Interpretations of Backoff

1. Interpretations of Backoff Menzo Wentink Intersil menzo.wentink@intersil.com Menzo Wentink, Intersil

2. Interpretations of Backoff • The interpretations of backoff differ in whether AIFS is considered to be a countdown slot, or not: • No, AIFS is not a countdown slot (AIFS is passive) • smallest backoff = 0 is the majority interpretation of 802.11-1999, with AIFS=DIFS • smallest backoff = 1 is the majority interpretation of TGE D0.1 • Yes, AIFS is a valid countdown slot (AIFS is active) • smallest backoff = 0 is not allowed, because it would lead to negative backoff • smallest backoff = 1 is the minority interpretation of 802.11-1999, with AIFS=DIFS • So there are three different backoff behaviors: • passive/AIFS/0 - majority interpretation of 802.11-1999 • passive/AIFS/1 - majority interpretation of TGE D0.1 • active/AIFS - minority interpretation of 802.11-1999 • Most people agree that the backoff counter is decremented by one if no energy is detected near the end of a valid countdown slot. If the counter zeroes, the station transmits in the next slot. Menzo Wentink, Intersil

3. Observations (1) • The majority interpretation of DCF in 802.11-1999 is passive/DIFS/0 • Deferred frames never go at DIFS, but always at or after DIFS+1, because the smallest possible remaining backoff is 1. Only when the initial backoff is 0, a transmission will occur at DIFS. So effectively, the slot immediately after DIFS is wasted for all deferrals. This was likely not the intent of the standard, but it is difficult to interpret the normative text in another way. • The majority interpretation of EDCF in TGE D0.1 is passive/AIFS/1 • Both deferred and smallest backoff frames in passive/AIFS/1 go at or after AIFS+1. This means that the effective arbitration time of passive/AIFS/1 is exactly equal to AIFS+1 (which in turn is equal to active/(AIFS+1), see next slide). • passive/AIFS/1 always transmits at or after AIFS+1, although the arbitration time of AIFS does not suggest so. Hence the actual arbitration time in D0.1 EDCF is AIFS+1. Menzo Wentink, Intersil

4. Observations (2) • TGE D0.1 has no QoS level equal to 802.11-1999 (i.e. passive/DIFS/0) • TGE D0.1 specifies that the backoff is passive/AIFS/1. The closest level to legacy (passive/DIFS/0) would be passive/PIFS/1 (the minority interpretation of DCF), because both levels have a lower transmission bound at DIFS. • Yet passive/PIFS/1 is a higher level than passive/DIFS/0, because it has a one slot countdown advantage after each deferral. The countdown for passive/PIFS/1 is resumed at DIFS (not PIFS, because it is passive), while passive/DIFS/0 only resumes at DIFS+1. Hence the difference between these two ‘DIFS’ levels becomes larger when the load increases, because there will be more deferrals. With every deferral, the backoff difference grows by 1 (see slide 6). • passive/AIFS/1  active/(AIFS+1) • passive/AIFS/1 has the exact same behavior as active/(AIFS+1). The slot after a passive AIFS could be interpreted as the ‘activator’ for (AIFS+1) when regarded as one slot (see slide 7). Both passive/AIFS/1 and active/(AIFS+1) never transmit before AIFS+1. • The advantage of using active/AIFS is the fact that AIFS is exactly equal to the effective arbitration time. This is not the case for passive/AIFS/1, because the effective arbitration time would be AIFS+1 instead of AIFS. Menzo Wentink, Intersil

5. Recommendations for TGE D0.1 The backoff issue can be solved in two ways: • Specify whether AIFS is active or passive – preferred solution • Specify explicitly whether AIFS is active or passive, the latter with smallest initial backoff equal to 0, i.e. active/AIFS or passive/AIFS/0. This covers for the full range of possible behaviors, including the minority and majority interpretations of legacy. This solution is preferred because AIFS will always correspond with the real arbitration time. • Whether AIFS is active or passive could either be made normative or be dynamic and specified in the QoS Parameter Set element, along with AIFS itself. The passive/DIFS/0 level must be available. • X-faxtor (passive/AIFS/x) • AIFS is passive (i.e. AIFS is not a countdown slot), but the smallest initial backoff x can be 0 or 1. This covers for the full range of possible behaviors, including the minority and majority legacy interpretations. However AIFS does not indicate the true arbitration time for x=1. • This solution is proposed by Mathilde Benveniste in doc. 11-01-243r1-E, i.e. backoff=r+x, r=random(0,CW) and x {0,1} • the x-factor could either be normative or be variable and specified in the QoS Parameter Set element, along with AIFS itself. The passive/DIFS/0 level must be available. Menzo Wentink, Intersil

6. passive/PIFS/1 vs. passive/DIFS/0 backoff counter: 3 3 2 1 1 0 * * * PIFS slot slot PIFS slot TX! passive/PIFS/1 backoff counter: 3 3 2 2 * DIFS slot DIFS passive/DIFS/0 medium busy medium idle deferral * = ‘active’ slot boundary The advantage of passive/PIFS/1 over passive/DIFS/0 grows with one slot for each deferral. Menzo Wentink, Intersil

7. passive/AIFS/1 vs. active/(AIFS+1) backoff counter: 3 3 2 1 1 0 * * * AIFS slot slot AIFS slot TX! passive/AIFS/1 backoff counter: 3 3 2 1 2 0 * * * AIFS+1 slot AIFS+1 TX! active/(AIFS+1) medium busy medium idle deferral * = ‘active’ slot boundary passive/AIFS/1 and active/(AIFS+1) are equivalent Menzo Wentink, Intersil