US 5,502,724 AGrant
Communication Medium Access Control Protocol for Wireless Communication
Issue Date:1996-03-26
•22 Claims
•6 Drawing Sheets
Abstract
A method is provided for controlling access to a communication medium intended for sharing by a least two stations to enable peer-to-peer communications therebetween. Initially, the communication medium is sensed at a first station to determine if the medium is in use. If the first station senses that the medium is not in use, the first station may then transmit a connection request message intended for a second station. Upon receiving at the second station the connection request message, the second station transmits a connection confirm message to the first station for the purpose of establishing a reservation of the transmission medium for use by the first and second stations. Thereafter, the first station transmits information as one or more data messages from the first station to the second station. Upon receiving the last of the data messages from the first station, the second station transmits an acknowledgement message to the first station. The first station, upon receiving the acknowledgement message, subsequently transmits a disconnect request message to the second station. Upon receiving the disconnect request message, the second station subsequently transmits a disconnect confirm message for reception by the first station for the purpose of ending the reservation of the medium.
Metadata
Assignee
- International Business Machines Corporation
Inventors
- Jeane S. Chen
- Stanley L. Frickes
- Bryan R. Hartlen
- Tom D. Kurata
- Jacek P. Maryan
- Dan N. McKay
- Timothy F. Williams
Application Information
Application Number:US 3387151
Filing Date:1994-11-14
Priority Date:1993-11-15
Art Unit:263
Classifications
IPC:
H04L 12413H04B 7204
Field of Search:
37085.1;85.2;85.3;85.4;85.5;85.15;94.1;94.2;95.1;95.3;110.1;85.7
Patent Drawings (6 sheets)
Description
Background of the Invention
1. Technical Field
Present invention generally relates to methods for transferring data between multiple independent stations. More particularly, the present invention relates to data transfer in a local area network between a number of data processing devices--such as personal computers, lap top computers, or palm top computers--where the devices use a wireless media--such as infrared, radio frequency or microwaves--or interconnecting cable, if desired.
2. Description of the Related Art
There are a number of techniques for communicating between independent data processing stations using well known medium such as infrared or radio frequency.
In an article entitled "A New Channel Access Message For Packet Radio," which was published in the proceedings of the ARRL Ninth Computer Networking Conference, London, Ontario, Canada, Sep. 22, 1990, ISDN0-87259-337-1 by Phil Karn, there is disclosed a channel access method for use by multiple packet radio stations on a shared simplex packet radio channel which permits different stations to initiate independent access to the channels and initiate conversations with other stations. The article proposes a technique for multiple access with collision avoidance to reduce the interference of stations with each other. However, there is no disclosure of any message for providing fairness of access to maximize the utilization of the medium by each station and reduce the waiting of each station for access to the medium; and there is no mechanism for making medium reservations between stations, nor is there a mechanism for terminating conversations.
U.S. Pat. No. 5,231,634, issued Jul. 27, 1993 and assigned to Proxim, Inc., discloses a medium access protocol for wireless LAN which requires a station intending to initiate communications to send reservation duration information in its request to the intended receiving station to reserve a fixed block of time for its proposed communications or conversation. This may pose some problems as there is no allowance provided for retries, and if there is a problem in communications, a new reservation is required which may necessitate delays because other stations may have initiated communications at the end of the reservation.
U.S. Pat. No. 4,661,902, issued Apr. 28, 1987 and assigned to Apple Computer, Inc., discloses a local area network with carrier sense collision avoidance. However, it does not disclose a means by which the stations not involved in the conversation acknowledge and obey a conversation reservation. The system disclosed is a hardwired system which does not address the problem of "hidden terminals," and can therefore depend wholly upon Carrier Sense Multiple Access With Collision Avoidance (CSMNCA). This reference shows the initiation of conversations by the transmission of a request to send message by the transmitting station and a clear to send message sent back to the transmitting station by the intended receiving station. The non-conversant stations perform a carrier sense to detect when the communication medium is free for their use. Thus, they are free to try to initiate conversations during the conversations of other stations.
Unlike a wired environment, in a wireless environment-such as infrared or radio frequency--not all stations are able to hear all the other stations. This is referred to as the "hidden terminal" problem. The problem with hidden terminals is that they need to be made aware of the reservation effected by a communicating pair of stations so that they do not interrupt the conversation and cause a collision. This problem occurs in wireless local area networks (LANs), as it is possible for one or more station (or terminal) to be intermittently shielded from the others--for instance, by being moved or by the interposition of an obstruction. This is particularly important in a portable environment, as a portable station, such as a lap top or palm top, are quite mobile and may move out of range.
Summary of the Invention
The foregoing and other problems are overcome by the method of the invention disclosed and claimed herein which provides a method of controlling peer-to-peer communications in networks where all stations or nodes are equally able to communicate with all other stations within range of their communications capability. The transmission medium to which this invention pertains may include infrared light, radio frequency, ultraviolet, or other electrical magnetic media. In the peer-to-peer communications to which this invention relates, the stations are typically, but not necessarily, in a local area network, and usually communicate on the same channel or frequency.
The invention provides a protocol for communication including:
reserving the media;
sending data;
receiving data;
preserving fair access to the media; and
reconciling collisions and other error conditions.
The invention provides a method for controlling access to communications medium shared by at least two stations to provide peer-to-peer communication there between. Communicating a message includes the steps in which the intended initiating station--a first station--senses the communication medium to determine if a communication medium is in use. If the first station finds that the communication medium is not in use, it transmits a connection request (CR) message directed to a second station. The message includes identification information of the first station. The receiving, or second station, receives the connection request message and transmits a connection confirm (CC) message which is intended for receipt by the first station. The connection confirm message includes identification of the second station and is intended to establish a reservation of the transmission medium for the communication of messages between the first and second station. Other stations(e.g., a third station) may receive the connection request or the connection confirm message, depending upon whether or not the other stations are fully connected, or are partially hidden. If they are close enough to the sending and receiving station fully connected, they will receive both the connection request and connection confirm messages.
Upon receipt of either the connection request or the connection confirm messages, the third station transitions into a wait state having a preselected time period during which the third station does not transmit messages on the medium until it detects the expiration of the reservation between the first and second stations by detecting a disconnect request (DR) message or a disconnect confirm (DC) message from said stations. If the wait state expires before the third station detects a disconnect request or disconnect confirm message, it will perform a carrier sense to determine if the medium is in use. If the medium is not in use, the third station returns to an idle state. If the medium is in use, the third station returns to a wait state having a predetermined period.
During the reservation or conversation between the first and second stations, the first station may transmit one or more data messages (XFR) to the second station. These messages may include multiple transmissions, and the second station may, upon receiving the last message, send an acknowledgement (XAK) message back to the first station where such an XAK message includes information identifying what data frames were received successfully. The first station, upon receiving an acknowledgment from the second station that one or more of the messages were not successfully received, may retransmit the unsuccessfully received data messages to the second station. The second station, upon receiving the last of the retransmitted data messages, may send a retry acknowledgment message to the first station where such an retry acknowledgment message includes information identifying which data messages were received successfully. The first station receives the retried acknowledgement message. If the acknowledgement message indicates that the transmission was still unsuccessful, then it may be necessary for the first station to retry sending the complete message at a later time; basically establishing another reservation.
The implementation of the invention herein may be achieved by software (e.g. routines or device drivers) or microcode at each station. Alternately, hardwired circuitry may be employed to implement the invention herein. These techniques will be apparent to those skilled in the art to which this invention relates.
Brief Description of the Drawings
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
FIG. 1 is a drawing of message frame contents used in communications according to the invention herein;
FIG. 2 is a drawing showing reservation based conversation flow without retry;
FIG. 3 is a drawing showing reservation based conversation flow with retry;
FIG. 4 is a drawing showing nonparticipating station flow;
FIG. 5 is a drawing showing typical timing of network accessing; and
FIG. 6 is a drawing depicting typical communication networks that are partially overlayed and have partially hidden stations.
Detailed Description of Preferred Embodiments
Referring to FIG. 1, it can be seen that the present invention transmits messages or data in data packets, typically known as "frames." The use of such frames for data transmission is well known. In the frames used in accordance with the described embodiment of the invention, each of the message frames includes a commonly structured header having the following fields:
The first field (DST) of the header comprises the Destination Station's address, which may be a broadcast, or a unique station address.
The second field (SRC) comprises the initiating station's address.
The third field (RSVD) is optional and may be reserved for other information.
The fourth field (REV) is also optional and may be used, for instance, to identify the version of the communication protocol used.
The fifth field (TYPE) identifies the type of frame (e.g., CR, CC, etc. . . . ) as described below.
There are six basic communication message types used in practicing the present invention. These message types are listed and described below. Each one uses the common header described above.
The Connection Request frame (CR) includes the following fields:
SEG SIZE which identifies the maximum size of the data field within the XFR frame (described below) to be used within the requested conversation which is to follow.
SEG CNT identifies which packet in a series of packets is being transmitted.
MAX SEG identifies how many XFR frames will be transmitted by the initiating station during the requested conversation.
REAL DST is the specific address for the targeted station with which a conversation is to be initiated.
The Connection Confirm frame (cc) includes the same SEG SIZE, SEG CNT, AND MAX SET fields and information as the Connection Request frame does; however, the REAL DST frame contains the specific address of the initiating station.
The Data Transfer frame (XFR), which is adapted to transfer the data or information from the initiating station to the receiving station, includes the following fields:
SEG CNT, MAX SEG, RSVD, and Data which have been defined previously in relation to the CR?CC frames and the header.
The Transfer Acknowledgement frame (XAK) contains a number of fields (in this case 3) which are used to identify which XFR frames have been received successfully or not.
The Disconnect Request frame (DR) includes a REAL DST field to identify the specific address for the targeted receiving station with which a conversation has been occurring; a number of fields (RSVD) for optional purpose.
The Disconnect Confirm frame (CC) includes a REAL field containing the specific address of the initiating station.
FIG. 2 depicts initiation of communications between two stations in a communication network in accordance with one embodiment of the present invention. Reservation of the medium, data transfer and termination of the reservation, and communications between the first and second station will be described.
Assuming for the purposes of this illustration that the transmission medium is unoccupied for the time being, and station one--which is identified as the transmitter in FIG. 2--and station 2--which has been identified as the receiver in FIG. 2--are both in the idle state. To initiate a conversation, station 1 sends out a connection request on the medium directed to station 2 and then enters a wait state, WCC, awaiting confirmation of receipt of the connection request from station 2. Upon receipt of a connection confirm message cc from station 2, station 1 changes wait states from WCC to the WXAK state.
In order to transfer data, station 1 transmits an XFR frame (containing data as described above) directed to station 2, which receives it and sends back an acknowledgment XAK, which station 1 receives. Station 2 transitions into a wait state WDR awaiting a disconnect request DR.
To terminate a communication reservation, station 1 transmits a disconnect request DR, which is received by station 2. Station 1 awaits a disconnect confirm DC from station 2. Upon receiving such a disconnect confirm DC from station 2, station 1 returns to an idle state. Station 2 returns to an idle state upon transmitting the disconnect confirm DC message to station 1.
Referring to FIG. 3--which depicts the progression of a conversation from initiation to termination with retry from an incomplete message or data transmission--it can be seen that the initial reservation sequence and the first date transmission sequence are the same as depicted in FIG. 2. However, the subsequent data transfer steps differ slightly, given the selective retransmission of XFR frames that were received incomplete, with errors, or not received at all.
The initiating station, upon receiving an XAK frame from the receiving station containing non-zero MSK fields, indicates one or more failed XFR frames and enters a retransmission cycle RXFR, in which it resends the XFR frames indicated by the MSK fields in the XAK frame.
After receiving the last retransmitted frame (RXFR frames), the reservation of the medium is terminated in the same manner as described with respect to FIG. 2.
FIG. 4 illustrates the possible transitions between states in a third station, which is a station not participating in the conversation between the aforementioned initiating (first station) and receiving stations (second station).
The transition indicated is designed to assure that the third station will not interfere with a reservation being established, and will be able to re-enter the contention period for use of transmission medium only after receiving reservation termination frames (DR, DC), Or following the elapse of the appropriate safety timer (WTS-T1, WTS-T2, WTS-Tn).
The random delays indicated in FIG. 4 are intended to achieve fairness in medium access among contending stations following the termination a reservation.
Referring to FIG. 5, there is depicted how one embodiment of the protocol of the present invention achieves fairness in medium contention for all participating stations. Before entering contention for the medium, every station waits for a random amount of time.
As can be seen, some stations may be partially hidden, or not hidden, from the transmissions of conversing stations 1 and 2.
Station 1, 3, 4, and 5 will wait a random time between zero and a maximum backoff time less interference gap time. Station 2 will wait a random time between interframe gap time and maximum backoff time.
These random intervals will equalize the opportunity amongst the stations to initiate conversations. The advantage that station 2 has is neutralized by imposing the randomization from a non-zero base.
The randomization of delay timer may be achieved in various ways. For instance, programs or hardware in each station may generate a random number that is used to derive a corresponding random time delay. The maximum time delay is chosen to minimize the chance that more than one station entering the contention period (contending with each other) at the same time, and to minimize the amount of idle medium time between conversations.
Occasionally, even with the randomizing techniques for obtaining fairness and avoiding more than one station attempting to initiate a conversation at the same time, a collision may occur when two stations send a connection request simultaneously. In this situation, stations within range of both initiating stations will be unable to process either connection request due to errors caused by the presence of the conflicting signals simultaneously on the medium, and these stations will remain in their current states. Partially hidden stations (i.e., stations receiving either one of said connection requests, while not being targeted by said requests) will transition into a wait state (see WTS timeout in FIG. 4) in anticipation of an impending conversation. After a random delay these stations will time-out and transition to an idle state.
The two colliding, initiating stations await receipt of connection confirm frames from their intended receiving stations in a wait state, WCC, which is shorter than WTS timeout of the non participating stations. When these states time out, each initiating station may then retry initiating a conversation. Further collisions will be avoided by the imposition of random delays in retransmitting.
In the event that unforeseen error conditions occur where one or more stations has determined that frames being received do not conform to the expected protocol sequences, it may be advantageous to require such stations to perform a stabilization procedure in which it transitions to, or remains in, an idle state, but is restricted from contending for the medium until a stabilization period--for instance 2WTS timeout periods--has lapsed. The same stabilization procedure may also prove advantageous for newcomers to a network.
The following section of the description of a preferred embodiment of the invention herein uses state tables to describe the processes carried out by stations employing the protocol of the invention. The following definitions are used.
Each cell in a state table designates (a) the next state to be reached in response to an occurrence of an event, given the station's initial state, and (b) the steps or transitions--which are noted in "()", e.g. "(1)"--required to move from the station's initial state to the next state-which is noted with letters (e.g. WXFR). Where more than one state name is listed in a cell (e.g. WCC, WDC), this implies alternative possibilities. Where no state name appears in a cell, and a "-" appears before the transition number (e.g. "-(7)"), this denotes that no change in state will occur in response to the event.
A ( ) symbol is used to designate a directed broadcast frame received that was addressed for another node. If the ( ) is not present, then the directed broadcast was addressed for the receiving node.
Transition Descriptions
In the transition descriptions that follow, some global state variables are used. They are defined as follows:
Time Definitions
The following list of timer definitions will be useful in understanding the relationship between states in both the reservation and the data transfer state tables, suggesting means of calculating appropriate values.
The following is a list of transmissions used in the following tables. ##SPC1##
Examples of Conversations Using the Preferred Embodiment of the Invention
This section provides a temporal view of how common conversations may progress through the defined state tables. This list of examples is not meant to be exhaustive, but to be representative of typical conversations.
The scenarios in this section assume that CR, CC, DR and DE frames are directed broadcast frames. All other frames, XFR, and XAK, are addressed and are not processed by a station unless the destination address matches the station's address. FIG. 6 shows the network and station relationships used in the scenario examples and Table 4 gives a brief explanation on how to read the scenario tables and what the fields in the columns mean.
Single Network Race Conditions 2
At this point the race condition is resolved and the conversation will end normally.
At this point both stations stay in WCC mode until the WCC.sub.-- t timeout occurs. Then they will backoff off for different random time periods before resending their cr's. The node with the shorter backoff will get the medium for transmission.
At this point the contention has been resolved and the session between X and Y will continue through xfr's and XAK to completion. Station I's WTS.sub.-- t timer will pop and it will move into an Idle state.
At this point the contention has been resolved and the session between Y and I will continue normally to completion.
Hidden Station Race Conditions
At this point the contention has been resolved and the session between X and Y will continue through xfr's and XAK to completion. Station Z will eventually hear the YdeX or its WTS.sub.-- t timer will pop. In either case it will move into an Idle state.
At this point the contention has been resolved and the session between X and Y will continue through xfr's and XAK to completion. Station Z's and Station J's WTS.sub.-- t timer will pop and they will move into an Idle state.
Hidden Station Collision Conditions
While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
Claims
What is claimed is:
1. A method for controlling access to a communication medium intended for sharing by at least two stations for peer-to-peer communications therebetween, said method comprising the steps of: sensing said communication medium at a first station to determine if said communication medium is in use; transmitting a connection request message from said first station to a second station if said first station determines that said communication medium is not in use; transmitting a connection confirm message from said second station to said first station for temporarily reserving said communication medium for use by only said first and second stations; transmitting data messages from said first station to said second station; transmitting an acknowledgement message from said second station to said first station in response to said second station receiving a last one of said data messages; transmitting a disconnect request message from said first station to said second station in response to receiving said acknowledgement message; and transmitting a disconnect confirm message from said second station to said first station in response to receiving said disconnect request message for terminating said reservation of said communication medium.
2. The method according to claim 1 further including the step of placing a third station in a wait state until the occurrence of a predetermined event, wherein said third station is restrained from transmitting messages on said communication medium while in said wait state.
3. The method according to claim 2 further including the step of maintaining said wait state until said third station detects termination of said reservation of said communication medium, or until a wait state timer expires.
4. The method according to claim 3 further including the step of maintaining said wait state until said wait state timer expires, or until said third station detects either said disconnect request message or said disconnect confirm message.
5. The method according to claim 2 further including the step of placing said third station in an idle state from said wait state in response to the occurrence of said predetermined event.
6. The method according to claim 1 wherein said data messages include sequence information for identifying said data messages, and wherein said step of transmitting data messages from said first station to said second station includes transmitting data messages having said sequence information from said first station to said second station.
7. The method according to claim 6 wherein said sequence information includes a total number of said data messages to be transmitted from said first station to said second station.
8. The method according to claim 7 further including the step of identifying, within said second station, a last data message by utilizing said sequence information.
9. The method according to claim 1 further including the step of placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations.
10. The method according to claim 9 wherein said step of placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations includes placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations, and expiration of a preselected period of time preselected for each station.
11. The method according to claim 10 wherein said preselected period of time includes a random delay.
12. A communications system for controlling access to a communication medium intended for sharing by at least two stations for peer-to-peer communications therebetween, said communications system comprising: means for sensing said communication medium at a first station to determine if said communication medium is in use; means for transmitting a connection request message from said first station to a second station if said first station determines that said communication medium is not in use; means for transmitting a connection confirm message from said second station to said first station for temporarily reserving said communication medium for use by only said first and second stations; means for transmitting data messages from said first station to said second station; means for transmitting an acknowledgement message from said second station to said first station in response to said second station receiving a last one of said data messages; transmitting a disconnect request message from said first station to said second station in response to receiving said acknowledgement message; and transmitting a disconnect confirm message from said second station to said first station in response to receiving said disconnect request message for terminating said reservation of said communication medium.
13. The communications system according to claim 12 further including means for placing a third station in a wait state until the occurrence of a predetermined event, wherein said third station is restrained from transmitting messages on said communication medium while in said wait state.
14. The communications system according to claim 13 further including means for maintaining said wait state until said third station detects termination of said reservation of said communication medium, or until a wait state timer expires.
15. The communications system according to claim 14 further including means for maintaining said wait state until said wait state timer expires, or until said third station detects either said disconnect request message or said disconnect confirm message.
16. The communications system according to claim 13 further including means for placing said third station in an idle state from said wait state in response to the occurrence of said predetermined event.
17. The communications system according to claim 12 wherein said data messages include sequence information for identifying said data messages, and wherein said means for transmitting data messages from said first station to said second station includes means for transmitting data messages having said sequence information from said first station to said second station.
18. The communications system according to claim 17 wherein said sequence information includes a total number of said data messages to be transmitted from said first station to said second station.
19. The communications system according to claim 18 further including means for identifying, within said second station, a last data message by utilizing said sequence information.
20. The communications system according to claim 12 further including means for placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations.
21. The communications system according to claim 20 wherein said means for placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations includes means for placing each station accessing said communication medium in a state in which it is free to secure a reservation of said communication medium, in response to termination of said reservation by said first and second stations, and expiration of a preselected period of time preselected for each station.
22. The communications system according to claim 21 wherein said preselected period of time includes a random delay.