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Fast Active Scan Proposals. Moo Ryong Jeong, Fujio Watanabe, Toshiro Kawahara DoCoMo USA Labs mrj@ieee.org Zhun Zhong Philips zhun.zhong@philips.com. Background. 03/416r0 presented an active scanning scheme having short scanning time*. *: Excludes time for sending probe request

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fast active scan proposals

Fast Active Scan Proposals

Moo Ryong Jeong, Fujio Watanabe, Toshiro Kawahara

DoCoMo USA Labs

mrj@ieee.org

Zhun Zhong

Philips

zhun.zhong@philips.com

Moo Ryong Jeong, DoCoMo USA Labs.

background
Background
  • 03/416r0 presented an active scanning scheme having short scanning time*

*: Excludes time for sending probe request

†: Varies depending on the size of SSID field and Supported Rates field

  • The cost of the short scanning time is increased implementation complexity

Moo Ryong Jeong, DoCoMo USA Labs.

goal of this presentation
Goal of This Presentation
  • This presentation provides:
    • Three schemes (modification to 03/416r0) that make trade-off between scanning time and implementation complexity at different levels

Moo Ryong Jeong, DoCoMo USA Labs.

current active scanning procedure
Current Active Scanning Procedure
  • Use of DCF in probe response transmission is source of time-consumption
  • DCF is used in current standard on assumption that
    • STA has no knowledge about neighbor APs
    • STA wants to obtain information on all neighbor APs
    • Probe request is broadcast so that any AP can hear and respond by sending probe response

Moo Ryong Jeong, DoCoMo USA Labs.

assumptions changed
Assumptions Changed
  • STA may have knowledge on possible neighbor APs
    • Site report, previous scanning results
  • STA may be interested only in specific APs
    • APs chosen from site report or previous scanning results
  • Probe request can be a directed frame (by standard modification) to a desired AP so that the AP can hear and respond within SIFS
  • We present fast active scanning methods which fits with changed assumptions

Moo Ryong Jeong, DoCoMo USA Labs.

basic ideas of fast active scan
Basic Ideas of Fast Active Scan
  • Probe request is sent as directed frame to a specific AP on the AP’s operation channel with BSSID in DA field and BSSID field
  • AP responds to directed probe request after SIFS
    • Response may be a probe response or an ACK
    • If the response is an ACK, AP sends probe response later
  • STA can learn availability/unavailability of AP after SIFS
  • If STA receives ACK, STA stay on the channel unlit it receives probe response

Moo Ryong Jeong, DoCoMo USA Labs.

original scheme 03 416r0
Original scheme (03/416r0)
  • AP sends probe response with broadcast destination address after SIFS deferral
    • Probe response is fast transmitted with high probability of success
  • Probe response considered as ACK to probe request
  • No retransmission of probe response

MinChannelTime

Scanning

Station

Probe to APi

APi

P Response

SIFS

Moo Ryong Jeong, DoCoMo USA Labs.

original scheme 03 416r08
Original scheme (03/416r0)
  • Shortest scanning time
    • ≈ 10 sec when AP is unavailable
    • SIFS plus transmission time of probe response ( 898 sec) when AP is avaiable
  • Most demanding in implementation

Moo Ryong Jeong, DoCoMo USA Labs.

alternative scheme 1
Alternative Scheme 1
  • AP sends ACK after SIFS period
  • When probe response is ready, send it with DIFS deferral (and random backoff)
  • Probe response is retransmitted unless ACK is received

MaxChannelTime

MinChannelTime

Scanning

Station

Probe to APi

Ack

APi

P Response

Ack

Other frames

SIFS

SIFS

DIFS

Random Backoff

Moo Ryong Jeong, DoCoMo USA Labs.

alternative scheme1
Alternative Scheme1
  • Scanning time
    • For unavailable AP, as short as in 03/416r0
    • For available AP, longer than 03/416r0 but shorter than conventional active scanning (collocated APs do not respond to probe request)
  • Least demanding in implementation

Moo Ryong Jeong, DoCoMo USA Labs.

alternative scheme 2
Alternative Scheme 2
  • AP sends ACK after SIFS period
  • When probe response is ready, send it with PIFS deferral
    • Facilitates fast and safe transmission of probe response
  • Probe response is retransmitted with DIFS and random backoff unless ACK is received

MaxChannelTime

MinChannelTime

Scanning

Station

Probe to APi

Ack

APi

P Response

Other frames

Ack

SIFS

SIFS

PIFS

Moo Ryong Jeong, DoCoMo USA Labs.

alternative scheme 212
Alternative Scheme 2
  • Scanning time
    • For unavailable AP, as short as in 03/416r0
    • For available AP, longer than 03/416r0 but shorter than alternative scheme 1 (priority access of probe response)
  • Less demanding than 03/416r0 but more demanding than alternative scheme 1

Moo Ryong Jeong, DoCoMo USA Labs.

knowledge on neighbor ap
Knowledge on Neighbor AP
  • Collection of neighbor AP information
    • Offline collection by operator
    • Realtime collection by AP
      • Reassociation message (TGf)
      • Beacon report from associated STAs (TGk)
      • Exchange neighbor AP information among access point
    • Realtime collection by STA
      • Results from previous scanning (passive or conventional active)
  • Delivery of neighbor AP information
    • Initial setup
    • Site Report request
    • Beacon
    • Probe response

Moo Ryong Jeong, DoCoMo USA Labs.

indicating probe request with fast active scan
Indicating Probe Request with Fast Active Scan
  • Use current probe request
    • AP’s MAC address in DA and BSSID fields in probe request as indication of fast active scan
  • Define new management frame subtype for probe request with fast active scan
  • Define new action category for probe request with fast active scan

Moo Ryong Jeong, DoCoMo USA Labs.

indicating fast active scanning capability of ap
Indicating Fast Active Scanning Capability of AP
  • Require all STAs capable of radio measurement be also capable of fast active scanning
    • Set B12 (radio measurement) in capability information field
    • Non-AP STA shall be compatible with all schemes
    • AP shall implement at least one of the three schemes
      • Implementation changes according to the fast active scanning needs
  • Define new bit in capability information field to indicate fast active scanning capability

Moo Ryong Jeong, DoCoMo USA Labs.

modification required
Modification Required
  • Addressing
    • Directed frame for probe request (all schemes)
    • Broadcast frame for probe response (03/416r0)
  • IFS
    • SIFS between probe request and probe response (03/416r0)
    • PIFS deferral in sending probe response (alternative 2)
  • ACK
    • Immediate probe response considered as ACK to probe request (03/416r0)
  • MaxChannelTime
    • Ignored if probe response is received

Moo Ryong Jeong, DoCoMo USA Labs.

conclusion
Conclusion
  • We considered improvement of active scanning
  • Our schemes speed up active scanning process when there is prior knowledge on neighbor APs (e.g. site report)
  • Proposed active scanning schemes are effective for:
    • Reducing overhead in active scan-based measurement
    • Fast handoff
  • AP implements one of the three schemes according to the fast active scanning needs

Moo Ryong Jeong, DoCoMo USA Labs.