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HC Recovery and Backoff Rules

HC Recovery and Backoff Rules. Sunghyun Choi and Javier del Prado Philips Research USA Briarcliff Manor, New York sunghyun.choi@philips.com. References. IEEE 802.11e/D1.3 IEEE 802.11-01/412r0: “ Aligning 802.11e HCF and 802.11h TPC operations ” by A. Soomro, S.Choi and J. del Prado

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HC Recovery and Backoff Rules

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  1. HC Recovery and Backoff Rules Sunghyun Choi and Javier del Prado Philips Research USA Briarcliff Manor, New York sunghyun.choi@philips.com

  2. References • IEEE 802.11e/D1.3 • IEEE 802.11-01/412r0: “Aligning 802.11e HCF and 802.11h TPC operations ” by A. Soomro, S.Choi and J. del Prado • IEEE 802.11-01/128r1: “HCF Channel AccessAnd Inter-BSS Channel Sharing” by J.-M. Ho, et al.

  3. Introduction Status as of 802.11e/D1.3

  4. Problem Statement • 802.11e/D2.0 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 desirable if Overlapping BSS (OBSS) exists • HC Backoff may result in collision with its own ESTAs in the QBSS

  5. 802.11e/D1.3 Says … • Clause 9.10 of D2.0 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 dot11CWmin[7] and dot11AIFS[7].”

  6. 802.11e/D2.0 Says … • And Clause 9.10.1.2 of D2.0 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 QoS (+)CF-Poll frame, and is the TXOP holder in case of a QoS data type frame transmitted during a CFB”

  7. Status as of 802.11e/D2.0 • …therefore, after sending a QoS (+)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

  8. But… • After sending QoS (+)CF-Poll frame, detecting PHY-CCA.indication(busy) is enough, and it is not needed to receive the frame correctly • Was the QoS (+)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!

  9. Proposed Solution

  10. Proposed Solution (1) • If PHY-CCA.indication (busy) is not detected within PIFS after the HC sending anyframe which requires response, e.g., QoS (+)CF-Poll, the HC may recover transmission or perform backoff both after PIFS • Which one to do is up to the HC algorithm, and out of the scope of 802.11e spec. • For example, the HC may want to backoff if it knows the OBSS existence.

  11. Example • HC backoff (if OBSS detected) • HC recovery

  12. Proposed Solution (2) • If PHY-CCA.indication (busy) is detected within PIFS after the HC sending a QoS (+)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 QoS (+)CF-Poll with another frame, e.g., from neighboring HC, so HC may perform backoff after PHY-CCA.indication (idle).

  13. Example • Successful TXOP grant • Potential collision with OBSS

  14. Proposed Solution (3) • If PHY-CCA.indication(busy) is detected within PIFS after the HC sending an RTS or non-CF-Poll QoS Data(+) frame: • If there were PHY-RXSTART.indication, PHY-CCA.indication(idle), and PHY-RXEND.indication, and the response is not received correctly, e.g., FCS error, 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

  15. Example • Successful frame transmission w/ unsuccessful response reception • Potential collision with OBSS

  16. HC Backoff Rules • For the random backoff procedure HC shall use: • Fixed MIB value: dot11AIFSHC = 1 (i.e., AIFSHC = PIFS) • Fixed MIB value: CWHC = dot11CWmin = dot11CWmax • CWHC could be 3 (ref. 01/128r1) • 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

  17. Summary • HC recovery or backoff depending on PHY-RXSTART.indication(.) as well as PHY-CCA.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

  18. Motion • Empower the editor to incorporate the normative text found in the word document 01/630r1, and to change wording to make the revised draft to be consistent with the above changes.

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