1. Neighborhood Capture and OBSS
July 2009
Neighborhood Capture and OBSS
Date:
Authors:
Name
Address
Company
Phone
Mathilde Benveniste
South Orange, NJ 07079
En-aerion
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M. Benveniste (En-aerion)

2. Neighborhood Capture and OBSS Mathilde Benveniste benveniste@ieee.org
July 2009
Neighborhood Capture and OBSS Mathilde Benveniste
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 /596, November 2001
M. Benveniste (En-aerion)

3. Outline Some Background: IEEE 802.11 MAC Protocols
July 2009
Outline
Some Background: IEEE 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)

4. 802.11 channel access mechanism, DCF
July 2009
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)

5. 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)

6. Higher LAN densities & mobility
July 2009
Higher LAN densities & mobility C
IEEE 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)

7. 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)

8. ‘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)

9. ‘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)

10. 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)

11. 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)

12. 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)

13. Synchronization proposal
July 2009
Synchronization proposal
Generalization of IEEE 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)

14. 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)