1. HCF Channel Access And Inter-BSS Channel Sharing
Jin-Meng Ho, Sid Schrum, Khaled Turki, and Donald P. Shaver
Texas Instruments Incorporated
12500 TI Blvd.
Dallas, Texas 75243
(214) (Ho)

2. HC Channel Access
The HC senses the channel before it attempts to initiate a CFB.
If the channel has been idle for one PIFS, the HC sends a frame.
If the channel is busy, the HC defers the transmission until one PIFS following the end of the busy period.
PIFS
HC senses
HC transmits
PIFS
HC senses
HC transmits
Channel Busy

3. HC Access Recovery -- I
If the HC expects a response following the end of a transmission from itself, and finds the channel to be busy within one SIFS but cannot correctly receive a frame, the HC shall not reclaim the channel before the end of the TXOP applicable to a WSTA.
The TXOP may be an explicitly allocated TXOP or the time required for sending a QOS CF-Ack frame.
The HC shall defer to the end of the busy period or the TXOP, whichever is longer, and then invocate the DCF backoff procedure using PIFS for DIFS and CWHCmin for CWmin.
CWHCmin is a MIB parameter of default value 3.
From another BSS HC HC
SIFS
SIFS
PIFS
PIFS
Backoff
Backoff
TXOP
TXOP
Channel
Busy
Channel
Busy
WSTA
WSTA
WSTA
TXOP > Busy Period
TXOP < Busy Period
Scenario 1
Scenario 2

4. HC Access Recovery -- II
If the HC expects a response following the end of a transmission from itself, and finds the channel to be idle after one SIFS, the HC shall access the channel by the backoff procedure specified above, with the PIFS measuring from the end of that transmission.
If the HC successfully receives a frame from the station that had failed to send the expected response to the HC, in the course of the backoff and within the applicable TXOP following the transmission, the HC shall ignore the backoff timer and consider the TXOP operation to have been successful.
Channel
Idle
Channel
Idle HC HC
SIFS
SIFS
WSTA
WSTA
Backoff
Backoff
PIFS
PIFS
From another BSS
Scenario 1
Scenario 2

5. WSTA Access Recovery
If a WSTA does not receive an expected response within one SIFS of the end of a transmission from itself, the WSTA shall reclaim the channel one PIFS after the end of that transmission if sufficient TXOP remains for completing transmission of a frame exchange sequence.
From another BSS
SIFS HC HC
WSTA
WSTA
PIFS
TXOP
From another BSS
Scenario 1
Scenario 2

6. Channel Sharing
An HC shall not extend, in duration, a CFB beyond aMaxCFBLimit defined in the MIB.
This enables channel sharing between inter-BSS stations.
This provides bandwidth for contention access.
An HC shall not regain the channel within aMinCFBSpacing (a MIB parameter) interval of the preceding CFB it supported.
Within a CFB, all transmissions are separated by a SIFS.
SIFS
PIFS
CFB length 
aMaxCFBLimit
Separation 
aMinCFBSpacing
CFB length 
aMaxCFBLimit
CFB
CFB

7. HC Contention Window
A small contention window is adequate for resolving HC-HC collision.
Such collision occurs between 2 or 3 cochannel BSSs in practice.
A three-way overlapping is equivalent to a two-way overlapping in reality.
A small contention window is adequate and desirable for resolving HC-STA collision.
The colliding STA (in an overlapping BSS) should have a larger backoff to allow the HC to reclaim the channel.
A small contention window is desirable for enabling the HC to have a higher priority access than non-HC stations after a collision.
Non-HC stations should wait for a longer time based on larger (E)DCF contention windows.
A small contention window is desirable for minimizing channel idle overhead.
HC should always use the same contention window to backoff regardless of the presence of overlapping BSSs.
This makes the inter-BSS channel sharing normative and fair.
The contention window need not be doubled as overlapping BSS collision is highly asymmetric and dynamic and may be resolved by a retransmission.

8. Normative Text

9. Normative Text (Cont)