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Overlapping coverage issues with 802.11n BSSs: CCA on extension channel

Overlapping coverage issues with 802.11n BSSs: CCA on extension channel. Mathilde Benveniste benveniste@ieee.org. Addresses TGn comments: 295, 704, 712, 1495, 1503, 1517, 1524, 1559, 1625, 1657, 1750, 2730, 2738, 3513, 3514, 3882,

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Overlapping coverage issues with 802.11n BSSs: CCA on extension channel

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  1. Overlapping coverage issues with 802.11n BSSs: CCA on extension channel Mathilde Benveniste benveniste@ieee.org Addresses TGn comments: 295, 704, 712, 1495, 1503, 1517, 1524, 1559, 1625, 1657, 1750, 2730, 2738, 3513, 3514, 3882, 4084, 4188, 4574, 4640, 4791, 4804, 6813, 7012, 7178, 7473, 7839, 7840, 7873, 7893, 7913, 7922, 7926, 8021, 8200, 8284, 10016, … Mathilde Benveniste, Avaya Labs

  2. Abstract • The available channels are not enough to provide all BSSs with overlapping coverage separate channels • BSSs must share channels; CSMA/CA -- CCA and virtual carrier sense included -- makes that possible • An extension channel without either will impact the co-existence of 40MHz 11n stations with both legacy and 11n peers • 40 MHz capable 11n devices make aggressive neighbors to legacy devices • Aggregate throughput losses would occur in OBSSs with 40 MHz channels relative to the throughput with 20 MHz channels only Mathilde Benveniste, Avaya Labs

  3. Outline • Overlapping BSS scenarios • CCA Considerations • 11n coexisting with legacy stations • Co-existing 11n stations • Virtual Carrier Sense Considerations [discussed in document 06-1522r0] • 11n coexisting with legacy stations • Co-existing 11n stations Mathilde Benveniste, Avaya Labs

  4. Mixed Case: Overlapped Coverage between Legacy and 40MHz 11n BSSs • Legacy and 40 MHz 11n devices found in BSSs with coverage overlap face two overlap situations: • The legacy device uses the control channel of the 40MHz 11n device • The legacy device uses the extension channel of the 40MHz 11n device Mathilde Benveniste, Avaya Labs

  5. 11n OBSS Case: Overlapped Coverage among 40MHz 11n BSSs In general, 40 MHz BSSs with coverage overlap face four overlap situations:* • BSSs use the same control and extension channels • BSSs use same extension channel but different control channels • The control channel of one BSS is the extension channel of the other [not possible according to D1.0] • BSSs use the same control channel but different extension channels *Note: Some of these cases may not be allowed by the specification of the draft Mathilde Benveniste, Avaya Labs

  6. No CCA Mathilde Benveniste, Avaya Labs

  7. CCA • CCA helps avoid collisions between legacy devices in OBSSs • CCA on the control channel can prevent collisions between 40MHz devices, but only in BSSs using the same control channel (that is, in 11n-OBSS cases 1 and 4 and Mixed case 1) • Otherwise (for 11n-OBSS cases 2 and 3 and Mixed case 2), collisions involving 40 MHz transmissions are possible • Such collisions cannot be prevented by using CCA on the control channel only Mathilde Benveniste, Avaya Labs

  8. Consequences for legacy devices • Backward compatibility is not observed • 11n 40 MHz devices seize the channel without regard for legacy transmissions • The performance of legacy technologies would thus be compromised, as they would be at a disadvantage in contending for the channel • Applications that cannot use 11n technology (due to battery or other constraints) would suffer Mathilde Benveniste, Avaya Labs

  9. Consequences for 11n devices • Increased collision rate • Transmissions on the extension channel would cause collisions to other 11n devices • With increased collision rates, increases in throughput realized with the extension channel would be taken up in retransmissions Mathilde Benveniste, Avaya Labs

  10. Fixes that would not work: • Channel segregation • Preventing extension channel sharing Mathilde Benveniste, Avaya Labs

  11. Channel segregation In order to avoid 11n OBSS collisions caused by 40 MHz transmissions, it has been suggested to designate certain channels to be used as control and different channels to be used as extension channels • Proposed scheme does not prevent collisions with legacy devices • The utilization efficiency of the available bandwidth would be reduced • Extension channels are less heavily used than control channels in mixed 20/40 BSSs • If all BSSs do not use 40 MHz, channels designated as extension channel would not be utilized • Preventing the use of such channels as control channels would effectively reduce the available RF spectrum • In areas with high WLAN density (e.g. metropolitan areas, enterprise, or campus), reducing the RF spectrum use efficiency reduces aggregate throughput Mathilde Benveniste, Avaya Labs

  12. Preventing extension channel sharing In order to avoid 11n OBSS collisions caused by 40 MHz transmissions, it has been suggested to avoid sharing of extension channels by two 11n BSSs • Proposed scheme does not prevent collisions with legacy devices • The utilization efficiency of the available bandwidth would be reduced • Extension channels are less heavily used than control channels in mixed 20/40 BSSs • Sharing of extension channels by BSSs could increase utilization of extension channels • Preventing BSSs from sharing extension channels would increase the number of extension channels needed and thus effectively reduce the available RF spectrum • In areas with high WLAN density (e.g. metropolitan areas, enterprise, or campus), reducing the RF spectrum use efficiency reduces throughput Mathilde Benveniste, Avaya Labs

  13. Fixes that would work:RequireCCA on extension channel Mathilde Benveniste, Avaya Labs

  14. CCA on extension channel CCA on the 20 MHz extension channel prevents a 40 MHz station from colliding with a transmission it can hear on the extension channel Mathilde Benveniste, Avaya Labs

  15. Conclusion • CCA on the extension channel is necessary for backward compatibility with legacy BSSs • CCA on the extension channel is needed to prevent collisions among 11n devices • Other fixes mentioned here would reduce the aggregate throughput realizable in high density WLAN areas (e.g. metropolitan areas, enterprise, or campus) • If CCA cannot be made mandatory for the extension channel • A higher aggregate throughput would be realized in high density areas by using only 20 MHz channels • High BSS throughput can be achieved with collocated 20 MHz radios Mathilde Benveniste, Avaya Labs

  16. Straw Poll Change “may sense CCA on the 20MHz extension channel” to “shall sense CCA on the 20MHz extension channel”. Language also needs to be added indicating that STAs shall not transmit 40MHz PPDUs unless CCA is clear on both the control and extension channels. For: 24 Against: 18 Mathilde Benveniste, Avaya Labs

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