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Overlapping coverage issues with 802.11n BSSs

Overlapping coverage issues with 802.11n BSSs. M. Benveniste. Abstract. Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to having no extension channel)

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Overlapping coverage issues with 802.11n BSSs

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  1. Overlapping coverage issues with 802.11n BSSs M. Benveniste M. Benveniste (Avaya Labs)

  2. Abstract • Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to having no extension channel) • Channel sharing by OBSS, enabled by CSMA/CA and virtual carrier sensing, addresses the limited availability of channels • Avoiding collisions in OBSS through RF resource management (i.e. restricting channel sharing by BSSs with coverage overlap) could decrease aggregate throughput • Virtual carrier is important for both control and extension channels M. Benveniste (Avaya Labs)

  3. Overlapping Coverage • With 40 MHz tx, BSSs with coverage overlap may share one or both of their channels • 4 overlap situations: • BSSs use the same control and extension channels • BSSs use same extension channel but different control channel • The control channel of one BSS is the extension channel of the other • BSSs use the same control channel but different extension channels • When one channel is shared, we have POBSS = partially overlapping BSS M. Benveniste (Avaya Labs)

  4. Virtual Carrier Sensing – RTS/CTS • RTS/CTS helps avoid collisions between 20 MHz tx in OBSSs (using the same control channel) • RTS/CTS can prevent collisions with 40 MHz tx only in POBSS cases 1 and 4 • POBSS cases 2 and 3 would be more common M. Benveniste (Avaya Labs)

  5. Channel sharing by BSSs with overlapping coverage • The available channels are not enough to provide all BSSs with overlapping coverage a separate channel • BSSs must share channels; CSMA/CA makes that easy • Since the extension channel is used less heavily than the control channel, it is better for BSSs with coverage overlap to • share the same extension channel • use one’s control channel as the other’s extension channel M. Benveniste (Avaya Labs)

  6. Sharing extension channel – POBSS Case 2 – results in collisions • BSSs A and B have coverage overlap; they use different control channels, but the same extension channel • AP A hears no transmission when station 1 is transmitting on the extension channel • A 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx 1 A B M. Benveniste (Avaya Labs)

  7. Sharing control/extension channel – POBSS Case 3 – results in collisions • BSSs A and B have coverage overlap; A uses as control channel B’s extension channel • If station 1 is transmitting on the extension channel, AP A hears no transmission • A 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx 1 A B M. Benveniste (Avaya Labs)

  8. Virtual Carrier Sense for POBSS • To avoid collisions in POBSSs, the extension channel should be monitored independently • RTS/CTS on both the control extension channel should be decoded within interference range and NAVs should be maintained M. Benveniste (Avaya Labs)

  9. Conclusions • Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to no extension channel) • Maintaining a NAV on both the extension and the control channel is desirable, as it enables channel sharing by POBSS with collision avoidance M. Benveniste (Avaya Labs)

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