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An Access Mechanism for Periodic Contention-Free Sessions

An Access Mechanism for Periodic Contention-Free Sessions. Mathilde Benveniste AT&T Labs, Research. CFS and PCFS: Definitions. Definitions

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An Access Mechanism for Periodic Contention-Free Sessions

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  1. 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 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. 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 • 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. PCFSChannel 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 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. Fixed Cycle Time 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. PCFS repetition interval TImer of AP1 expires & is reset AP1 sets timer=CP 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 notconflict AP2 sets timer=CP Timer of AP2 expires & is reset PIFS PIFS PIFS PIFS SIFS SIFS SIFS SIFS PCFS of AP2 PCFS of AP2 PCFS of AP1 PCFS of AP1 Non-conflicting PCFSs M. Benveniste -- AT&T Labs, Research

  9. 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 IBNAVis 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 of AP1 CFS of AP2 PCFS NAV IBCP M. Benveniste -- AT&T Labs, Research

  10. Option: Contiguous PCFSs 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 byidle gaps = PIFS) This option would eliminate the need for stations to maintain TBTT M. Benveniste -- AT&T Labs, Research

  11. 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

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