1. An Access Mechanism for Periodic Contention-Free Sessions
doc.: IEEE /457
November 2001
November 2001
An Access Mechanism for Periodic Contention-Free Sessions
Mathilde Benveniste
AT&T Labs, Research
M. Benveniste -- AT&T Labs, Research

2. CFS and PCFS: Definitions
November 2001
CFS and PCFS: Definitions
Definitions
Contention free session (CFS) Any frame exchange sequence that may occur without contention following a successful channel access attempt. A CFS may involve one or more stations. A CFS may be initiated by any station.
A Contention-Free Burst and an RTS/CTS exchange are both examples of a CFS
Periodic contention free period (PCFS) A CFS that must occur at regular time intervals.
A Contention-Free Period is an example of a PCFS
Both PCFSs and CFSs are needed; the former for periodic traffic, the latter in order to use channel time efficiently, as channel availability permits
CFS/PCFS origin
For simplicity, but without loss of generality, we assume that CFSs/PCFSs are initiated by APs only
M. Benveniste -- AT&T Labs, Research

3. November 2001
Overview
This is one of 5 independent submissions relating to HCF access and OBSS mitigation.
These are:
(1) CPMA: An Access Mechanism for Contention-Free Sessions
CPMA a protocol for prioritized contention-based access
(2) An access mechanism for Periodic Contention-Free Sessions 
Regularly spaced sessions
(3) ‘Shield’: Protecting High-Priority Channel Access Attempts
Prevents CFS/PCFS corruption in case of collision with an (E)STA
(4) ‘Neighborhood capture’ in wireless LANs
Preventing a capture effect
(5) HCF Access Mechanisms: Inter-BSS NAV protection
Virtual carrier sense for CFS/PCFSs
M. Benveniste -- AT&T Labs, Research

4. Introduction
November 2001
A PCFS is a special case of a CFP that is started by an PC
Ideally we want:
PCFSs to have priority access over (E)DCF transmissions
(E)DCF transmissions to access channel at assigned priority
PCFSs to be able to regain control of the channel periodically conflict-free at pre-specified time intervals
No conflicts with PCFSs or CFSs from other BSSs or (E)DCF transmissions
PCFSs can be generated by the PCF method in the present standard; however, additional measures are needed to avoid collisions with PCFSs or CFSs from other BSSs
An optional feature is presented that eliminates the need for stations to keep track of Target Beacon Transmission Time
M. Benveniste -- AT&T Labs, Research

5. PCFS Channel Access Mechanism: Key features
doc.: IEEE /457
November 2001
November 2001
PCFS Channel Access Mechanism: Key features
This prioritized distributed medium access protocol consists of 3 features
Fixed Cycle Time
Reduces conflicts with PCFSs from other BSSs
Interleaving PCFSs/CFSs
Reduces conflicts with CFSs from other BSSs
Staggered Start-up
Contiguous sequences of PCFSs to deter collisions with (E)STAs
Relationships:
Features 1 and 2 can be used without 3
M. Benveniste -- AT&T Labs, Research

6. Assumptions Inter-BBS NAV protection
doc.: IEEE /457
November 2001
November 2001
Assumptions
Inter-BBS NAV protection
We assume that there exists a mechanism for ‘busy’ channel detection (detection of the start and end of a CFS)
‘Ideal’ backoff conditions
We assume fully overlapped BSSs or partially overlapping BSSs with IBNAV protection
Otherwise, we assume ‘parallel’ backoff [see paper on “Neighborhood Capture”]
M. Benveniste -- AT&T Labs, Research

7. PCFS repetition interval
Fixed Cycle Time
November 2001
Repeating in cycles of length CP (CFPPeriod*DTIM),
Periodic contention-free sessions (PCFSs) are generated, one from each overlapping BSS
PCFS attempts occur at the fixed specified time spacing since the start of the previous cycle
Each active AP sets a timer at CP; a PCFS is initiated when the timer expires
The timer is reset to CP (if there is no data to transmit); this starts a new cycle
Access is attempted with the shortest AIFS possible
(E)DCF transmissions are attempted by their assigned priority
A new HC can get started and resolve possible collisions through a random backoff
If channel is busy at designated start time, PCFS is shortened by the time lost
PCFS repetition interval
Foreshortened
PCFS
PCF
PCF
Busy channel
PCFS
PCFS
Contention Period
Contention Period
M. Benveniste -- AT&T Labs, Research

8. Non-conflicting PCFSs
November 2001
PCFS repetition interval
AP1 sets
timer=CP
TImer of AP1
expires & is reset
AP2 sets
timer=CP
Timer of AP2
expires & is reset
PIFS
PIFS
PIFS
PIFS
SIFS
SIFS
SIFS
SIFS
PCFS of AP1
PCFS of AP2
PCFS of AP1
PCFS of AP2
Non-conflicting PCFSs
Future PCFSs will not conflict, given a sequence of non-conflicting PCFSs
Because their previous PCFSs did not conflict, the follower AP’s starting time is different from that of the leader’s
Provided that the CFPMaxDuration<CP/number of interfering BSS, the PCFSs will not conflict
M. Benveniste -- AT&T Labs, Research

9. Interleaving PCFSs/CFSs
November 2001
Interleaving PCFSs/CFSs
Both CFSs and PCFSs use the shortest AIFS
CFSs and PCFSs from different BSSs may collide
To lessen the contention between them, the CFS length sent to update the NAV and IBNAV is increased by a period IBCP (inter-BSS contention period)
APs will attempt to access the channel during the IBCP only for a PCFS, while they will wait for NAV and IBNAV expiration before attempting a CFS
IBCP duration >= slot time to enable carrier sensing
PCFS
CFS of AP2
PCFS of AP1
NAV
IBCP
M. Benveniste -- AT&T Labs, Research

10. Option: Contiguous PCFSs
November 2001
Contiguity [gaps not longer than PIFS] can be used to prevent DCF transmissions from conflicting with new PCFSs
PCFSs from interfering BSSs can be made contiguous by a ‘staggered start-up’ procedure similar to CPMA
If PCFSs are all the same size, future PCFSs will be contiguous, given a sequence of contiguous PCFSs (consecutive and separated by idle gaps = PIFS)
This option would eliminate the need for stations to maintain TBTT
M. Benveniste -- AT&T Labs, Research

11. November 2001
QoS Management
PCFSs provide regular access to the channel for periodic traffic
The use of PCFSs alone cannot not provide efficient dynamic bandwidth allocation
CFSs generated on a contention-basis must complement PCFSs
PCFSs and CFSs access the channel at top priority; with the shortest AIFS
To be assured timely access, only PCFSs will attempt access of the channel during the IBCP - the time interval added at the close of the NAV
QoS requirements are met by each AP scheduling its traffic as follows:
Periodic traffic is transmitted in PCFSs
Non-periodic traffic is placed either in a PCFSs or in its allotted CFS according to traffic priority
Delay-sensitive traffic is scheduled first, followed by traffic of lower priorities
M. Benveniste -- AT&T Labs, Research