Wireless LANs and neighborhood capture

• US 20030174690A1
• Filed: 09/27/2002
• Published: 09/18/2003
• Est. Priority Date: 11/02/2001
• Status: Active Grant

• Current Assignee: AT&T Intellectual Property II LP (AT&T, Inc.)
• Original Assignee: AT&T Corporation (AT&T, Inc.)
• Inventors: Benveniste, Mathilde
• Granted Patent: US 7,280,517 B2
• Time in Patent Office: Days
• US Class Current: 370/350
• CPC Class Codes: H04W 56/00 Synchronisation arrangements
• CPC Class Codes: H04W 74/02 Hybrid access
• CPC Class Codes: H04W 74/0808 using carrier sensing, e.g. carrier sense multiple access [CSMA]
• CPC Class Codes: H04W 84/12 WLAN [Wireless Local Area Networks]

Abstract

Overlapped wireless LAN cells in a medium have an equal chance at establishing a session on the medium. A first member station in the first cell transmits a timing packet containing a timestamp value, which is received at a second member station in the second cell. This synchronizes member stations in the first and second cells to interrupt transmissions at a global channel release instant corresponding to the timestamp value. The member stations in the first and second cells then have the opportunity to contend for access to the medium following the global channel release instant, using a slotted CSMA/CA access method. Each of the member stations in the first and second cells has a superframe clock that is synchronized based on the timestamp value, thereby establishing a periodic global channel release instant during each of a plurality of periodic superframes. The member stations can then periodically interrupt transmissions at the periodic global channel release instant to contend for the medium. The periodic global channel release instant occurs at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical voice and video applications.

Claims

1. A method to enable overlapping first and second wireless LAN cells in a medium to have an equal chance at establishing a session on the medium, each cell including a respective plurality of member stations, comprising:

transmitting by a first member station in the first cell, a timing packet containing a timestamp value;

receiving the timing packet at a second member station in the second cell;

interrupting transmissions by member stations in the first and second cells at a global channel release instant corresponding to said timestamp value; and

permitting member stations in the first and second cells to contend for access to said medium following said global channel release instant.

2. The method of claim 1, which further comprises:

synchronizing a superframe clock in member stations in the first and second cells based on said timestamp value;

establishing a periodic global channel release instant at member stations in the first and second cells during each of a plurality of periodic superframes based on said clock; and

periodically interrupting transmissions at member stations in the first and second cells at said periodic global channel release instant.

3. The method of claim 2, which further comprises:

periodically permitting member stations in the first and second cells to contend for access to said medium following said periodic global channel release instant.

4. The method of claim 3, which further comprises:

said periodic global channel release instant occurring at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical voice applications.

5. The method of claim 3, which further comprises:

said periodic global channel release instant occurring at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical video applications.

6. The method of claim 3, which further comprises:

said member stations in the first and second cells contending for access to said medium using a slotted CSMA/CA access method following said periodic global channel release instant.

7. The method of claim 6, which further comprises:

conducting a distributed coordination function (DCF) session by one of said member stations in the first and second cells that has successfully accessed the medium following said periodic global channel release instant.

8. The method of claim 6, which further comprises:

conducting a Tiered Contention Multiple Access (TCMA) session by one of said member stations in the first and second cells that has successfully accessed the medium following said periodic global channel release instant.

9. The method of claim 6, which further comprises:

conducting a point coordination function (PCF) session by one of said member stations in the first and second cells that has successfully accessed the medium following said periodic global channel release instant.

10. The method of claim 6, which further comprises:

requiring that the medium be free of all activity at the termination of a busy period;

detecting completion of a busy period before a next periodic global channel release instant; and

foreshortening a subsequent busy period in order to ensure termination of the subsequent busy period at a next periodic global channel release instant.

11. A method of distributed dynamic bandwidth allocation for mitigating the effects of neighborhood capture in an infrastructure WLAN, comprising the steps of:

time-slotting of a communications channel into superframes of superframe duration (SFDuration) by a MAC protocol, wherein said superframes comprise control and data frames and busy periods (BPs), wherein said BPS comprise contention-free periods (CFPs) and contention-free bursts (CFBs);

assigning each BP an Arbitration-Time Inter-Frame Space (AEFS);

further assigning each enhanced distributed coordination function (EDCF) station AEFS values according to said station'"'"'s priority classification;

further assigning CFPs and CFBs the shortest possible AEFS values;

further assigning available channels to cells;

polling stations within a cell by an access point (AP) to determine if said stations within said cell have communications traffic to transmit;

attempting to gain access to said communications channel by means of a contention-based scheme using a short reservation packet by said AP if said AP determined that said stations within said cell had communications traffic to transmit;

performing frame exchanges if channel access is gained;

gaining access to said communications channel by one of waiting for a backoff period and re-attempting to gain access to said communication channel in the event that a transmission by another cell is less than a full time slot if channel access was not gained and waiting to attempt to gain access until a next time slot;

performing a global channel release (GCR) of all channels at pre-specified intervals; and

continuing to perform the above steps.

12. The method according to claim 11, wherein said contention-based scheme is based on carrier-sensing.

13. The method according to claim 11, wherein said BPs are foreshortened if necessary to meet GCR requirements.

14. The method according to claim 11, wherein said GCR may occur less frequently than after each BP if BPs are shorter than said SFDuration.

15. The method according to claim 11, wherein said distributed dynamic bandwidth allocation scheme gives all co-channels an equal opportunity to contend for said communication channel.

16. The method according to claim 11, wherein said GCR is performed at regularly spaced intervals.

17. The method according to claim 11, wherein communication traffic loads are balanced across cells and re-use groups.

18. The method according to claim 11, wherein cells in a multiple cell WLAN are synchronized to avoid BPs straddling a boundary of said superframe.

19. The method according to claim 18, wherein stations within a cell are synchronized with said cell'"'"'s access point (AP).

20. The method according to claim 18, wherein neighboring cells synchronize with frames sent by stations in an overlapping coverage area of said neighboring cells.

21. The method according to claim 18, wherein time offsets are corrected by stations transmitting a special frame carrying a timestamp field.

22. The method according to claim 21, wherein each station updates its clock upon receipt of said special frame having said timestamp field.

23. The method according to claim 19, wherein synchronization between cells is accomplished via a wired distribution system in communication with said APs.

24. A multiple-cell wireless local area network including overlapping first and second wireless LAN cells in a medium, each cell including a respective plurality of member stations, comprising:

a first member station in the first cell, transmitting a timing packet containing a timestamp value;

a second member station in the second cell receiving the timing packet;

said member stations in the first and second cells interrupting transmissions at a global channel release instant corresponding to said timestamp value; and

said member stations in the first and second cells contending for access to said medium following said global channel release instant.

25. The network of claim 24, which further comprises:

said member stations in the first and second cells synchronizing a superframe clock based on said timestamp value;

said member stations in the first and second cells establishing a periodic global channel release instant during each of a plurality of periodic superframes based on said clock; and

said member stations in the first and second cells periodically interrupting transmissions at said periodic global channel release instant.

26. The network of claim 25, which further comprises:

said member stations in the first and second cells periodically contending for access to said medium following said periodic global channel release instant.

27. The network of claim 26, which further comprises:

said periodic global channel release instant occurring at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical voice applications.

28. The network of claim 26, which further comprises:

said periodic global channel release instant occurring at intervals that are sufficiently close to meet delay and jitter restrictions for time-critical video applications.

29. The network of claim 26, which further comprises:

said member stations in the first and second cells contending for access to said medium using a slotted CSMA/CA access method following said periodic global channel release instant.

30. The network of claim 29, which further comprises:

said member stations in the first and second cells detecting completion of a busy period before a next periodic global channel release instant; and

said member stations in the first and second cells foreshortening a subsequent busy period in order to ensure termination of the subsequent busy period at a next periodic global channel release instant.

31. The network of claim 29, which further comprises:

one of said member stations in the first and second cells conducting a distributed coordination function (DCF) session, said one of said member stations having successfully accessed the medium following said periodic global channel release instant.

32. The network of claim 29, which further comprises:

one of said member stations in the first and second cells conducting a Tiered Contention Multiple Access (TCMA) session, said one of said member stations having successfully accessed the medium following said periodic global channel release instant.

33. The network of claim 29, which further comprises:

one of said member stations in the first and second cells conducting a point coordination function (PCF) session, said one of said member stations having successfully accessed the medium following said periodic global channel release instant.

34. A multiple-cell wireless local area network (WLAN) using distributed dynamic bandwidth allocation for mitigating the effects of neighborhood capture, comprising:

a wired distribution network; and

a plurality of cells, each cell having a plurality of stations, wherein one of said stations in each cell is an access point (AP), and further wherein communications between said plurality of stations is via said AP and where said plurality of stations are within one of a same cell and a different cell, and further wherein communications between one of said stations and said wired distribution network is via said AP, and further wherein stations in some different cells overlap;

wherein said wired distribution network allocates time-slots of a communications channel into superframes of superframe duration (SFDuration) using a MAC protocol, wherein said superframes comprise control frames and data frames and busy periods (BPs), wherein said BPs comprise contention-free periods (CFPs) and contention-free bursts (CFBs);

further wherein said wired distribution network assigns each BP an Arbitration-Time Inter-Frame Space (AEFS);

further wherein said wired distribution network assigns each enhanced distributed coordination function (EDCF) station AIFS values according to said station'"'"'s priority classification;

further wherein said wired distribution network assigns CFPs and CFBs the shortest possible AIFS values;

further wherein stations within each cell are polled by their respective AP to determine if said stations within said cell have communications traffic to transmit;

further wherein each said AP attempts to gain access to said communications channel by means of a contention-based scheme using a short reservation packet if each said AP determines that said stations within said cell have communications traffic to transmit;

further wherein each said AP that gained access to said communications channel performs frame exchanges with said wired distribution network;

further wherein said AP that had communications traffic to transmit and that did not gain access to said communications channel re-attempts to gain access to said communications channel by one of waiting for a backoff period and attempting to gain access to said communication channel in the event that a transmission by another cell is less than a full time slot and waiting to attempt to gain access until a next time slot;

further wherein each said AP and said wired distribution network perform a global channel release (GCR) of all channels at pre-specified intervals.

35. The WLAN according to claim 34, wherein said contention-based scheme is based on carrier-sensing.

36. The WLAN according to claim 34, wherein said BPs are foreshortened if necessary to meet GCR requirements.

37. The WLAN according to claim 34, wherein said GCR may occur less frequently than after each BP if BPs are shorter than said SFDuration.

38. The WLAN according to claim 34, wherein said distributed dynamic bandwidth allocation scheme gives all co-channels an equal opportunity to contend for said communication channel.

39. The WLAN according to claim 34, wherein said GCR is performed at regularly spaced intervals.

40. The WLAN according to claim 34, wherein communication traffic loads are balanced across cells and re-use groups.

41. The WLAN according to claim 34, wherein cells in said multiple-cell WLAN are synchronized to avoid BPs straddling a boundary of said superframe.

42. The method according to claim 41, wherein stations within a cell are synchronized with said cell'"'"'s access point (AP).

43. The WLAN according to claim 41, wherein neighboring cells synchronize with frames sent by stations in an overlapping coverage area of said neighboring cells.

44. The WLAN according to claim 41, wherein time offsets are corrected by stations transmitting a special frame carrying a timestamp field.

45. The WLAN according to claim 44, wherein each station updates its clock upon receipt of said special frame having said timestamp field.

46. The WLAN according to claim 45, wherein synchronization between cells is accomplished via a wired distribution system in communication with said APs.