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Overlapping coverage issues with 802.11n BSSs

Overlapping coverage issues with 802.11n BSSs. Mathilde Benveniste benveniste@ieee.org. Introduction. Use of an extension channel should not lower aggregate throughput of a WLAN system because of the increased collisions it may cause (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 Mathilde Benveniste benveniste@ieee.org M. Benveniste (Avaya Labs)

  2. Introduction • Use of an extension channel should not lower aggregate throughput of a WLAN system because of the increased collisions it may cause (relative to having no extension channel) • Collisions arise because of channel sharing, which is necessary in OBSS due to the limited number of channels • Avoiding collisions in OBSS through RF resource management (i.e. preventing channel sharing by BSSs with coverage overlap) could decrease aggregate throughput • Channel sharing is enabled by CSMA/CA and virtual carrier sense (both help avoid collisions) and are important for both the control and the extension channel • This presentation focuses on the importance of virtual carrier sense on the extension channel for 11n OBSS • The impact on legacy devices and the importance of CSMA/CA are addressed elsewhere M. Benveniste (Avaya Labs)

  3. Overlapping Coverage • With 40 MHz tx, BSSs with coverage overlap may share either 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 *Note: Case 3 is not possible for the specification in Draft D01 M. Benveniste (Avaya Labs)

  4. Virtual Carrier Sensing – RTS/CTS • RTS/CTS (and NAV) help avoid collisions between 20 MHz tx in OBSSs (using the same control channel) • RTS/CTS on the control channel can prevent collisions with 40 MHz tx, but only in OBSS cases 1 and 4 • For POBSS cases 2 and 3, collisions could not be prevented for 40 MHz tx through RTS/CTS sent on the control channel only M. Benveniste (Avaya Labs)

  5. Channel sharing by BSSs with overlapping coverage • The available channels are not enough to provide separate channels to all BSSs with overlapping coverage • 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 [POBSS Case 2], or • use one’s control channel as the other’s extension channel [POBSS Case 3] 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 • Its 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx • Multiple retransmissions could ensue, tying up the channel • Similarly, B’s 40 MHz transmissions collide with a 40 MHz Tx from station 2 to A 2 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 • AP A hears no transmission when station 1 is transmitting on the extension channel • Its 40 MHz transmission when CCA (on the control channel) indicates inactivity on extension channel would cause collision with station 1’s tx • Multiple retransmissions could ensue, tying up the channel • B’s 40 MHz transmissions will collide with a Tx from station 2 to A 2 1 A B *Note: Case 3 is not possible for the specification in Draft D01 M. Benveniste (Avaya Labs)

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

  9. Conclusions • Use of an extension channel should not lower aggregate throughput of a WLAN system (relative to having no extension channel) as a result of increased collision rates • Maintaining a NAV on the extension channel is desirable, as it helps avoid collisions and facilitates channel sharing by POBSS M. Benveniste (Avaya Labs)

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