Automatic Circuit Switching Device Restorable Without a Hit and Its Method

1. Field of the invention

The present invention relates to an automatic circuit switching device for data communication, particularly to an automatic circuit switching device which switches over from a private circuit serving as a regular circuit to a public data exchange circuit, such as an Integrated Services Digital Network (ISDN) network circuit, arranged to be connected as a stand-by circuit, in the case of deterioration of the quality and disconnection of the regular circuit.

2. Description of Related Art

FIG. 1 is a block diagram showing an automatic circuit switching device of the conventional type.

The automatic circuit switching device of the conventional type usually accommodates private circuit 2 used as regular circuit through modem 23.sub.1, 23.sub.2 and an ISDN network circuit prepared for use as stand-by circuit 4 to back up the operation through corresponding terminal adaptor circuits 25.sub.1, 25.sub.2 at the legs of switching units 22.sub.1, 22.sub.2 which are connected to the each subscriber terminal 21.sub.1, 24.sub.2.

In FIG. 1, when the regular circuit between modems 23.sub.1 and 23.sub.2 is disconnected due to some trouble, data transmission between subscriber terminals 21 on the side of the host machine and subscriber terminal device 24 is stopped, thereby abnormality is usually detected at subscriber terminal 21 through monitoring of communications protocol. After abnormality is detected, a maintainer of the system operates circuit switching units 22.sub.1 and 22.sub.2 manually to connect a data communication bus to stand-by circuit 4 to back up the system.

In order to switch the circuit automatically, output signals of carrier detecting circuits provided respectively in modems 23.sub.1 and 23.sub.2 are used. Since the output signal of this carrier detecting circuit turns to "ON" when carrier is detected on regular circuit 2 and turns to "OFF" when the carrier is disconnected, modems 23.sub.1, 23.sub.2 detect the disconnection of the carrier only when the regular circuit is disconnected due to some trouble by keeping carrier constantly in the sending state.

Therefore, the automatic circuit switching is conducted by operating circuit switching units 22.sub.1, 22.sub.2 when the disconnection of carrier is detected using the output signal of the carrier detecting circuit as the control signal.

When regular circuit 2 is restored, the carrier is also sent again and the output of the carrier detecting circuit turns "ON", thereby it is possible to switch back both circuit switching units 22.sub.1 and 22.sub.2 automatically through the output signal of the carrier detecting circuit as the control signal.

When there are identification bits which can show presence of transmitted data in a data transmission units in use, the automatic switching of the circuit can be performed in the same way by using the data transmission units instead of modems 23.sub.1 and 23.sub.2.

However, the automatic circuit switching device of the conventional type fails in the normal connection of data (bit irregularity). In the worst case, restarting of the host and subscribers terminal is required when it is switched back from the stand-by circuit to the regular circuit either manually or automatically, because the regular circuit and stand-by circuit are not typically synchronized with each other.

An object of the present invention is to provide an automatic circuit switching device which can solve the above problem, and consequently can synchronize data transmission between the regular circuit and the stand-by circuit, and further can switch back the above circuits without any hit, i.e. short break of the data transmission.

Another object of the present invention is to provide an automatic circuit switching device which can perform switch back operation without any hit and any restriction of the characteristics of the transmission system and terminal device.

In order to achieve the objects described above, the automatic circuit switching device according to the present invention comprises monitoring means to monitor a signal indicating the normality of a regular circuit, means for switching over the connection of a subscriber terminal from a regular circuit to a stand-by circuit through a terminal adaptor circuit when detecting disconnection of the regular circuit, means for synchronizing the regular circuit with the stand-by circuit in use, means for switching back the connection of the subscriber terminal from the stand-by circuit to the regular circuit when synchronization is achieved. The automatic circuit switching device described above providing an ISDN network as its stand-by circuit is included in the present invention.

Further, the automatic circuit switching device described above having monitoring means which monitors a signal indicating the reception of the carrier for detecting the normality of the regular circuit is also included in the present invention.

Still further, the automatic circuit switching device according to the present invention may have monitoring means comprising a detector which outputs an instruction signal to establish connection of the stand-by circuit when the regular circuit is disconnected and outputs an instruction signal to start comparison when the regular circuit is restored from the disconnection thereof, according to the result of monitoring of the signal indicating the normality of the regular circuit, the terminal adaptor circuit establishes the stand-by circuit through the stand-by circuit terminating equipment and outputs an instruction signal to switch over the connection after establishing the stand-by circuit, the means for synchronizing both circuits comprises first and second variable delay circuits which delay the signals received from the regular circuit through the regular circuit terminating equiment and from the stand-by circuit throught the terminal adaptor circuit according to the designated delay time, and a comparator which inputs the delay time to the two delay circuits based on the comparison start instruction and outputs a switch back instruction signal when the output signal of two delay circuits coincide for a predetermined period of time, the means for switching over and switch back are comprised in a switching circuit which switches over the connection to the stand-by circuit based on the switch-over instruction outputted from the terminal adaptor circuit and switches back the connection to the regular circuit based on the switch back instruction outputted from the comparator.

Another aspect of the present invention includes a method of automatic circuit switching comprising the steps of monitoring a signal indicating the normality of a regular circuit, switching over the connection of subscriber terminal from a regular circuit to a stand-by circuit when detecting disconnection of the regular circuit, synchronizing the regular circuit with the stand-by circuit in use when the connection of the subscriber terminal from the stand-by circuit to the regular circuit after the synchronization is achieved. The method of automatic circuit switching described above may monitor the signal outputted corresponding to the reception of the carrier of the signal indicating the normality of the regular circuit.

The method of automatic circuit switching described above may synchronize the restored regular circuit with the stand-by circuit by controlling each variable delay circuit joined to the regular circuit and the stand-by circuit.

The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended to define the limits of the invention.

FIG. 1 is a block diagram showing an example of an automatic circuit switching device of the conventional type.

FIG. 2 is a block diagram of an embodiment of the automatic circuit switching device of the present invention.

FIG. 3 is a flow chart showing operation of detectors 14.sub.1 and 14.sub.2 of the automatic circuit switching device shown in FIG. 2.

FIG. 4 is a flow chart showing operation of comparators 18.sub.1 and 18.sub.2 of the automatic circuit switching device shown in FIG. 2.

FIG. 5 is a flow chart showing operation of terminal adapters 15.sub.1 and 15.sub.2 of the automatic circuit switching device shown in FIG. 2.

FIG. 6 is a flow chart showing operation of switching circuits 19.sub.1 and 19.sub.2 of the automatic circuit switching device shown in FIG. 2.

An embodiment of the present invention will be described with reference to the attached drawings.

FIG. 2 is a block diagram of an embodiment of the automatic circuit switching device of the present invention.

Automatic circuit switching devices 10.sub.1, 10.sub.2 are connected with subscriber terminals through connectors 11.sub.1, 11.sub.2, and switch the connections with regular circuit 2 through connectors 12.sub.1, 12.sub.2 and data-circuit-terminating equipment (DCE) 1.sub.1, 1.sub.2, to stand-by circuit 4 established by ISDN through modular jacks 13.sub.1, 13.sub.2 and data service units (DSU) 3.sub.1, 3.sub.2 respectively. Further, automatic circuit switching devices 10.sub.1 and 10.sub.2 each have detectors 14.sub.1, 14.sub.2, terminal adaptors 15.sub.1, 15.sub.2, first and second delay circuits 16.sub.1, 16.sub.2, 17.sub.1, 17.sub.2, comparators 18.sub.1, 18.sub.2, and switching circuits 19.sub.1, 19.sub.2. Detectors 14.sub.1, 14.sub.2 each output an instruction signal to establish a stand-by circuit when detecting the disconnection of regular circuit 2 by monitoring stoppage of the signal which is outputted from data-circuit-terminating equipment 1.sub.1, 1.sub.2. Detectors 14.sub.1, 14.sub.2 then output an instruction signal to start comparison of synchronization when regular circuit 2 is restored from the disconnection state. Terminal adaptor circuits 15.sub.1, 15.sub.2 each establish stand-by circuit 4 through data service units 3.sub.1, 3.sub.2 based on the instruction to establish a stand-by circuit and output a switching instruction signal. First and second delay circuits 16.sub.1, 16.sub.2, 17.sub.1, 17.sub.2 each receive communication signals inputted through data-circuit-terminating equipment 1.sub.1, 1.sub.2 (DCE) and terminal adaptor circuits 15.sub.1, 15.sub.2, and delay the above signals for the period designated by comparators 18.sub.1, 18.sub.2. Comparators 18.sub.1, 18.sub.2 each set the delay times by adjusting the variable delay circuits 16.sub.1, 16.sub.2 according to the instruction to start the comparison sent from detectors 14.sub.1, 14.sub.2 so that the delay times of the regular circuit and stand-by circuit through delay circuits 16.sub.1, 16.sub.2, 17.sub.1, 17.sub.2 may become equal. Comparators 18.sub.1, 18.sub.2 then output the instruction signal to switch back to the regular circuit when they coincide each other for more than a predetermined period. Switching circuits 19.sub.1, 19.sub.2 each switch over the connections of subscriber terminals from regular circuit 2 to delay circuits 17.sub.1, 17.sub.2 in accordance with the switch over instruction signal outputted from terminal adaptor circuits 15.sub.1 15.sub.2 and switch back the connections of the subscribers terminals from delay circuits 17.sub.1, 17.sub.2 to delay circuits 16.sub.1, 16.sub.2 based on the switch back instruction signal outputted from comparators 18.sub.1, 18.sub.2.

FIG. 3 is a flow chart showing the operation of detectors 14.sub.1, 14.sub.2 of the automatic circuit switching device shown in FIG. 2. FIG. 4 is a flow chart showing the operation of comparators 18.sub.1, 18.sub.2 of the automatic circuit switching device shown in FIG. 2. FIG. 5 is a flow chart showing the operation of terminal adapters 15.sub.1, 15.sub.2 of the automatic circuit switching device shown in FIG. 2. FIG. 6 is a flow chart showing the operation of switching circuits 19.sub.1, 19.sub.2 of the automatic circuit switching device shown in FIG. 2.

The process by which automatic circuit switching operation is performed through monitoring signals such as signal RS for requesting the transmission of the carrier current, signal CS for showing that DCE 1.sub.1, DCE 1.sub.2 is ready to receive data and-signal CD for showing reception of the carrier will first be described.

In this embodiment, the carrier is always kept transmitting in data-circuit-terminating equipment 1.sub.1, 1.sub.2.

When data transmission is performed normally by using data-circuit-terminating equipment 1.sub.1, 1.sub.2, they transmit carrier on the circuit when they receive carrier sent from respective subscriber terminals and the transmission unit of the receiving side turns on the signal CD upon receiving the carrier.

Since automatic circuit switching devices 10.sub.1, 10.sub.2 each are provided between subscriber terminals and data-circuit-terminating equipment 1.sub.1, 1.sub.2, detectors 14.sub.1, 14.sub.2 each determine that normal operation is maintained while carrier detecting signal CD is detected (steps 31, 32), then detectors 14.sub.1, 14.sub.2 do not output instruction to establish a stand-by circuit. When no carrier is detected due to a trouble such as disconnection of circuit, data-circuit-terminating equipment 1.sub.1, 1.sub.2 turn off signal CD (step 31). Detectors 14.sub.1, 14.sub.2 each output the instruction signal to establish the stand-by circuit to terminal adaptor circuits 15.sub.1 15.sub.2 when they detect that signal CD is off for more than a predetermined period (steps 31, 33).

As shown in FIGS. 5 and 6 when receiving the instruction signal to establish stand-by circuit from detectors 14.sub.1, 14.sub.2 (step 51), terminal adaptor circuits 15.sub.1, 15.sub.2 establish stand-by circuit 4 through the ISDN network (steps 52, 53), and switching circuits 19.sub.1, 19.sub.2 each switch over the connection of the subscriber terminals from regular circuit 2 to stand-by circuit 4 through delay circuit 17.sub.1, 17.sub.2 based on the switch over instruction signal outputted from terminal adaptor circuits 15.sub.1, 15.sub.2 (step 54). Receiving the switch over instruction signal (step 61), switching circuits 19.sub.1, 19.sub.2 determine the received signal is the switch over instruction (step 62), and connect the subscriber terminals to stand-by circuit 4 (step 63).

As shown in FIG. 3, detectors 14.sub.1, 14.sub.2 monitor signal CD continuously even in the switched off or released state of the regular circuit 2. When regular circuit 2 is restored and the carrier is transmitted, carrier detection signal CD of data-circuit-terminating equipment 1.sub.1, 1.sub.2 becomes ON. When detectors 14.sub.1, 14.sub.2 each receive signal CD for more than a predetermined period (step 31), they output comparison start instruction signal to comparators 18.sub.1, 18.sub.2 (step 34) when they determine that the regular circuit is not in service (step 32).

When comparators 18.sub.1, 18.sub.2 each receive the comparison start instruction signal from detectors 14.sub.1, 14.sub.2 (step 41), they set a fixed delay time on delay circuits 17.sub.1, 17.sub.2 in advance (step 42), and set an initial delay time which is determined in advance on delay circuits 16.sub.1, 16.sub.2 (step 43). The first comparison between the outputs of delay circuits 16.sub.1, 16.sub.2 and delay circuits 17.sub.1, 17.sub.2 are performed respectively (step 44). When the results of this comparison coincide for a determined period (bit numbers) (step 45), comparators 18.sub.1, 18.sub.2 each instruct delay circuits 16.sub.1, 16.sub.2 to fix the delay time and concurrently instruct switching circuits 19.sub.1, 19.sub.2 to perform switchback operation (step 46), and also instruct terminal adaptor circuits 15.sub.1, 15.sub.2 to disconnect the back-up circuits 4 (step 47). When the above comparison results do not coincide, comparators 18.sub.1, 18.sub.2 reset the delay time by adding the predetermined additional time to the initial delay time on delay circuits 16.sub.1, 16.sub.2 and perform a second comparison. This comparison will be repeated until the comparison results coincide for the predetermined period (bit numbers).

Now, for example, assume the delay time of the regular circuit to be 50.00 [ms], of stand-by circuit to be 49.50 [ms], of fixing delay circuits 17.sub.1, 17.sub.2 to be 2.00 [ms], and the initial delay times of delay circuits 16.sub.1, 16.sub.2 to be set to 0.00 [ms] and the delay times of delay circuits 16.sub.1, 16.sub.2 to be increased by 0.05 [ms] per comparison. After 30 repetitions of the above comparison, switch back of the circuit is performed without hit or disconnection (bit irregularity). When switch back is operated according to the conventional system, disconnection of data of 0.50 [ms] occurs.

Differences between the delay times smaller than 0.05 [ms] are to be absorbed by automatic circuit switching devices 1.sub.1, 1.sub.2 and terminal adaptors 15.sub.1, 15.sub.2.

As described above, it is necessary to set an appropriate fixed delay time on delay circuits 17.sub.1, 17.sub.2 to make this operation effective, even when the delay time of the stand-by circuit is less than that of the regular circuit.

In order to decide the delay time to be set on delay circuits 16.sub.1, 16.sub.2, it is possible to apply various commonly known methods other than the method shown in the present embodiment.

When switching circuits 19.sub.1, 19.sub.2 each receive the switch back instruction from the comparators (step 61), they connect the subscriber terminals to regular circuit at their corresponding delay circuits 16.sub.1, 16.sub.2 (steps 62, 64), whereby the back up state is canceled and the connection returns to the normal operation state.

Although the invention has been described in its preferred from with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.