1. Fast Active Scan Proposals
May 2003
doc.: IEEE /xxxr0
July 2003
Fast Active Scan Proposals
Moo Ryong Jeong, Fujio Watanabe, Toshiro Kawahara
DoCoMo USA Labs
Zhun Zhong
Philips
Moo Ryong Jeong, DoCoMo USA Labs.
Moo Ryong Jeong, DoCoMo USA Labs.

2. May 2003
doc.: IEEE /xxxr0
July 2003
Background
03/416r0 presented an active scanning scheme having short scanning time*
Conventional active scan
Fast active scan
Long preamble 1 Mbps
Short preamble 2 Mbps
When AP is unavailable
MinChannelTime ( 670 sec)
MinChannelTime ( 10 sec)
When AP is available
MaxChannelTime ( 15 msec)
SIFS + time for sending probe response
(= 586 ~ 898 sec) †
(= 298 ~ 454 sec) †
*: 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.
Moo Ryong Jeong, DoCoMo USA Labs.

3. Goal of This Presentation
July 2003
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.

4. Current Active Scanning Procedure
July 2003
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.

5. Assumptions Changed STA may have knowledge on possible neighbor APs
July 2003
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.

6. Basic Ideas of Fast Active Scan
May 2003
doc.: IEEE /xxxr0
July 2003
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
Or PIFS
Moo Ryong Jeong, DoCoMo USA Labs.
Moo Ryong Jeong, DoCoMo USA Labs.

7. May 2003
doc.: IEEE /xxxr0
July 2003
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
Or PIFS
Scanning
Station
Probe to APi
APi
P Response
SIFS
Moo Ryong Jeong, DoCoMo USA Labs.
Moo Ryong Jeong, DoCoMo USA Labs.

8. Original scheme (03/416r0) Shortest scanning time
May 2003
doc.: IEEE /xxxr0
July 2003
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
Or PIFS
Moo Ryong Jeong, DoCoMo USA Labs.
Moo Ryong Jeong, DoCoMo USA Labs.

9. Alternative Scheme 1 AP sends ACK after SIFS period
July 2003
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
Ack
Other frames
P Response
SIFS
SIFS
DIFS
Random Backoff
Moo Ryong Jeong, DoCoMo USA Labs.

10. Alternative Scheme1 Scanning time Least demanding in implementation
July 2003
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.

11. Alternative Scheme 2 AP sends ACK after SIFS period
July 2003
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
Ack
Other frames
P Response
SIFS
SIFS
PIFS
Moo Ryong Jeong, DoCoMo USA Labs.

12. Alternative Scheme 2 Scanning time
July 2003
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.

13. Knowledge on Neighbor AP
July 2003
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.

14. Indicating Probe Request with Fast Active Scan
July 2003
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.

15. Indicating Fast Active Scanning Capability of AP
July 2003
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.

16. Modification Required
July 2003
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.

17. Conclusion We considered improvement of active scanning
May 2003
doc.: IEEE /xxxr0
July 2003
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.
Moo Ryong Jeong, DoCoMo USA Labs.