Fast Active Scan for Measurement and Handoff

1
Fast Active Scan for Measurement and Handoff

• Moo Ryong Jeong, Fujio Watanabe,
• Toshiro Kawahara
• DoCoMo USA Labs
• mrj_at_ieee.org

2
Background

• Channel scanning is a time-consuming process
• Passive scan several beacon intervals per
  channel
• Active scan 15 msec per channel and 30 - 400
  msec in total reported in
• Arunesh Mishra, Minho Shin, William Arbaugh,
  An Empirical Analysis of the IEEE 802.11 MAC
  Layer Handoff Process, Submitted to ACM CCR
• Beacon request requires measurements on beacon or
  probe response obtained from scanning
• Scanning is used for handoff
• Time-consuming scanning results in large
  measurement overhead and long handoff
  interruption time

3
Goal of This Presentation

• This presentation provides
• Problem analysis in current standard on active
  scanning
• Improved active scanning proposal in terms of
  scanning time

4
Current Active Scanning Procedure

• Move to a channel to be examined
• Perform basic medium access procedure of DCF, and
  broadcast probe request frame
• Broadcast destination and broadcast BSSID
• If the medium was never busy for MinChannelTime,
  scan the next channel
• Otherwise, process any probe response frame
  received by MaxChannelTime, and scan the next
  channel

5
Why Time-consuming? (1/3)

• Sending probe request
• Time-consuming because medium access is based on
  DCF
• But, DCF must be used since scanning STA cannot
  be authorized for high priority medium access
• Receiving probe response
• Should wait for MinChannelTime or MaxChannelTime
• MinChannelTime and MaxChannelTime are long
• We want to improve this!!!

6
Why Time-consuming? (2/3)

• If MinChannelTime and MaxChannelTime are
  improperly set too short
• STA can miss probe responses
• Causes AP to retransmit probe response
• MinChannelTime should be sufficiently long to
  receive probe response deferred in maximum ( 670
  ?sec)
• MinChannelTime ? DIFS CW aSlotTime
• MaxChannelTime should be sufficiently long to
  receive (and send ACKs to) probe responses of as
  many APs as possible

7
Why Time-consuming? (3/3)

• 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
• These assumptions may not be true in many cases

8
Changed Assumptions

• STA may have knowledge on neighbor APs
• STA may be interested only in specific APs
• Probe request can be directed (by standard
  modification) so that only desired AP can hear
  and respond
• We provide novel fast active scanning method
  which best fits with changed assumptions

9
Fast Active Scan (1/5)

• Prerequisite
• Prior knowledge on APs to be examined
• Operation channels
• MAC address (BSSID)
• Capability of fast active scan
• Both AP and STA are capable of fast active scan
• STA can indicate in probe request that fast
  active scan is now used

10
Fast Active Scan (2/5)

• Basic idea
• Probe request is sent to a specific AP on its
  operation channel designating as the sole
  responder
• Designated AP sends probe response after SIFS
  deferral
• STA waits for probe response only for SIFS

11
Fast Active Scan (3/5)

• Move to the operation channel of AP to be
  examined
• Perform basic medium access procedure of DCF, and
  send probe request to specific AP designating as
  the sole responder
• Directed frame, or broadcast frame with MAC
  address of the AP in BSSID field
• If the medium was never busy for MinChannelTime,
  scan the next AP
• Otherwise, process any probe response frame
  received, and scan the next channel

12
Fast Active Scan (4/5)

• Sending probe response
• AP turns into fast active scanning mode after
  identifying it from probe request
• Probe request and probe response are tightly
  coupled so that no frame can intervene

• SIFS between probe request and response
• No random backoff
• AP does not expect to receive ACK for probe
  response

13
Fast Active Scan (5/5)

• STA can learn unavailability of AP when
  MinChannelTime passes with idle medium
• MinChannelTime can be a small value such as
  ACKTimeout ( 10 ?sec)
• It also can learn availability of AP and get
  measurement result after receiving probe response
• SIFS plus transmission time of probe response (?
  898 ?sec)
• STA does not send ACK to probe response

14
Advantages (1/2)

• Short scanning time
• Excludes time for sending probe request
• Varies depending on the size of SSID field and
  Supported Rates field

15
Advantages (2/2)

• Probe response is transmitted with high
  probability of success
• No need to send ACK
• No need to retransmit probe response

16
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
• On demand
• Beacon
• Probe response

17
Indicating Probe Request with Fast Active Scan

• Use current probe request
• Directed address or non-broadcast BSSID 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

18
Indicating Fast Active Scanning Capability of AP

• Define new bit in capability information field to
  indicate fast active scanning capability
• Require all AP capable of radio measurement be
  also capable of fast active scanning
• Set B12 (radio measurement) in capability
  information field

19
Modification Required

• Addressing
• Directed frame or specific BSSID for probe
  request
• IFS
• SIFS (or PIFS) between probe request and probe
  response
• ACK
• No ACK to probe response (by STA)
• No ACK expectation for probe response (by AP)
• No retransmission for unacknowledged probe
  response
• MaxChannelTime
• Ignored or not used
• New management subtype or new action category
• New management frame and new action frame should
  be classified into Class 1
• New capability information bit

20
Implication on Measurement

• Beacon request and report per AP basis
• Measurement only on the desired APs
• Even when they are per channel basis, measurement
  time is reduced
• Short interruption time
• Low overhead

21
Implication on Handoff

• Scanning per AP basis
• Scans only the desired APs (e.g. handoff
  candidates)
• Short interruption time
• Facilitates fast handoff

22
Conclusion

• We considered improvement of active scanning
• Our approach speeds up active scanning process
  when there is prior knowledge on neighbor APs
• Proposed active scanning method is effective for
• Reducing overhead in active scan-based
  measurement
• Fast handoff