November 2001

M. Benveniste -- AT&T Labs, Research

Slide 1Submission

doc.: IEEE 802.11-01/569r1

An Access Mechanism for Periodic Contention-Free Sessions

Mathilde BenvenisteAT&T Labs, Research

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 2Submission

doc.: IEEE 802.11-01/569r1

CFS and PCFS: DefinitionsDefinitions

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.

– When restricting the time to the next access attempt, the channel cannot be used sooner, even if needed and available; it limits efficiency of channel re-use

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 3Submission

doc.: IEEE 802.11-01/569r1

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 SessionsCPMA 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 AttemptsPrevents CFS/PCFS corruption in case of collision with an (E)STA

(4) ‘Neighborhood capture’ in wireless LANsPreventing a capture effect

(5) HCF Access Mechanisms: Inter-BSS NAV protectionVirtual carrier sense for CFP/CFBs

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 4Submission

doc.: IEEE 802.11-01/569r1

Introduction

• A PCFS is a special case of a CFP– For simplicity, but without loss of generality, we assume that PCFSs are

initiated by APs

• 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 either 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

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 5Submission

doc.: IEEE 802.11-01/569r1

PCFS Channel Access Mechanism: Key features

This prioritized distributed medium access protocol consists of 3 features

1 Fixed Cycle Time Reduces conflicts with PCFSs from other BSSs

2 Interleaving PCFSs/CFSs Reduces conflicts with CFSs from other BSSs

3 Staggered Start-up (optional) Contiguous sequences of PCFSs to deter collisions with (E)STAs

Assumption:We assume that there exists a mechanism for ‘busy’ channel detection (detection of

the start and end of a CFS)

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 6Submission

doc.: IEEE 802.11-01/569r1

Fixed Cycle TimeRepeating 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, if there is collision, resolve such collisions through a random backoff

If channel is busy at designated start time, PCFS is shortened by the time lost

PCFSContention Period

PCFS repetition interval

Busy channelPCF

Foreshortened PCFS

PCF

Contention PeriodPCFS

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 7Submission

doc.: IEEE 802.11-01/569r1

AP2 sets timer=CP

PCFS of AP1

PIFS

PCFS of AP2

AP1 sets timer=CP

PIFS

SIFS SIFS

PCFS of AP1

PIFS

PCFS of AP2

PIFS

SIFS SIFS

TImer of AP1expires & is reset

Timer of AP2expires & is reset

PCFS repetition interval

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

Non-conflicting PCFSs

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 8Submission

doc.: IEEE 802.11-01/569r1

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

NAV IBCP

PCFS

Contention Period

CFS

Contention Period

PCFS

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 9Submission

doc.: IEEE 802.11-01/569r1

Option: Contiguous PCFSs

Contiguous PCFs [gaps = PIFS] prevent DCF transmissions from conflicting with new PCFSs

– This option can eliminate the need for stations to maintain TBTT

Assumptions:• We assume fully overlapping BSSs or partially overlapping BSSs with IBNAV protection• Otherwise, we assume ‘parallel’ backoff [see paper on “Neighborhood Capture”]• PCFSs are all the same size [see paper on “Neighborhood Capture” for the value of

fixed-size PCFSs]

PCFSs from interfering BSSs can be made contiguous by a ‘staggered start-up’ procedure similar to CPMA

Given a sequence of contiguous PCFSs (separated by idle gaps = PIFS), future PCFSs will be contiguous if PCFSs are all the same size

November 2001

M. Benveniste -- AT&T Labs, Research

Slide 10Submission

doc.: IEEE 802.11-01/569r1

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 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