Neighborhood Capture and OBSS July 2009 Neighborhood Capture and OBSS Date: 2009-07-31 Authors: Name Address Company Phone Email Mathilde Benveniste South Orange, NJ 07079 En-aerion 973-761-6105 benveniste@ieee.org M. Benveniste (En-aerion)

July 2009 Neighborhood Capture and OBSS Mathilde Benveniste, Ph.D. benveniste@ieee.org Based on publication “Wireless LANs and ‘Neighborhood Capture’”, M. Benveniste, Proceedings PIMRC 2002, Lisboa, Portugal, September 2002 [http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.11.5392 ] 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)

Conclusions Mobility requires contiguous coverage by WLAN cells July 2009 Conclusions Mobility requires contiguous coverage by WLAN cells Increasing coverage or user density leads to OBSS Available radio resources are not adequate to avoid OBSS CSMA MAC protocols used in 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)