Neighborhood Capture and OBSS July 2009 Neighborhood Capture and OBSS Date: 2009-07-14 Authors: Name Address Company Phone Email Mathilde Benveniste South Orange, NJ 07079 En-aerion 973-761-6105 benveniste@ieee.org Notice: This document has been prepared to assist IEEE 802.11. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.11. Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures <http:// ieee802.org/guides/bylaws/sb-bylaws.pdf>, including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair <stuart.kerry@philips.com> as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.11 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at <patcom@ieee.org>. M. Benveniste (En-aerion)

Neighborhood Capture and OBSS Mathilde Benveniste benveniste@ieee.org July 2009 Neighborhood Capture and OBSS Mathilde Benveniste benveniste@ieee.org Based on publication “Wireless LANs and ‘Neighborhood Capture’”, M. Benveniste, Proceedings PIMRC 2002, Lisboa, Portugal, September 2002 An earlier version of paper appeared as Submission IEEE 802.11-01/596, November 2001 M. Benveniste (En-aerion)

Outline Some Background: IEEE 802.11 MAC Protocols July 2009 Outline Some Background: IEEE 802.11 MAC Protocols Radio resources management and OBSS ‘Neighborhood Capture’ – a new capture effect description and its impact on QoS Our proposed solution: Global Channel Release Implementation M. Benveniste (En-aerion)

802.11 channel access mechanism, DCF July 2009 802.11 channel access mechanism, DCF CSMA A STA transmits a new frame if the medium is idle for a period >= DIFS (DCF inter-frame space) If medium is busy, transmission is deferred for a random backoff delay Backoff countdown starts/resumes following a busy condition after medium is idle for a period >= DIFS reduces the delay by 1 for every slot medium is idle is interrupted when medium becomes busy STA transmits when backoff expires DIFS Backoff Delay DIFS Busy Medium Contention Window Next Frame Slot time Immediate access when medium is free >= DIFS Defer Access Decrement Backoff as long as medium is idle M. Benveniste (En-aerion)

802.11e channel access mechanism - EDCF July 2009 802.11e channel access mechanism - EDCF Next Frame Slot time Busy Medium SIFS DIFS/AIFS Lower priority AIFS > DIFS AIFS[i] PIFS Backoff-Window Top priority AIFS = PIFS AIFS[0]/ DIFS Legacy STA AIFS = DIFS AIFS[1]/ A station must wait a priority- dependent idle period to count down backoff delay or transmit Other differentiation mechanisms are also employed* Independent queues are maintained for different priorities All contention-free access involves EDCF M. Benveniste (En-aerion)

Higher LAN densities & mobility July 2009 Higher LAN densities & mobility C IEEE 802.11 b/a/g/e … B A C B Higher LAN densities & mobility Contiguous cells & Channel Reuse Channel must be allocated to BSSs as in cellular systems M. Benveniste (En-aerion)

OBSS, a Radio Resources Management Problem July 2009 OBSS, a Radio Resources Management Problem Co-channel interference B C A B C C A B B C Channel reuse locations without interference A B A Assignment of 3 channels B C B But, too few channels & the ad-hoc placement of Access Points Independent operators Independent operators cause interference among co-channel cells -> Overlapping BSS (OBSS) M. Benveniste (En-aerion)

‘Neighborhood Capture’ July 2009 OBSS + CSMA = 4 5 6 3 2 1 a b 9 7 8 c Cell a: stations 1, 2 and 3 Cell b: stations 4, 5 and 6 Cell c: stations 7, 8 and 9 Co-channel group (use same channel) Reuse group: Cells a & c (no interference between them) Stations in a & c do not hear each other’s transmissions Stations in b hear transmissions from a and c Even though HCCA may be used within the cell, the HCs (APs) access the channel using CSMA/CA ‘Neighborhood Capture’ M. Benveniste (En-aerion)

‘Neighborhood Capture’ Defining ‘Neighborhood Capture’ July 2009 Impact of OBSS on CSMA Our example: Stations in cells a & c can transmit or count down their backoff timers concurrently Channel access & backoff countdown freeze in cell b during transmissions in cells a or c Time Cell a Cell b Cell c Cells a and c ‘capture’ the channel NEIGHBORHOOD CAPTURE: mutually non-interfering co-channel cells deprive co-channel neighbors of access M. Benveniste (En-aerion)

The result of Neighborhood Capture is deleterious to QoS Impact on QoS July 2009 5 Balancing loads across cells maximum synergy among cells in same reuse group worsens capture effect Balancing loads across reuse groups fairer access but unstable channel use increased delay and jitter a 2 b 3 4 6 1 11 7 d 12 c 8 9 10 2 reuse groups Co-channel group: Cell a, b, c and d Reuse group 1: Cells a and c Reuse group 2: Cells b and d The result of Neighborhood Capture is deleterious to QoS M. Benveniste (En-aerion)

All cells have equal access to the channel July 2009 Our proposed remedy Global Channel Release (GCR) The channel must be free at pre-specified times -TBPend All station clocks are synchronized All cells have equal access to the channel Cell a Cell b Cell c Time TBPend TBPend TBPend TBPend TBPend TBPend TBPend Slotting the channel into super-frames helps with periodic traffic M. Benveniste (En-aerion)

Slotted Channel Super-frame Structure July 2009 Slotted Channel Super-frame Structure Channel time is divided into super-frames Transmissions may not straddle the super-frame boundary BP= BUSY PERIOD a contention-free frame sequence (e.g. (E)DCF Tx, CFB, CFP) AIFS BCKOFF Max BPLength Foreshortened BP BP BP BP BP TBPend TBPend SUPERFRAME SUPERFRAME The channel is released at the end of each super-frame M. Benveniste (En-aerion)

Synchronization proposal July 2009 Synchronization proposal Generalization of IEEE 802.11 functions Stations synchronize with their AP’s time stamped beacons Neighbouring co-channel APs synchronize through frames sent by stations in the overlapping coverage area of two cells Co-channel APs synchronize with the Timing Synchronization Function – used by IBSS stations now AP1 Clock time 10 Synchronization Process APs exchange time-stamped frames If received stamp time exceeds own clock time, adjust clock Initialize clock to 0 when powering on AP2 Clock time 0 10 AP3 Clock time 3 10 M. Benveniste (En-aerion)

July 2009 Conclusions Increasing user density and mobility will require contiguous coverage by WLAN cells Available radio resources are not adequate to avoid OBSS in such systems CSMA MAC protocols used with OBSS lead to ‘Neighborhood Capture’ - a serious problem for QoS Global Channel Release (GCR) can mitigate the capture effect Implementation of GCR possible through channel slotting and synchronization M. Benveniste (En-aerion)