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					July 2009                                         doc.: IEEE 802.11-09/0844r1


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




Submission                              Slide 1                  M. Benveniste (En-aerion)
July 2009                                               doc.: IEEE 802.11-09/0844r1




               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


Submission                               Slide 2                      M. Benveniste (En-aerion)
July 2009                                     doc.: IEEE 802.11-09/0844r1


                                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




Submission                          Slide 3               M. Benveniste (En-aerion)
July 2009                                                          doc.: IEEE 802.11-09/0844r1


 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                  Contention Window                 Next
                                     Medium                                                  Frame
                                                              Slot time
Immediate access when
 medium is free >= DIFS

                                     Defer Access           Decrement Backoff as
                                                            long as medium is idle

Submission                                        Slide 4                            M. Benveniste (En-aerion)
 July 2009                                                 doc.: IEEE 802.11-09/0844r1


  802.11e channel access mechanism - EDCF

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


 Submission                                Slide 5                         M. Benveniste (En-aerion)
  July 2009                                 doc.: IEEE 802.11-09/0844r1
                    A

                                   Higher LAN densities
      C                                 & mobility

                        B
                                              IEEE 802.11 b/a/g/e …

                                                       B

                                              A
                                                                       A
Higher LAN densities & mobility
                                                       C
                                               B

                                                                       B
                                                       A




Contiguous cells & Channel Reuse
  Channel must be allocated to BSSs as in cellular systems

  Submission                      Slide 6                  M. Benveniste (En-aerion)
   July 2009                                          doc.: IEEE 802.11-09/0844r1

                                                  OBSS, a Radio
Co-channel               B

interference
                             C                     Resources
                         A
                    B                          Management Problem
                C        C
                    A
                B        B
                    C
                A
                                 Channel reuse locations
                                  without interference
     Assignment of 3 channels                               B
                                                                        A

                                                                    B

                                                                C
                But, too few channels &                                         B



                    the ad-hoc placement
                        of Access Points                   Independent operators



   cause interference among co-channel cells -> Overlapping BSS (OBSS)
   Submission                              Slide 7                  M. Benveniste (En-aerion)
July 2009                                                      doc.: IEEE 802.11-09/0844r1


                                                           OBSS + CSMA =

                                         9
                                                           Co-channel group
                         5
                                     7                     (use same channel)
             a       2       b           c
         3
                             4   6           8
                                                      Cell a: stations 1, 2 and 3
                 1                                    Cell b: stations 4, 5 and 6
                                                      Cell c: stations 7, 8 and 9



        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’
Submission                                       Slide 8                   M. Benveniste (En-aerion)
    July 2009                                      doc.: IEEE 802.11-09/0844r1
                             Defining
                      ‘Neighborhood Capture’
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


 Cell a                                                                  Cells a and
                                                                          c ‘capture’
Cell b                                                                   the channel
 Cell c

                                                                      Time



   NEIGHBORHOOD CAPTURE: mutually non-interfering co-channel
                                  cells deprive co-channel neighbors of access
    Submission                          Slide 9                M. Benveniste (En-aerion)
 July 2009                                               doc.: IEEE 802.11-09/0844r1


                                             Impact on QoS

                   a   2
                            5               Balancing loads across cells
                                    b
         3                      4       6   • maximum synergy among cells in same
                   1                           reuse group
              11                7
                                            • worsens capture effect
               d       12
                                c           Balancing loads across reuse groups
                            9           8
          10                                • fairer access but unstable channel use
                                            • increased delay and jitter

        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
 Submission                                  Slide 10                M. Benveniste (En-aerion)
   July 2009                                                        doc.: IEEE 802.11-09/0844r1

                              Our proposed remedy

 Global Channel Release (GCR)
                       The channel must be free at pre-specified times -TBPend
                                            All station clocks are synchronized


Cell a                                                                                         All cells
                                                                                             have equal
Cell b
                                                                                              access to
Cell c                                                                                      the channel

                                                                                         Time
                    TBPend   TBPend   TBPend   TBPend      TBPend   TBPend   TBPend




                Slotting the channel into super-frames helps with periodic traffic
   Submission                                   Slide 11                          M. Benveniste (En-aerion)
 July 2009                                                      doc.: IEEE 802.11-09/0844r1

                              Slotted Channel

Super-frame Structure
                                   Channel time is divided into super-frames
                    Transmissions may not straddle the super-frame boundary

                                                                BP= BUSY PERIOD
     AIFS BCKOFF             Max BPLength
                                                                  a contention-free
                                                                   frame sequence
                                                            (e.g. (E)DCF Tx, CFB, CFP)
                            Foreshortened BP

     BP        BP      BP          BP



      TBPend                                           TBPend



                     SUPERFRAME                                     SUPERFRAME


                     The channel is released at the end of each super-frame
 Submission                                 Slide 12                        M. Benveniste (En-aerion)
 July 2009                                                doc.: IEEE 802.11-09/0844r1

                 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

              Clock time 10 AP1
                                                                 Synchronization Process
                                                   APs exchange time-stamped frames
 Clock time 0 AP2     10                           If received stamp time exceeds own
                                                                  clock time, adjust clock
                                                  Initialize clock to 0 when powering on
              Clock time 3 AP3    10


 Submission                            Slide 13                        M. Benveniste (En-aerion)
July 2009                              doc.: IEEE 802.11-09/0844r1

                      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




Submission                  Slide 14               M. Benveniste (En-aerion)

				
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