November 2001 doc IEEE 802 11 01630 r
November 2001 doc. : IEEE 802. 11 -01/630 r 0 HC Recovery and Backoff Rules Sunghyun Choi and Javier del Prado Philips Research USA Briarcliff Manor, New York sunghyun. choi@philips. com Submission 1 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 References • IEEE 802. 11 e/D 1. 3 • IEEE 802. 11 -01/412 r 0: “Aligning 802. 11 e HCF and 802. 11 h TPC operations ” by A. Soomro, S. Choi and J. del Prado • IEEE 802. 11 -01/128 r 1: “HCF Channel Access And Inter-BSS Channel Sharing” by J. -M. Ho, et al. Submission 2 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Introduction Status as of 802. 11 e/D 1. 3 Submission 3 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Problem Statement • 802. 11 e/D 1. 3 has both recovery and backoff rule of HC, which look inconsistent • HC backoff is not needed if the HC has the full control over the medium • However, backoff is needed if Overlapping BSS (OBSS) exists • HC Backoff may result in collision with its own ESTAs in the QBSS Submission 4 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 802. 11 e/D 1. 3 Says … • Clause 9. 10 of D 1. 3 reads: “A HC may perform a backoff following an interruption of a frame exchange sequence due to lack of an expected response from an (E)STA, or due to detection of CCA busy within a CFP or CFB. This backoff shall occur under the rules of the EDCF, using dot 11 CWmin[7] and dot 11 AIFS[7]. ” Submission 5 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 802. 11 e/D 1. 3 Says … • And Clause 9. 10. 1. 2 of D 1. 3 reads: “ESTAs, including the HC, are required to respond within any frame exchange sequence after a SIFS period. If the beginning of reception of an expected response, as detected by the occurrence of PHY-CCA. indication(busy) at the ESTA which is expecting the response, does not occur during the first slot time following SIFS, that ESTA may initiate recovery by transmitting after PIFS from the end of the last transmission. This recovery after PIFS is only permitted by the ESTA expecting the response. This ESTA is the HC in case of a Qo. S (+)CF-Poll frame, and is the TXOP holder in case of a Qo. S data type frame transmitted during a CFB” Submission 6 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Status as of 802. 11 e/D 1. 3 • …therefore, after sending a Qo. S (+)CF-Poll frame, the HC senses the medium and: – If PHY-CCA. indication busy is not detected • The HC may recover transmission by sending a second frame (ref. 9. 10. 1. 2) • The HC may perform backoff (ref. 9. 10) – If PHY-CCA. indication busy is detected • The HC remains silent during the rest of the CFB (ref. 9. 10. 1. 2) • The HC may perform backoff (ref. 9. 10); but, will not due to 9. 10. 1. 2 Submission 7 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 But… • After sending Qo. S (+)CF-Poll frame, detecting PHY -CCA. indication(busy) is enough, and it is not needed to receive the frame correctly • Was the Qo. S (+)CF-Poll received correctly? • Is the frame received by the HC from the Polled (E)STA? • If these two things are not true, it results in the loss of the TXOP. But, no way to find it out! Submission 8 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Proposed Solution Submission 9 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Proposed Solution (1) • If PHY-CCA. indication (busy) is not detected within PIFS after the HC sending any frame which requires response, e. g. , Qo. S (+)CFPoll, the HC may recover transmission or perform backoff – Which one to do is up to the HC algorithm, and out of the scope of 802. 11 e spec. – For example, the HC may want to backoff if it knows the OBSS existence. Submission 10 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Proposed Solution (2) • If PHY-CCA. indication (busy) is detected within PIFS after the HC sending a Qo. S (+)CF-Poll frame: – The HC needs to have PHY-RXSTART. indication(. ) before PHY-CCA. indication (idle). – If PHY-RXSTART. indication(. ) happens, the HC assumes that the TXOP was granted successfully. – If PHY-RXSTART. indication(. ) does not happen, it could mean a collision of the Qo. S (+)CF-Poll with another frame, e. g. , from neighboring HC, so HC may perform backoff after PHY-CCA. indication (idle) Submission 11 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Proposed Solution (3) • If PHY-CCA. indication(busy) is detected within PIFS after the HC sending an RTS or non-CFPoll Qo. S Data(+) frame: – If there were PHY-RXSTART. indication(. ), PHYCCA. indication(idle), and PHY-RXEND. indication(. ), and the response is not received correctly, the HC may retransmit the frame or transmit another frame after SIFS from PHY-CCA. indication(idle) – If there was not PHY-RXSTART. indication(. ) before PHY-CCA. indication (idle), it may perform backoff as it may mean a collision Submission 12 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 HC Backoff Rules • For the random backoff procedure HC shall use: – – AIFSHC = PIFS (a fixed MIB value) CWmin = CWmax = CWHC (a fixed MIB value) CWHC could be 3 (ref. 01/128 r 1) The random backoff will reduce collisions with other HCs • The HC may set AIFS [i] for every i to > PIFS+ CWHC – Increasing the AIFS [i] will avoid collisions with EDCF access Submission 13 S. Choi, Philips Research
November 2001 doc. : IEEE 802. 11 -01/630 r 0 Summary • HC recovery or backoff depending on PHYRXSTART. indication(. ) as well as PHYCCA. indication(. ) • Random backoff with fixed contention window size for HCs to avoid collisions with other HCs • Increasing AIFS[i] for EDCF access to avoid collision with HC • A best-effort & simple approach to handle OBSS problem Submission 14 S. Choi, Philips Research
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