1. LB97 20/40 BSS Coexistence Date: 2007-05-24 Authors: July 2007
doc.: IEEE /2021r0
July 2007
LB97 20/40 BSS Coexistence
Date:
Authors:
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Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

2. July 2007
doc.: IEEE /2021r0
July 2007
Abstract
An explanation of what is in , and more generally, what is found in TGn draft subclause
Matthew Fischer, Broadcom
Matthew Fischer, Broadcom

3. July 2007
20/40 MHz BSS Coexistence
TGn draft contains rules for 20/40 MHz BSS operation
Coexistence with 20 MHz BSS
Means to prohibit 20/40 MHz BSS operation
See sublcauses , x
r5 contains modifications to these subclauses
A couple of significant changes
Change in BSS Width Trigger conditions
Change from AP-directed Overlapping BSS Scanning to STA-initiated OBSS Scanning
Move 20/40 BSS Coexistence Management Action frame to Spectrum Management Action category to achieve Class 1 designation
A lot of other minor changes that do not change the basic function
See listing near the end of the presentation
Matthew Fischer, Broadcom

4. New terms to assist in clarification of applicability
July 2007
New terms to assist in clarification of applicability
STA vs AP
STA in 2.4 GHz vs 5 GHz
HT vs non-HT
IBSS vs infrastructure
E.g. FC-HT-STA-19
Forty MHz Capable HT STA operating on a Clause 19 channel
NOTE FC = 40 MHz CAPABLE – not necessarily operating as 40 MHz
3 Definitions
Matthew Fischer, Broadcom

5. July 2007
Before starting a 20/40 BSS
FC-HT-AP shall scan all channels in the band
Rules for 2.4 GHz:
A 20/40 MHz BSS is allowed only if:
The Primary channel of the 20/40 MHz BSS matches the primary channel of all other BSS(s) in the channel range [ (P+S)/2 – 20, (P+S)/2 + 20]
Forty MHz Intolerant bit has not been found == 1
Rules for 5 GHz:
No restrictions, but recommendation for choosing the Primary and Secondary channels:
1st choice: Primary and secondary channels both in an unoccupied channel
2nd choice: Primary co-located with an existing primary and the secondary channel in an unoccupied channel
3rd choice: Primary co-located with an existing primary and the secondary co-located with an existing secondary
4th choice: Other
Scanning requirements for 20/40 MHz BSSs
Matthew Fischer, Broadcom

6. FC-HT-STA-19-initiated Overlapping BSS Scanning
July 2007
FC-HT-STA-19-initiated Overlapping BSS Scanning
Overlapping BSS Scan operation
FC-HT-STA-19 periodically scan for Beacons on all channels in the 2.4 GHz band
Only when associated with an FC-HT-AP-19
But independently of the current BSS operating width
I.e. if AP is capable of changing BSS to 20/40 MHz, then scanning must be performed
Exception for low-bandwidth STA
Low bandwidth = variable percentage of medium time, as set by the AP, within the range 0.0% through 3.0%
a Scanning and reporting requirements for 40 MHz capable STA
Matthew Fischer, Broadcom

7. FC-HT-AP-19 control of Overlapping BSS Scanning
July 2007
FC-HT-AP-19 control of Overlapping BSS Scanning
FC-HT-AP-19 controls the parameters of the Overlapping BSS Scanning operations
E.g. frequency of scanning operation, time to scan per channel, low-bandwidth exclusion value, etc.
Transmits OBSS Scan Parameters element to change parameter values
Associated STA accept the AP-supplied parameter values
Use defaults if AP does not supply any values
Default Scan Interval = 1800 seconds
a Scanning and reporting requirements for 40 MHz capable STA
a Overlapping BSS Scan Parameters element
Matthew Fischer, Broadcom

8. FC-HT-STA-19 BSS Width Trigger events
July 2007
FC-HT-STA-19 BSS Width Trigger events
A) Detection of “Legacy” Beacon
No HT Capabilities element present
B) Detection of Forty MHz Intolerant bit == 1
In any frame
The detecting FC-HT-STA-19 reports these events to the associated FC-HT-AP-19 using the 20/40 BSS Coexistence Management Action frame
Containing 0 or more AP Channel Report elements
And in the 20 MHz BSS Width request bit of the 20/40 BSS Coexistence Management field
Switching between 40 MHz and 20 MHz
Matthew Fischer, Broadcom

9. FC-HT-AP-19 BSS Width Trigger events
July 2007
FC-HT-AP-19 BSS Width Trigger events
A) Detection of “Legacy” Beacon
No HT Capabilities element present
B) Detection of Forty MHz Intolerant bit == 1
In any frame
C) Detection of 20 MHz BSS Width Request == 1
In a frame that is addressed to the FC-HT-AP-19 and sent by a STA that is ASSOCIATED with that AP
The detecting FC-HT-AP-19 responds to any of these events by:
Re-examining Primary/Secondary channel overlap/co-location rules
Using AP Channel Report information
Switching channels or switching from 20/40 MHz BSS operation to 20 MHz BSS operation, if necessary
Switching between 40 MHz and 20 MHz
Matthew Fischer, Broadcom

10. Responses to specific Trigger events
July 2007
Responses to specific Trigger events
Legacy Beacon
Channel of legacy beacon examined to determine if an overlap exists as defined by the rules
Forty MHz Intolerant bit
Disallows any 20/40 MHz operation in the 2.4 GHz band
20 MHz BSS Width Request bit
Switching between 40 MHz and 20 MHz
Matthew Fischer, Broadcom

11. Trigger response hysteresis
July 2007
Trigger response hysteresis
Single trigger event detection causes change
E.g. in response to ONE detected event, BSS must change channel or switch from 20/40 MHz BSS to 20 MHz BSS (if necessary)
Multiple OBSS Scan intervals with NO trigger events required to revert to 20/40 MHz BSS operation
The multiple is determined by MIB attribute dot11BSSWidthChannelTransitionDelayFactor, with a minimum value of 4 and default value of 4
I.e. cannot revert to 20/40 MHz BSS operation until there have been FOUR successive scans with no BSS Width Trigger events
Switching between 40 MHz and 20 MHz
Matthew Fischer, Broadcom

12. 20/40 MHz BSS Control Bits Forty MHz Intolerant bit
July 2007
20/40 MHz BSS Control Bits
Forty MHz Intolerant bit
HT Capabilities element
20/40 BSS Coexistence Management Action frame
STA sets the bit to ONE whenever dot11FortyMHzIntolerant MIB attribute is TRUE
i.e. whenever a STA wants
As per trigger condition b), can be received/detected/obeyed by any BSS (not just the sender’s BSS)
20 MHz BSS Width Request
Sent by STA to associated AP
Only received/detected/obeyed by AP of associated STA
Switching between 40 MHz and 20 MHz
Matthew Fischer, Broadcom

13. New Element – OBSS Scan Parameters
July 2007
New Element – OBSS Scan Parameters
Overlapping BSS Scan Parameters element
Allows AP control over the parameters of the Overlapping BSS Scanning operations performed by associated STA
Includes values to force setting of all MIB attributes associated with OBSS Scanning
E.g. dot11BSSWidthTransitionDelayFactor, dot11OBSSScanInterval, etc.
Matthew Fischer, Broadcom

14. New Element – 20/40 BSS Coexistence
July 2007
New Element – 20/40 BSS Coexistence
20/40 BSS Coexistence Management element
Formerly a field of the 20/40 BSS Coexistence Management Action frame, converted to an element for greater application space
Note that 20/40 BSS Coexistence Management Action frame was formerly called HT Information Exchange Action frame
Contains
Forty MHz Intolerant bit
20 MHz BSS Width Request bit
Information Request bit
Matthew Fischer, Broadcom

15. MIB attributes [units] (default value, range)
July 2007
MIB attributes [units] (default value, range)
dot11FortyMHzOptionImplemented (FALSE)
dot112040BSSCoexistenceManagement (FALSE)
dot11FortyMHzIntolerant (FALSE)
dot11BSSWidthTriggerScanInterval [seconds] (1800, )
dot11BSSWidthChannelTransitionDelayFactor (4, 4-100)
dot11OBSSScanPassiveDwell [TU] (20, )
dot11OBSSScanActiveDwell [TU] (10, )
dot11OBSSScanPassiveTotalPerChannel [TU] (200, 200 – 10000)
dot11OBSSScanActiveTotalPerChannel [TU] (20, 20 – 10000)
dot11OBSSScanActivityThreshold [bp i.e. 1%/100] (300, 0-300)
i.e. 300 bp <=> 3%
Matthew Fischer, Broadcom

16. Miscellaneous changes in 11-07-0614r5
July 2007
Miscellaneous changes in r5
Add Extended Capabilities element to various frames
E.g. to Beacons, Association Request, etc.
Extended Capabilities element has existed for years – TGn draft proposes to add a bit for signalling support of the 20/40 BSS Coexistence Management action frame
Add 20/40 BSS Coexistence Management element to various frames
Change HT Information Exchange Action frame to 20/40 BSS Coexistence Management Action frame
And change the HT Information field to the 20/40 BSS Coexistence Management element
Rearrange subclauses for readability
Fix Class 1 and Class 3 designations
Matthew Fischer, Broadcom

17. References 11-07-0614r6 Draft-P802.11n_D2.0.pdf
July 2007
doc.: IEEE /2021r0
July 2007
References r6
Draft-P802.11n_D2.0.pdf
Draft P802.11n_D2.02.pdf
P REVma-D9.0-red-line-all-pages.pdf
Matthew Fischer, Broadcom
Matthew Fischer, Broadcom