Operational flow is directed from decision block 422 to decision block 426 if the incoming transmission is not a dynamic reassignment for the transceiver. At decision block 426, it is determined whether the incoming transmission is a disablement command for the transceiver. If the incoming transmission is a disablement command, operational flow is directed to step 428 where the transceiver is disabled. Operational flow then proceeds to step 410 for reassignment of the transceiver to its home channel, and then to step 304 for continued monitoring of the system.

The initiate system communication request step 306 is depicted in detail in FIGS. 16a and 16b. Operational flow is first directed to decision block 450 where it is determined whether the push-to-talk switch has been depressed by the operator of the transceiver. If the push-to-talk switch has not been depressed, operational flow is directed to step 452 where the transmitter is unkeyed, and operational flow is returned to step 304 for continued monitoring of the system activity. Operational flow is directed from decision block 450 to decision block 454 if the push-to-talk switch of the transceiver is depressed by the operator. At decision block 454, it is determined whether the radio has been disabled. If the radio has been disabled, operational flow proceeds from decision block 454 to decision block 460 where it is determined whether the push-to-talk switch is still depressed. If the push-to-talk switch is still depressed, and the radio has been disabled, operation flow remains at block 460. Once the operator releases the push-to-talk switch, operational flow is directed to step 462 where the busy tone and intercept tone is released and the operational flow is directed to step 304 for continued monitoring of the system activity.

Operational flow is directed from decision block 454 to decision block 456 if the push-to-talk switch is depressed and the transceiver has not been disabled. The priority of the desired outgoing transmission is compared to the current system access priority. It will be appreciated that the determination of access priority is being made at the transceiver, prior to any outgoing transmission being made by the transceiver. If the transmission priority in the transceiver is not equal to or higher than the current system access priority, operational flow is directed to step 458 where the busy tone is initiated.

Operational flow is directed from decision block 456 to decision block 470 if the priority of the desired outgoing transmission is equal to or higher than the current system access priority. At decision block 470, it is determined whether an auxiliary or telephone group has been received. If the auxiliary or telephone group has been received, operational flow is directed from decision block 470 to step 500 for communication request acknowledgement, since the handshaking requirements of steps 472-492, and 460, 462 are not required in full duplex operation of the transceiver.

It will be appreciated by those skilled in the art, that if full duplex operation is not provided for, a handshaking procedure must be followed to initiate communications. The handshaking procedure is initiated by directing operational flow from decision block 470 to step 472 where the transceiver is directed to a free channel. Operational flow next proceeds to step 474 where one control message is transmitted from the transceiver to the repeater initiating the call. Also in step 474, the record of the number of attempts made by the transceiver to transmit a control message to a repeater is incremented by one. It will be appreciated that the record of transmitting attempts ("TXAMPTS") is zeroed out at step 306 where the system communication request is initiated.

Operational flow is next directed to step 476 where the transceiver attempts to obtain a control message back from the repeater in response to the transmission at step 474. The attempt at step 476 continues for approximately 250 milliseconds. Operational flow is next directed to decision block 478 where it is determined whether or not a control message was received and decoded by the transceiver. If a control message was decoded, operational flow proceeds to decision block 484 where it is determined whether or not the received control message was in fact a response to the request initiated at step 474. Assuming the response was valid, operational flow is directed from block 484 to step 500 where the communication request is acknowledge and full communication is allowed.

If no control message is decoded at decision block 478, operational flow is directed to block 480 where it is determined how many attempts at transmitting a control message from the transceiver (step 474) have been made. If a predetermined number of attempts (for instance 10) have not been made, operational flow is redirected to decision block 456 to set up another attempt at establishing a handshake between the transceiver and the repeater. If more than 10 attempts have been made, operational flow is directed to step 482 where an intercept tone is presented by the transceiver to the operator, indicating that the handshake was not accomplished, and communications have not been established. Operational flow then proceeds from step 482 to decision block 460 where it is determined whether the push-to-talk switch has been depressed. If the push-to-talk switch is depressed, operational flow is hung at decision block 460 until the push-to-talk switch is released. Once the push-to-talk switch is released, operational flow proceeds from decision block 460 to step 462 where the intercept tone and the busy tone are turned off. Operational flow then returns to step 304 for continued monitoring of system activity.

Returning to decision block 484, if a control message is received by the transceiver, but the control message received is not a proper response to the handshake, operational flow is directed to decision block 486. It will be appreciated that receiving any response to the handshake, even if not proper response, is preferable to receiving no response, because it indicates to the transceiver that it is in communications range. At decision block 486, the transceiver determines whether the control message received indicates that the same channel is already occupied. Operational flow is directed to step 488, if the channel is so occupied, where the free channel information and system access priority information is saved from the received control message. Operational flow is then redirected to step 450 where the handshaking procedure will again be initiated.

Operational flow is directed to decision block 490 if the test of block 486 is not met. Operational flow will be directed from block 490 to step 492 and then back to step 476 if there have been less than two consecutive tries to decode a control message received in response to the control message transmitted at step 474 by the transceiver. If there have been two consecutive attempts at decoding, operational flow is returned to block 450 where another attempt at handshaking will be made.

The communication request is acknowledge at step 500, when a successful handshake is made. Operational flow is then directed to block 502. Decision block 502 is a user selectable feature. A proceed tone can be enabled if the user of a transceiver wants to hear an audible signal when a communication request has been acknowledged. If the proceed tone feature is enabled, operational flow proceeds from its decision block 502 to step 504 where the audible tone is initiated, and is then directed to step 506. If the proceed tone feature is not enabled, operational flow proceeds directly from decision block 502 to step 506 and the transceiver control message and voice circuits are enabled.

Operational flow proceeds from step 506 to decision block 508 where it is determined whether the push-to-talk switch is active. The operational flow will stay at step 508 as long as the push-to-talk switch is depressed, or until a predetermined time-out period expires. Once the push-to-talk switch is released, operational flow proceeds to step 512 where the voice circuit is disabled, and then to step 514 where one control message with a turnoff code is transmitted. Operational flow then proceeds to decision block 516 to determine whether the transceiver has been set for full duplex operation. The transceiver is maintained at the channel it is currently on (step 518) if communications are full duplex. The transceiver is reset to its home channel (step 520), if full duplex operations are not required.

The operation of a mobile transceiver 12 equipped with the alternate home channel feature of the present invention will now be described with reference to the flow diagrams depicted in FIGS. 19-23. This description will highlight the differences between the present invention and the prior signaling and switching protocol described in connection with FIGS. 14-17 previously described. For ease of comparison, those operational flow steps that have not changed are again designated by reference numerals from 200-599, the operational flow steps that have changed or are new are designated by reference numerals 700-799.

If mobiles transceivers 10 were limited to receive update messages from just their home repeater means 14 and that repeater means 14 became inoperative, all mobile transceivers 10 assigned to that repeater means 14 would be out of service. To prevent this from happening, the present invention uses one repeater means 14, hereinafter referred to as the means, to provide backup for the other repeaters means 14. The alternate home repeater means transmits update messages for all calls occurring at a given site or coverage zone. All mobiles assigned to that site normally can receive update messages from either the alternate home repeater or their home repeater. Therefore, if the home repeater fails, the mobiles assigned to that repeater can still monitor the alternate home repeater for the update information needed to place and receive calls. In the preferred embodiment of the present invention, there will be no more than 2 seconds of missed communication, maximum, in the event that a home repeater were to fail.

As the preferred embodiment of the present invention repeater means 14 is implemented by software and control changes to both the mobile transceiver means 10 and the existing no additional hardware or circuitry is required. For existing LTR systems, some hardware changes may be necessary to bring the repeater means of the LTR systems up to the level of the repeater means 14 of a MultiNet system.

The following terms will be used in describing the preferred embodiment of the present invention.

Site--A set of repeaters located in the same general vicinity or coverage zone and connected to the same high speed data bus.

Home Repeater--A mobile transceiver means 10 has one of the site repeaters assigned as its "home" repeater. The home repeater/group ID information is used to identify mobiles when standard dispatch calls are placed.

System--Normally applies to the 1-16 systems selectable by the mobile system select switch on the mobile transceivers. Each system can be programmed with a different home repeater, group ID codes, and other transceiver operating parameters.

Alternate Home Repeater--One repeater in a site that is designated to transmit update messages occurring on all other repeaters at the site. This repeater is available for voice traffic, but it is not normally assigned as the home repeater for any mobiles.

Monitor Repeater--This is the repeater that a mobile is currently monitoring for update messages. This repeater may be either the mobile's home repeater or the site alternate home repeater. The mobile transceiver determines which repeater (home or alternate home) is to be monitored. Generally, the mobile monitors the last repeater that a valid data message was detected on. The monitor repeater is always used to make a call unless it is busy. When it is busy, the mobile is trunked to one of the other site repeaters to make the call. Selection of this repeater is random, so any site repeater may be used.

Each selectable system of a mobile transceiver will be programmed with two channel numbers. One for the home repeater and the other for the site alternate home repeater. When the mobile power is turned on, the mobile determines which repeater to monitor by scanning both the home and alternate home channels. When valid data is detected on either channel, that channel becomes the monitor channel. Both the home and alternate home channels have an equal chance of being the monitor channel. The monitor channel will not be changed unless valid control messages are no longer detected on the monitor channel or a system programmed with a different site number is selected. Transceiver performance is not affected by which channel (home or alternate home) is used as the monitor channel.

Each selectable system of a mobile transceiver must be programmed with the site number of the repeaters that are accessed by that system. For proper mobile operation, selectable systems with the same site number must be programmed with the same alternate home channel. The alternate home channel normally does not have any mobiles or systems assigned to it because these mobile would not have any other alternate home channel available for their backup. However, if the ID codes of the alternate home repeater are needed, these codes may be assigned without any loss of calling efficiency for the site.

It should be noted that the operation of the present invention with the alternate home channel feature only affects the transmission of control signals in the subaudio range. Voice and data information is still transmitted in the same manner as previously described for land mobile trunked transmission communication systems. The voice or data information will be transmitted over the GoTo channel if the monitor channel (either home channel or alternate home channel) is busy.

FIG. 19, broadly depicts in flow diagram format, how a mobile transceiver 10 communicates in the communications system of the present invention with the alternate home channel feature when operating in non-scan mode. Operation of the mobile transceiver 10 in scan mode is described in connection with FIGS. 23 and 24. The transceiver is powered up in the initialize radio step 300. Operational flow is then immediately directed to the step 702 wherein the transceiver 10 selects the monitor channel it will start using from either its home channel or the alternate home channel. Operational flow then proceeds to step 704, where the transceiver 10 listens for incoming control signals transmitted on the monitor channel. As described in detail above, the transceiver will be monitoring the subaudio range of a channel, in this case the monitor channel or other temporarily assigned channel, for signaling information relating to system priority, channel availability, etc., that is broadcast by the repeaters in the network.

Operational flow proceeds to step 706, to initiate a system transmission request, when the operator of the transceiver desires to transmit. The request to communicate is acknowledged in step 708 through a return control signal transmitted by the assigned repeater, and operational flow proceeds to step 710 where the transmission takes place. Once the transmission has been completed the transmitter is released in step 712, and operational flow returns to step 704 where the transceiver continues to check the monitor channel for activity.

The Determine Monitor Channel step 702 of FIG. 19 is described in greater detail in FIG. 20. Briefly, step 702 will switch back and forth between the home channel and the alternate home channel until activity in the form of a properly decoded control message is detected on the subaudio range of one of the two channels. At step 714, the current channel to be checked for activity is arbitrarily set to the home channel to start checking with the home channel. Operational flow proceeds to block 716 to determine whether the transceiver squelch has been opened for the current channel. If the squelch is not open on the current channel (i.e. the transceiver is not receiving any signals), operational flow is directed to decision block 718 to determine whether the squelch has been closed for a predetermined switch time period (such as 20 milliseconds). If not, the squelch check on the current channel is continued. If so, operational flow is directed to step 720 where the current channel is flipped to the non-current channel and operation flow returns to block 716. If the squelch is open on the current channel, then the transceiver attempts to obtain a control message at block 722, in the same manner as done in connection with block 326. At decision block 724, a check is made to see if the control message was properly decoded, similar to block 328. If the control message was not properly decoded, operational flow proceeds to block 720 to flip the current channel and return to block 716. If the control message was properly decoded, operational flow proceeds to block 726 where the current channel is set to the monitor channel. A monitor counter is then set with a count value equal to approximately 2 seconds. The use of the monitor counter will be described in connection with step 704 set forth hereinafter. The transceiver is now ready to begin checking the system for activity at step 704.

The Check system for Activity step 704 of FIG. 19 is described in greater detail in FIGS. 21a through 21c. Operational flow first proceeds to decision block 730 to determine whether the transceiver squelch has been opened for the channel currently being monitored, the monitor channel. If the squelch is not open, operational flow is directed to decision block 322 to determine whether the squelch has been closed for a predetermined time frame (such as 250 milliseconds). If so, operational flow is directed to step 324, where the monitor channel is marked as being free, if the squelch has been closed for the predetermined time period. Operational flow then proceeds to block 732. If the squelch has not been closed for the appropriate predetermined time, operational flow is directed from decision block 322 directly to block 732, without marking the channel as free. Requiring the squelch to be closed for a predetermined time before marking a channel as free prevents false indications of channel status caused by spurious noise that may be received by the transceiver.

At block 732, the monitor counter is decremented and at decision block 734, the monitor counter is checked to see if it is greater than zero. Operation flow proceeds back to block 320 if the monitor counter is not zero and to block 736 if the monitor counter is fully decremented. In essence, the loop created by blocks 730, 322, 324, 732 and 734 operates as a timing loop to check the monitor channel for activity for a given period of time, i.e. the length of time for monitor counter to decrement to zero. If there is no activity on the monitor channel, then the monitor channel is switched to the non-monitor channel as described next with respect to FIG. 21a-2 to briefly check to see if there is any activity on the non-monitor channel and, if so, switch the non-monitor channel to the monitor channel. By this process, the monitor channel is checked a first relatively long time period (2 seconds) and the non-monitor channel is then briefly check for a second relatively short time period (20 msecs).

At block 736, the channel currently being checked is temporarily switched to the non-monitor channel and the free channel is set equal to zero to insure that a free channel is acquired before communication takes place. Operation flow proceeds to block 738 to determine whether the transceiver squelch has been opened for the channel currently being monitored, now the non-monitor channel. If the squelch is not open on the non-monitor channel, operational flow is directed to decision block 740 to determine whether the squelch has been closed for a predetermined switch time period (such as 20 milliseconds). If not, the squelch check on the non-monitor channel is continued. If so, operational flow is directed to step 742 where the channel currently being checked is flipped back to the monitor channel. At block 744, the monitor counter is reset to two seconds and operational flow returns to block 720. If the squelch is open on the non-monitor channel, then the transceiver attempts to obtain a control message at block 746, similar to clock 326. At decision block 748, a check is made to see if the control message was properly decoded, similar to block 328. If the control message was not properly decoded, operational flow proceeds to block 742 and 744 to flip back to the monitor channel and return to block 730. If the control message was properly decoded, operational flow proceeds to block 750 where the non-monitor channel is set to the monitor channel. Operational flow then reenters the check system for activity loop 704 at block 752 in FIG. 15a-l.

Operational flow is directed from decision block 730 to step 326 if the squelch is determined as being open (i.e. it is determined that the transceiver is receiving signals). At step 326, the transceiver applies the system key, described herein above, and attempts to decode the control message received by the transceiver. The transceiver will attempt to decode the control message for a predetermined time of approximately 250 milliseconds. Operational flow is then directed to decision block 328 to determine whether the control message was decoded. If nothing was decoded, operational flow proceeds to step 732.

Operational flow is directed from decision block 328 to step 752 where the monitor counter is reset. If a valid control message was decoded, then there is system activity on the monitor channel and the transceiver should keep checking the monitor channel for activity. At decision block 330, if it is determined that a proper control message has been decoded, the transceiver will determine whether the incoming transmission is addressed to it. If the incoming transmission is addressed to the transceiver, operational flow proceeds to step 340 where the incoming signal is received and processed.

Operational flow is directed from decision block 330 to step 332, if the received transmission is not addressed to the transceiver. The free channel and system access priority information included in the control message is at that point saved by the transceiver. Operational flow is then directed to decision block 334 where it is determined whether the push-to-talk switch of the transceiver is depressed (indicating the operator of the transceiver desires to make a transmission) or not. If the push-to-talk switch is depressed, operational flow is directed to step 706 where the transceiver initiates a system communication request. It will be appreciated that the free channel and system access priority data saved in step 332 will be used in the system communication request procedure.

Operational flow is returned from decision block 334 to step 730 via blocks 732 and 734, i the push-to-talk switch is not depressed, where the transceiver continues to monitor for incoming transmission.

FIG. 21b depicts in detail the active receive information step 340 of FIG. 21a. At step 342, in FIG. 21b, the control message received by the transceiver is analyzed to determine which channel the transceiver needs to change frequencies to receive the transmission intended for it, and the transceiver is switched to that channel. Operational flow is next directed to decision block 344 where it is determined whether the transceiver is being addressed as part of a "normal" group of transceivers (i.e. a predetermined group of listeners such as a squad of police cars, fleet of taxi cabs, etc.), or whether the transmission being received by the transceiver is for a special group functions. If the transmission being received by the transceiver is for a special group function, operational flow proceeds to step 400 (detailed in FIG. 21c).

Operational flow is directed from decision block 344 to decision block 346 if the control message is addressed to a normal group of users. Decision block 346 determines whether the transceiver has been disabled or not. As described hereinabove, the transceiver can be remotely disabled from either receiving or transmitting. For instance, if a particular user is no longer an authorized user, the transceiver can be automatically disabled. The position of decision block 346 in relation to the determination of whether the group ID is a "normal" group is important in that a special group function as detailed in FIG. 21c, can be processed by the transceiver whether or not the transceiver has been disabled for normal voice communications or not. If the transceiver has been disabled, operational flow proceeds to step 704 for further monitoring of the system.

Operational flow proceeds from decision block 346 to step 348 if the transmission received by the transceiver is for a normal group ID and the transceiver has not been disabled. At step 348, the receive audio on the transceiver is opened, portions of the control message are saved by the transceiver for comparison in later steps, and a record entitled "data words try" (for reference later in the program at Step 364) is set to zero.

Operational flow proceeds from step 348 to decision block 350 where the transceiver determines if the squelch continues to be open. If the squelch has been closed, operational flow is directed to decision block 352 to determine whether the squelch has been closed for a predetermined time (such as 250 milliseconds). If the squelch has not been closed for the predetermined amount of time, operational flow returns from decision block 352 to decision block 350 to determine whether the squelch has been opened in the interim. The 250 millisecond delay in operational flow is used to prevent disconnection of the transceiver due to momentary fades in the incoming signal, or similar short interruptions of the signal. If the squelch has been closed for the predetermined amount of time, operational flow proceeds to step 364.

Operational flow proceeds from decision block 350 to step 360 if the transceiver squelch remains open. At step 360, the transceiver tries to again decode the control message. This is a repeat of the decode function performed at step 326. The second decode operation is required to ensure that the transceiver remains on the assigned channel through the transmission. Operational flow proceeds from step 360 to decision block 362 to determine whether successful decoding of the control message was accomplished. If there was not a successful decoding, operational flow is directed from decision block 362 to step 364 where the "data words try" record referred to in step 348 is incremented by one. Operational flow then proceeds to decision block 366 where it is determined whether there have been ten attempts to decode the control message. If there have not been ten attempts to decode the control message, operational flow is returned from decision block 366 to decision block 350 where, if the squelch still remains open, another attempt at decoding the control message is made. If ten unsuccessful tries at decoding the control message are made, operational flow is directed from decision block 366 to steps 354 and 356 where the receive audio is closed, the transceiver is directed back to its home channel, and operational flow again returns to the monitor system activity step 704.

Once the control message has been successfully decoded, operational flow proceeds from decision block 362 to decision block 370, where the control message decoded in step 360 is compared to the control message saved in step 344. The comparison is required because control messages are continuously transmitted by the system, and it is possible that the repeater could interleave different control signals on the same channel during the time the transceiver is receiving a transmission. Comparing the control signals received during a transmission to the control signal that initiated reception of the transmission is required so that the transceiver will not shift to receive a second transmission having a different control signal. If the control message is the same as the saved control message, and if the "go to" field of the control message is the turn off code (indicating the end of the received transmission), operational flow is directed to steps 354 and 356 where the receive audio is closed, the transceiver is set to the home channel, and is cleared of any alternate home and telephone settings. Operational flow then proceeds to step 704 for continued monitoring of system activity.

Operation flow is directed to decision block 372 if the tests of block 370 are not both met. Operational flow is directed from decision block 372 to step 374 if the control message decoded at step 360 is the same as the control message saved in step 348, and the "go to" field is a hang code. The receive audio is closed in step 374 and operational flow is returned to step 704 for continued monitoring of system activity. It will be noted that the operation flow does not reset the transceiver to the home channel when the long code is detected. The option to "hang" the transceiver on a channel, while closing the receive audio, can be used to hold the channel to maintain the priority of an individual call.

Operation flow is directed to decision block 376 if the test of block 372 are not met. If the control message decoded in step 360 is the same as the message saved in step 348, operational flow is redirected to step 342. It will be appreciated that operational flow will stay in a loop to step 394 throughout a received transmission as long as the received control message remains unchanged.

Operational flow is directed to decision block 378 if the test of block 376 is not met. The transceiver can be preprogrammed with two separate group IDs. If the group ID of the control message decoded in step 360 is one of the two preprogrammed group IDs, operational flow is directed from block 378 to step 342. If the test of block 378 is not met, operational flow is directed to step 362.

The processing of special group IDs, step 400, is depicted in detail in FIG. 21c. Operational flow is first directed to decision block 402 where it is determined whether the special group ID is an "all call" ID (i.e., all transceivers are to receive the communication). if the special group ID is an "all call" ID, operational flow is directed back to decision block 346 of FIG. 21b. If the transceiver has not been disabled (decision block 346), the incoming transmission is processed for reception by the transceiver as described above.

Operational flow is directed from decision block 402 to decision block 404 if a special group ID is not an "all call" ID. Block 404 determines whether or not there is an "interrogate" special group ID presented by the control message. The interrogate function instructs the transceiver to immediately respond to the transmitting repeater with the present status of the transceiver. In particular, the operator of the transceiver can selectively input a message to be automatically transmitted to the repeater as to what the operator is presently doing (for example, the operator is not in the vehicle, the operator has completed an assigned job, or the operator is waiting for a new assignment). Operational flow is directed to the decision block 406 if the test of block 404 is met. At block 406, it is determined whether the transceiver has been disabled from the interrogate function. For instance, if the transceiver had been stolen, and the system no longer wishes to track the location of the mobile, the interrogate function of the transceiver could be disabled. If the transceiver has not been disabled from the interrogate function, operation flow proceeds from decision block 406 to step 408 where the preprogrammed response to the interrogate special group ID is transmitted.

Operational flow is directed from decision block 404 to decision block 412 if the group ID is not an "interrogate" signal for the transceiver. Operational flow is then directed from decision block 412 to step 410 if the transceiver has been disabled. The transceiver is set to its monitor channel at step 750 and is directed back to step 704 for continued monitoring of system activity.

Operational flow is directed from decision block 412 to decision block 414, where it is determined whether the special group ID is an auxiliary call for the transceiver, if the transceiver has not been disabled. If the incoming transmission is an auxiliary call, operational flow is directed from decision block 414 to step 416 where the incoming call is marked for full duplex communications to handle the auxiliary call. Operational flow is then directed from step 416 to decision block 346 of FIG. 21b.

Operational flow is directed from decision block 414 to decision block 418 if it is determined that the incoming transmission is not an auxiliary call for the transceiver. At decision block 418 it is determined whether the incoming transmission is a telephone call for the transceiver. If the incoming transmission is a telephone call for the transceiver, operational flow is directed from decision block 418 to step 420 where the incoming transmission is marked for full duplex communications to handle the telephone call. Operational flow is then directed from step 420 to decision block 346 of FIG. 21b.

Operational flow is directed to decision block 422 if the test for block 418 is not met. At block 422 it is determined whether the incoming transmission is a dynamic reassignment for the transceiver. If the incoming transmission is a dynamic reassignment fort the transceiver, operational flow is directed to step 424 where the dynamic reassignment occurs, as described hereinabove. Operational flow is then directed from step 424 to step 410 and is then returned to step 704 for continued monitoring of the system activity.

Operational flow is directed from decision block 422 to decision block 426 if the incoming transmission is not a dynamic reassignment for the transceiver. At decision block 426, it is determined whether the incoming transmission is a disablement command for the transceiver. If the incoming transmission is a disablement command, operational flow is directed to step 428 where the transceiver is disabled. Operational flow then proceeds to step 759 for reassignment of the transceiver to its monitor channel, and then to step 704 for continued monitoring of the system.

The initial system communication request step 706 is depicted in detail in FIGS. 22a-22d. At decision block 760, a test is made to see if the free channel=zero and if the current channel=non-monitor channel. If not, operational flow may proceed directly to block 450. If so, operational flow proceeds to step 762 where the transceiver is moved to the monitor channel. Operational flow is then directed to decision block 450 where it is determined whether the push-to-talk switch has been depressed by the operator of the transceiver. If the push-top-talk switch has not been depressed, operational flow is directed to step 452 where the transmitter is unkeyed, and operational flow is returned to step 704 for continued checking of the system activity. Operational flow is directed from decision block 450 to decision block 454 if the push-to-talk switch of the transceiver has been depressed by the operator. At decision block 454, it is determined whether the radio has been disabled. If the radio has been disabled, operational flow proceeds from decision block 454 to decision block 460 where it is determined whether the push-to-talk is still depressed. If the push-to-talk switch is still depressed, and the radio has been disabled, operation flow remains at block 460. Once the operator releases the push-to-talk switch, operational flow is directed to step 462 where the busy tone and intercept tone is released and the operational flow is directed to step 704 for continued checking of the system activity.

Operational flow is directed from decision block 454 to decision block 456 if the push-to-talk switch is depressed and the radio has not been disabled. The priority of the desired outgoing transmission is compared to the current system access priority. It will be appreciated that the determination of access priority is being made at the transceiver, prior to any outgoing transmission being made by the transceiver. If the transmission priority in the transceiver is not equal to or higher than the current system access priority, operational flow is directed to step 458 where the busy tone is initiated.

Operational flow is directed from decision block 456 to decision block 470 if the priority of the desired outgoing transmission is equal to or higher than the current system access priority. At decision block 470, it is determined whether an auxiliary or telephone group has been received. If the auxiliary or telephone group has been received, operational flow is directed from decision block 470 to step 708 for communication request acknowledgement, since the handshaking requirements are not required in full duplex operation of the transceiver.

It will be appreciated by those skilled in the art that if full duplex operation is not provided for, a handshaking procedure must be followed to initiate communications. Referring now to FIG. 22c, the handshaking procedure is initiated by directing operational flow from decision block 470 to step 770 where the transceiver is directed to a free channel as hereinafter described. At decision block 770, the free channel is checked to see if it is set. If the free channel is set (i.e., free not equal zero), a free channel has already been set and the transceiver may go to the free channel at step 772 nd then continue on to block 474. If the free channel is not set (i.e., free=0), then the transceiver checks the monitor channel for activity at decision block, similar to block 738. In this case, the transceiver is directed to use the monitor channel acts as the fall back communication path. If there is no activity on the monitor channel (i.e., no squelch open), operational flow proceeds to decision block 776 to see if we have checked the monitor channel for 20 msecs. Until the timeout is reached, operational flow loops between blocks 774 and 776. When the timeout is reached, operational flow proceeds to step 778 to set the free channel to the monitor channel and then go to the free channel at step 772. If the squelch is found to be open at decision block 774, then an attempt is made to decode the control message on the monitor channel at step 780, similar to step 326. If the control message is valid at decision block 782 (similar to block 328), then the free channel in the control message is loaded as the free channel at step 784 and operational flow returns to step 474 through step 772. If the control message is not valid, a check is made at decision block 786 to see if three consecutive control messages have been attempted to be decode. If so, then the transceiver is not able to go to the free channel indicated by the control signal and the default condition, use the monitor channel as the free channel, is selected at step 788. If less than three attempts have been made to decode the control message, then operational flow goes back to step 1780 to look for another valid control message.

Once the transceiver is directed to a free channel, operational flow next proceeds to step 474 (FIG. 22a-l) where one control message is transmitted from the transceiver to the repeater initiating the call. Also in step 474, the record of the number of attempts made by the transceiver to transmit a control message to a repeater is incremented by one. It will be appreciated that the record of transmitting attempts ("TXAMPTS") is zeroed out at step 706 where the system communication request is initiated.

Operational flow is next directed to step 476 where the transceiver attempts to obtain a control message back from the repeater in response to the transmission at step 474. The attempt at step 476 continues for approximately 250 milliseconds. Operational flow is next directed to decision block 478 where it is determined whether or not a control message was received nd decoded by the transceiver. If a control message was decoded, operational flow proceeds to decision block 484 where it is determined whether or not the received control message was in fact a response to the request initiated at step 474. Assuming the response was valid, operational flow is directed from block 484 to step 708 where the communication request is acknowledged and full communication is allowed.

If no control message is decoded at decision block 478, operational flow is directed to block 480 where it is determined how many attempts at transmitting a control message from the transceiver (step 474) have been made. If a predetermined number of attempts (for instance 10) have not been made, operational flow is redirected to decision block 456 to set up another attempt at establishing a handshake between the transceiver and the repeater. If more than 10 attempts have been made, operational flow is directed to blocks 790-798 where the free channel is switched back and forth between the home channel and the alternate home channel in an attempt to gain access to the system for the communication request.

AT decision block 790, a total timeout value is checked to see if the transceiver has attempted to gain access for more than a predetermined time period (for example, 5 seconds). If so, the step 791 turns on a busy tone to indicate that system access cannot be gained. Normally, a human operator would release the PTT button after this time period; however, in the event that the transceiver is being used with a modem to communicate data information signals, some type of signal must be received by the modem to indicate that the modem should discontinue attempting to access the system. In the situation where the mobile transceiver is provided with automatic registration capabilities, this would be one point where the transceiver could assume that it had moved out of the coverage zone of a given site. An attempt could then be made to register in another coverage zone.

If the total timeout is not exceeded, decision block 792 checks to see if the free channel is set to the alternate home channel. If so, the free channel is switched to the home channel at step 794 and the communication request will be attempted again. If not, decision block 796 checks the opposite situation, where the free channel is set to the home channel. If so, then the free channel is switched to the alternate home channel at block 798 and, again, the communication request is retried. Operational flow then proceeds to decision block 460 where it is determined whether the push-to-talk switch has been depressed. If the push-to-talk switch is depressed operational flow is hung at decision block 460 until the push-to-talk switch is released. Once the push-to-talk switch is released, operational flow proceeds from decision block 460 to step 462 where the intercept tone and the busy tone are turned off. Operational flow then returns to step 704 for continued checking of system activity.

Returning to decision block 484, if a control message is received by the transceiver, but the control message received is not a proper response to the handshake, operational flow is directed to decision block 486. It will be appreciated that receiving any response to the handshake, even if not proper response, is preferable to receiving no response, because it indicates to the transceiver that it is in communications range. At decision block 486, the transceiver determines whether the control message received indicates that the same channel is already occupied. Operational flow is directed to step 488, if the channel is so occupied, where the free channel information and system access priority information is saved from the received control message. Operational flow is then redirected to step 450 where the handshaking procedure will again be initiated.

Operational flow is directed to decision block 490 if the test of block 486 is not met. Operational flow will be directed from block 490 to step 492 and then back to step 476 if there have been less than two consecutive tries to decode a control message received in response to the control message transmitted at step 474 by the transceiver. If there have been two consecutive attempts at decoding, operational flow is returned to block 450 where another attempt at handshaking will be made.

Referring now to FIG. 23, the communication request is acknowledged at step 708, when a successful handshake is made. At step 504, an audible proceed tone is initiated and, at this point, voice or data information can begin. This is the Communication Takes Places 710 step identified in the overall block diagram shown in FIG. 19.

Operational flow proceeds from step 506 to decision block 508 where it is determined whether the push-to-talk switch is active. The operational flow will stay at step 508 as long as the push-to-talk switch is depressed, or until a predetermined time out period expires. Once the push-to-talk switch is released, there is a Release of Communication 712 as shown in the overall block diagram in FIG. 19. The operational flow proceeds to step 512 where the voice circuit is disabled, and then to step 514 where one control message with a turnoff code is transmitted. Operational flow then proceeds to decision block 516 to determine whether the transceiver has been set for full duplex operation. The transceiver is maintained at the channel it is currently on (step 518) if communications are full duplex. The transceiver is reset to its monitor channel (step 520), if full duplex operations are not required.

System scan or scan mode is a user-selectable feature that is standard on the mobile transceivers. Scan mode is enabled by a switch (not shown) on the front panel of the mobile transceiver means 12. An LED indicator also on the front panel will display when the scan mode is selected. The microphone of the mobile transceiver must be on-hook for scanning to occur in the scan mode. When the transceiver is actually scanning, dashes (--) will be displayed on the system and groups displays, instead of the number of the system or group being used. Scanning is a sequential monitoring of all programmed systems in a mobile transceiver for activity. When a call is detected on any one of the systems being scanned, scanning stops and the system and group number of the call are displayed. The normal operation of scan mode in present LTR systems is described in detail in the manual entitled "LTR.RTM. 86/505, 510, 515 and 520 Service Manual", Part No. 242l-186xx-0001 (2nd printing, July 1988), available from E. F. Johnson Company, Waseca, Minn., which is incorporated herein by reference.

There are two types of scan mode operation that may be used with the present invention: single Site Scan and Multiple Site Scan. Single Site Scan is available only for system having the repeater means 14 of the preferred embodiment of the present invention and scans calls that are to be detected only on a single site. When there are several sites that may be scanned, the Single Site Scan will use the site of the revert system programmed in the mobile transceiver to scan.

When scanning is occurring, the transceiver normally monitors the alternate home channel for the single site. The control messages on the alternate home channel are decoded and checked against the home repeater/group ID codes of all selectable systems programmed in the mobile transceiver with the selected site number to determine if there is a match. If there is a match, the transceiver will open its squelch and monitor the indicated transmission. Because it is not necessary for the transceiver to switch frequencies to monitor all of the home channels for a given site, the operation of the scan mode using the present invention is greatly enhanced.

In the event that no activity is detected on the alternate home channel for a given period of time (for example, 10 seconds), the transceiver will begin a traditional scanning of all of the home repeaters of all systems programmed with the site to be scanned. When the home repeaters are scanned, control message are decoded only for as long as necessary to detect all calls on that repeater. The alternate home channel also continues to be scanned and normal scanning of only the alternate home channel is resumed if activity is detected on it. This feature ensures that scanning will occur even if the repeater for the alternate home channel is not on the air.

In Multiple Site Scan, more than one site and conventional and LTR channels may also be scanned. For all sites that have repeaters using the Multi-Net system of the preferred embodiment of the present invention, the alternate home channel will be scanned as just described. For conventional or LTR channels, the last selected channel on these systems will be used for the scan. While the transceiver must make the necessary frequency changes for scanning the conventional and LTR channels, the advantage of scanning the alternate home channel for all other systems is not lost.

Although the description of the preferred embodiment has been presented, it is contemplated that various changes could be made without deviating from the spirit of the present invention. Accordingly, it si intended that the scope of the present invention be dictated by the appended claims rather than by the description of the preferred embodiment.