1. Overlapping coverage issues with 802.11n BSSs
April 2006
Overlapping coverage issues with n BSSs
Mathilde Benveniste
M. Benveniste (Avaya Labs)

2. Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

3. Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

4. Virtual Carrier Sensing – RTS/CTS
Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

5. Channel sharing by BSSs with overlapping coverage
Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

6. Sharing extension channel – POBSS Case 2 – results in collisions
Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

7. Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

8. Virtual Carrier Sense for POBSS
Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company

9. Month 2002
doc.: IEEE /xxxr0
April 2006
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)
John Doe, His Company