1. Signaling of intolerance for 40 MHz transmissions
January 2007
doc.: IEEE /0049r1
January 2007
Signaling of intolerance for 40 MHz transmissions
Date:
Authors:
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Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

2. January 2007
doc.: IEEE /0049r1
January 2007
Abstract
There are various opinions regarding the ability of 20 MHz-only capable devices to coexist with 40 MHz transmissions on overlapping channels. In order to accommodate the various possible solutions to the decision to allow or forbid 40 MHz transmissions, some signaling mechanism is needed. This presentation offers a signaling mechanism and proposes a set of rules to use in making the decision to allow or forbid 40 MHz transmissions.
Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

3. Detection of neighboring devices
January 2007
Detection of neighboring devices
Detect 40 MHz intolerant devices
AP cannot detect all overlap cases
“Marginal overlap” case is important
Requires STA participation
Therefore the detection is performed by:
STA and AP
Matthew Fischer, Broadcom

4. STA in 40MHz BSS detects Legacy BSS (Intolerant of 40MHz)
January 2007
STA in 40MHz BSS detects Legacy BSS (Intolerant of 40MHz)
STA_B 40 or 20
AP 20 only
Inform
AP 40 or 20
STA informs AP40 of neighboring intolerant BSS
STA asserts its intolerant bit
AP40 falls back to 20MHz mode but does not declare intolerance
Matthew Fischer, Broadcom

5. 40MHz AP detects Non-Overlapping Legacy BSS
January 2007
40MHz AP detects Non-Overlapping Legacy BSS
AP 20 only
AP 40 or 20
AP 40 sees beacons from neighboring legacy AP20
Shows case where AP scans non-primary channel for beacons
AP40 falls back to 20MHz mode but does not declare intolerance
Matthew Fischer, Broadcom

6. 40MHz AP detects Non-Overlapping Legacy BSS through traffic
January 2007
40MHz AP detects Non-Overlapping Legacy BSS through traffic
AP 20 only
STA_C 20 only
AP 40 or 20
AP 40 sees traffic from neighboring legacy AP20
AP40 falls back to 20MHz mode but does not declare intolerance
Matthew Fischer, Broadcom

7. 40MHz STA detects Non-Overlapping Legacy BSS through traffic
January 2007
40MHz STA detects Non-Overlapping Legacy BSS through traffic
AP 20 only
STA_C 20 only
AP 40 or 20
Inform
STA_A 40 or 20
STA 40 sees traffic from neighboring legacy AP20
STA 40 signals to AP 40 to fall back to 20 MHz mode
AP40 falls back to 20MHz mode but does not declare intolerance
Matthew Fischer, Broadcom

8. Classification rules Devices that may be 40 MHz intolerant:
January 2007
Classification rules
Devices that may be 40 MHz intolerant:
Legacy b
Proposed protection mechanisms are viewed as insufficient
Legacy g
Same answer
TGn devices
May indicate intolerance for various reasons
Information to be used to classify:
Absence of HT Capability element
New bit in HT Capability element: Forty_MHZ_Intolerant
Matthew Fischer, Broadcom

9. Behavior rules Assuming that intolerant devices are discovered
January 2007
doc.: IEEE /0049r1
January 2007
Behavior rules
Assuming that intolerant devices are discovered
BSS must be switched to 20 MHz-only operation if:
The AP detects intolerant BSS overlap
Only covers Full overlap case
Any STA in the BSS detects intolerant BSS overlap
Takes care of marginal overlap case
Provided that STA can signal overlap to AP
See new Forty_MHZ_Intolerant bit
Another AP indicates “intolerance”
New Forty_MHZ_intolerance bit may be set by an HT-AP
Recovery period before BSS may switch back to 40 MHz operation
Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

10. January 2007
Finite propagation
AP that switches to 20 MHz does NOT set the Forty_MHZ_Intolerant bit
This prevents a chain of propagation of the fall to 20 MHz operation
Matthew Fischer, Broadcom

11. 40 MHz -> 20 MHz fallback January 2007 Detection
Look for presence/absence of HT Capability element
Any channel
New bit in HT Capability element: Forty_MHZ_Intolerant
New bit can be used by intolerant TGn APs to require neighbors to switch from 20/40 to 20
Look for traffic from any overlapping BSS on channels
Channels that overlap the secondary
Classification rules
Intolerant devices
HT Capability element is absent (e.g b/g)
OR HT Capability element is present with Forty_MHZ_Intolerant = 1
Behavior rules
40 MHz prohibited if:
Detect Intolerant BSS
STA conveys detection of marginal overlap case to its AP:
Dissociate and re-associate with Forty_MHZ_Intolerant = 1
OR Send MGMT action frame with Forty_MHZ_Intolerant = 1
New HT Information Exchange MGMT Action frame
Matthew Fischer, Broadcom

12. Recovery to 40 MHz operation
January 2007
Recovery to 40 MHz operation
BSS must wait for thirty minutes with NO “intolerant” detection before reverting to 40 MHz operation
Matthew Fischer, Broadcom

13. OBSS Scanning Only STA are required to scan One scan every 30 minutes
January 2007
OBSS Scanning
Only STA are required to scan
STAs scan according to requests from AP
Beacon Request Measurement request mgmt action
APs may also scan
One scan every 30 minutes
Minimum dwell time per channel
Communicated through scan request
Matthew Fischer, Broadcom

14. January 2007
Important points
Legacy association does NOT cause switch to 20 mhz operation
Legacy BSS does cause switch to 20 mhz
Switch to 20 mhz operation does NOT propagate endlessly
Can go one hop, then stops
Effected through Forty_MHZ_Intolerant bit
NOT through BSS width setting
AP could deny association based on Forty_MHZ_Intolerant = 1
But still must respect the bit in any received frame
Matthew Fischer, Broadcom