doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 HCF Access through Tiered Contention Mathilde Benveniste AT&T Labs, Research

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 2 In agreement with other proposals EDCF can be used by HC to access the channel HC is the highest priority class - uses shortest AIFS Backoff is needed to deal with Inter-BSS interference

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 3 Concern If there is a collision between HCs and there is random backoff, ESTAs can access the channel before the HC –too long a backoff could leave idle slots; (PIFS + 1)=DIFS allows an ESTA to transmit first –too short a backoff causes collisions between HCs

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 4 Remedy Ideal Tag Scheduling with deterministic backoff will remove idle gaps; ESTAs cannot access the channel before the HC avoid most collisions

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 5 Tag scheduling Tags are labels, such as 1, 2, 3, …, assigned to different BSSs Interfering BSS are assigned different tags Tags correspond to different backoff delays >> Collisions are diminished Each backoff delay increases by 1 slot With compact re-use of tags, there are no idle slots >> (E)DCF stations cannot seize the channel

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 6 Frame 2Frame 1 Time 1 Channel - 3 AP groups AP 2 AP 3 AP 1 AP 2 AP 3 Deterministic backoff procedure

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 7 Tag scheduling cont’ed To give each BSS (or group of BSS with the same tag) a fair chance at the channel, the tags are ordered in a way that is known a priori; this is the order of channel access. Increasing backoff delays are associated with decreasing tag rank in this ordering. For instance, a cyclic permutation for three tags, t=1, 2, 3, would give the following channel access ordering: 1, 2, 3 for the first frame, 3, 1, 2 next, and then 2, 3, 1.

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 8 Frame 2Frame 1 Time 1 Channel - 3 AP groups AP 2 AP 3 AP 1 Frame 3Frame 4Frame 5 Frame 6 Fair access through permutation of tags

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 9 QoS Management Delay-sensitive traffic will be sent first, followed by traffic with lower priority. Two or more backoff values associated with each tag, a shorter value for the higher priority traffic and longer ones for lower priority. As long as all the deferral time interval for low-priority traffic are longer than the deferral time intervals for higher priority traffic of any tag, top-priority packets are generally transmitted before any lower-priority packets.

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 10 Frame 2Frame 1 Time AP 1 AP 2 AP 3 AP 2 AP 3 AP 1 highlow priority Two Priority Classes

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 11 Tag re-use Non-interfering BSS are assigned the same tags Tag assignment may be either –pre-planned; APs are placed on a grid and tag assignment follows a re-use pattern (a) –ad-hoc; APs are located where convenient (b) and tag assignment is treated like Dynamic Channel Selection

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 12 Tag re-use examples t1 t2 t3 t4 t1 t3 t2 t1 t2 t3 t1 t2 t1 t3 t1 t2 t3 t2 t1 t2 t3 t1 t2 t3 t1 t2 t1 t3 t1 t2 t3 t2 t1 t2 t3 1 t2 t3 t1 (a)(b)

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 13 Inter-BSS NAV protection The NAV prevents contention from (E)DCF stations during a contention-free session (The NAV specifies the duration of the session.) A new NAV, Inter-BSS NAV (IBNAV), prevents contention from the HC in an interfering BSS All ESTAs echo the beacon received from an HC (own or neighboring) The IBNAV is set by an HC either –when the beacon is transmitted by the HC, or –when echoed by ESTAs by sending the CFTR

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 14 AB A 1 Overlapping BSS and CFTR A, B are HCs; A 1 is an ESTA associated with A, but hears both HCs When A 1 receives a beacon –it sends a contention-free time response (CFTR), containing the duration of the contention-free period found in the beacon –ESTAs update their IBNAV –HCs update their IBNAV

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 15 IBCP -- Increasing the protection of NAV/IBNAV The NAV and IBNAV use different contention-free session length to further prevent contention from DCF traffic during HC channel access The NAV expires later than the IBNAV by a period IBCP (inter-BSS contention period) IBCP enables the HCs to attempt access without contention from DCF traffic IBCP can be used with both deterministic and random backoff

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 16 Setting IBCP for Deterministic Backoff When using deterministic backoff with Tag Scheduling, IBCP is set <= the maximum number of tags. IBCP protection supplements the protection offered by “not so compact” tag re-use Therefore, assigning tags becomes easier (the simplest way to assign tags is to give a different tag to each BSS -- i.e. without re-use) The best results will involve some tag re-use and a small IBCP value

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 17 Collision possibilities Collisions are possible because of variable session lengths – Retry with a backoff will resolve collisions Collisions are also possible if interfering BSSs get the same tag –Change tags to avoid collisions (adaptive tag selection); –and resolve collisions with random backoff with contention windows (and/or offsets) that increase inversely with the rank of the associated tag; –or start with random backoff, instead of deterministic, using contention windows (and/or offsets) that increase inversely with the rank of the associated tag

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 18 Setting IBCP for Random Backoff If random backoff is used, IBCP is set to the contention window size A reasonable contention window size must be used to avoid collisions between HCs Given reasonable re-use of the tags, the channel time devoted to the IBCP would be less with deterministic backoff delays, as compared to random Random backoff should be used for resolution of collisions arising from interfering tag assignment

doc.: IEEE /171 Submission July 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 19 Conclusions Inter-BSS interference can be avoided through Tiered Contention by assigning HCs top priority Tag Scheduling and deterministic backoff sequence access of BSS fairly, quickly, and with the fewest collisions The CFTR frame and the IBNAV prevent interfering BSSs from engaging in overlapping contention-free sessions HCs are assured access before (E)DCF stations through tag re-use and the IBCP time Retry and backoff resolve any collisions between HCs, should they occur (low probability)