Preemptive packet for maintaining contiguity in cyclic prioritized multiple access (CPMA) contention-free sessions

• US 20030174664A1
• 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 7,245,605 B2
• Time in Patent Office: Days
• US Class Current: 370/317
• CPC Class Codes: H04W 24/04 Arrangements for maintaining operational condition
• 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

An access point transmits a preemptive peg packet when it has no data to transmit in order to maintain the contiguity of its transmission timing position with respect to the timing position of other contention-free sessions (CFS) transmitted by other access points in an existing, periodic sequence. The cyclic prioritized multiple access (CPMA) method establishes the transmission timing position of contention-free sessions (CFS) between overlapping first and second wireless LAN cells contending for the same medium. Each cell includes a respective plurality of member stations. If an access point has no traffic, it will transmit a short, preemptive pegging packet and reset its backoff timer. In this manner, no gaps longer than the distributed coordination function (DCF) Interframe Space (DIFS) are left idle. This prevents other stations from using DCF contention to seize the channel, until all access points have completed one contention-free session (CFS) per periodic cycle.

Claims

1. A method for reducing interference between overlapping first and second wireless LAN cells in a medium, each cell including a respective plurality of member stations, comprising:

coordinating by a first access point station in the first cell a periodic sequence of first contention-free sessions, each said session including multiple bursts with other member stations in the first cell, and retaining control of the medium by said first access point station by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session;

coordinating by said second access point station in the second cell a periodic sequence of second contention-free sessions, each said session including multiple bursts with other member stations in the second cell, and retaining control of the medium by said second access point station by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session;

transmitting by said first access point station a preemptive peg packet when it has no data to transmit, to maintain the contiguity of its transmission timing position with respect to a timing position of said second contention-free sessions transmitted by said second access point station; and

said preemptive peg packet preventing gaps to be left idle for longer than a distributed coordination function (DCF) Interframe Space (DIFS);

whereby other stations are prevented from using DCF contention to seize the channel, until said first and second access points have completed one contention-free session (CFS) per periodic cycle.

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

setting by the first access point a backoff timer to a fixed deterministic post-backoff delay, which has a value of Bkoff times a fixed number of idle time slots, the value of Bkoff being greater than a number of overlapping cells;

counting down the backoff timer by the first access point;

transmitting a second contention-free session by the first access point when the backoff timer expires; and

resetting the backoff timer to the value of Bkoff to start a new cycle.

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

said backoff timer being counted down using a shortest possible interframe space.

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

said interframe space being a Priority Interframe Space (PIFS).

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

said interframe space being a minimum arbitration interframe space (AIFS).

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

listening by the second access point in the second cell to said activity in the medium and detecting an end to one of said first contention-free sessions indicated by an interval longer than a PIFS idle interval following an end to said activity in the medium;

setting a second post-backoff delay by said second access point to transmit a minimal interval after said first contention-free session of said first access point; and

coordinating by said second access point in the second cell a second contention-free session, said second session including multiple bursts with other access points in the second cell, and retaining control of the medium by said second access point by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session.

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

said second post-backoff delay being a minimum arbitration interframe space (AIFS).

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

initiating by said second access point a contention-free session when said second post-backoff delay is counted down to zero; and

resetting said second post-backoff delay to start a new cycle.

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

separating consecutive ones of said first and second contention-free sessions by idle gaps which are less than or equal to a Priority Interframe Space (PIFS).

10. The method of claim 9, in which said setting said second post-backoff delay further comprises:

detecting an idle period X=PIFS+x, x>

0 at time t; and

setting said second post-backoff delay at time t, to a value of Bkoff−

x.

11. A wireless communications system having reduced interference between overlapping first and second wireless LAN cells in a medium, each cell including a respective plurality of member stations, comprising:

a first access point station in the first cell;

said first access point station coordinating in the first cell a periodic sequence of first contention-free sessions, each said session including multiple bursts with other member stations in the first cell, and retaining control of the medium by said first access point station by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session;

a first access point station in the first cell;

said second access point station coordinating in the second cell a periodic sequence of second contention-free sessions, each said session including multiple bursts with other member stations in the second cell, and retaining control of the medium by said second access point station by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session;

said first access point station transmitting a preemptive peg packet when it has no data to transmit, to maintain the contiguity of its transmission timing position with respect to a timing position of said second contention-free sessions transmitted by said second access point station; and

said preemptive peg packet preventing gaps to be left idle for longer than a distributed coordination function (DCF) Interframe Space (DIFS);

whereby other stations are prevented from using DCF contention to seize the channel, until said first and second access points have completed one contention-free session (CFS) per periodic cycle.

12. The system of claim 11, which further comprises:

said first access point setting a backoff timer to a fixed deterministic post-backoff delay, which has a value of Bkoff times a fixed number of idle time slots, the value of Bkoff being greater than a number of overlapping cells;

said first access point counting down the backoff timer;

said first access point transmitting a second contention-free session when the backoff timer expires; and

said first access point resetting the backoff timer to the value of Bkoff to start a new cycle.

13. The system of claim 12, which further comprises:

said backoff timer being counted down using a shortest possible interframe space.

14. The system of claim 13, which further comprises:

said interframe space being a Priority Interframe Space (PIFS).

15. The system of claim 13, which further comprises:

said interframe space being a minimum arbitration interframe space (AIFS).

16. The system of claim 12, which further comprises:

said second access point listening in the second cell to said activity in the medium and detecting an end to one of said first contention-free sessions indicated by an interval longer than a PIFS idle interval following an end to said activity in the medium;

said second access point setting a second post-backoff delay to transmit a minimal interval after said first contention-free session of said first access point; and

said second access point coordinating in the second cell a second contention-free session, said second session including multiple bursts with other access points in the second cell, and retaining control of the medium by said second access point by using interframe spaces sufficiently short between the bursts that the multiple bursts appear to contending stations to be a single instance of activity in the medium during a session until an end of a session.

17. The system of claim 16, which further comprises:

said second post-backoff delay being a minimum arbitration interframe space (AIFS).

18. The system of claim 16, which further comprises:

said second access point initiating a contention-free session when said second post-backoff delay is counted down to zero; and

said second access point resetting said second post-backoff delay to start a new cycle.

19. The system of claim 16, which further comprises:

said second access point separating consecutive ones of said first and second contention-free sessions by idle gaps which are less than or equal to a Priority Interframe Space (PIFS).

20. The system of claim 19, in which said setting said second post-backoff delay further comprises:

said second access point detecting an idle period X=PIFS+x, x>

0 at time t; and

said second access point setting said second post-backoff delay at time t, to a value of Bkoff−

x.