1. Fujio Watanabe, Moo Ryong Jeong, Toshiro Kawahara
Month 2002
doc.: IEEE /xxxr0
November 2004
AP Scanning
Fujio Watanabe, Moo Ryong Jeong, Toshiro Kawahara
DoCoMo USA Labs
Fujio Watanabe, DoCoMo USA Labs
John Doe, His Company

2. Background Channel scanning is a time-consuming process
November 2004
Background
Channel scanning is a time-consuming process
Passive scan: several beacon intervals per channel (e.g., >100ms)
Active scan: 15 msec per channel
Channel scanning scheme is within/without scope of TGr?
VoIP scenario
Measurement Results
SSID Overlap
Related Documents
Fast Active Scan for Measurement and Handoff (03/416r0)
Fast Active Scan Proposal (03/623r0, 03/623r2)
Fujio Watanabe, DoCoMo USA Labs

3. VoIP Scenario (FOMA/WLAN)
November 2004
VoIP Scenario (FOMA/WLAN)
Presence
Server
Intra
Server
Business
Application Server AS
SIP Server
PSTN
Router AP AP
Fixed
Phone
Fast Roaming
Reference:
Fujio Watanabe, DoCoMo USA Labs

4. Channel Scanning Time Scanning time represents
Month 2002
doc.: IEEE /xxxr0
November 2004
Channel Scanning Time
Scanning time represents
majority of Transition Process
Time
Reference : Slide 13.
[3] J. Spilman et al, “Test Methodology for Measuring BSS Transition Time,” IEEE /0748r1
Fujio Watanabe, DoCoMo USA Labs
John Doe, His Company

5. SSID Overlap SSID can be set to any value SSID:
November 2004
SSID Overlap
SSID can be set to any value
Non-Intentional duplication
Identical name can be used accidentally by multiple parties (e.g., same office people)
Intentional duplication
Malicious AP copies SSID as well as BSSID
SSID:
Can be used to distinguish between networks
Does not insure accessibility to network
Fujio Watanabe, DoCoMo USA Labs

6. November 2004
Problems
Time consuming scanning results in large measurement overhead and potential handoff interruption.
Miss-selection of AP (same SSID) causes potential handoff interruption.
Critical for Voice Applications
Fujio Watanabe, DoCoMo USA Labs

7. Current Active Scanning Procedure
Month 2002
doc.: IEEE /xxxr0
November 2004
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 (SSID overlap) can hear and respond by sending probe response
Fujio Watanabe, DoCoMo USA Labs
John Doe, His Company

8. Unicast Probe Request Prior knowledge on neighbor APs assumed
Month 2002
doc.: IEEE /xxxr0
November 2004
Unicast Probe Request
Prior knowledge on neighbor APs assumed
Probe request is sent as a directed frame to a specific AP on the AP’s operation channel with BSSID in DA field and BSSID field
AP responds to the directed probe request after SIFS
Response shall be an ACK frame
AP sends the probe response later
Probe response may be omitted if requested in the probe request
STA can learn availability/unavailability of the desired AP after SIFS
When STA receives ACK, the desired AP is available
If probe response is not requested, STA may leave the channel since the scanning for the AP is done
If probe response is requested, STA stay on the channel unlit it receives the probe response from the AP
When STA does not receive ACK, the desired AP is not available
Or PIFS
Fujio Watanabe, DoCoMo USA Labs
John Doe, His Company

9. Advantages Short Scanning Time
November 2004
Advantages
Short Scanning Time
Filter out APs with the same SSID (non-intentional)
Fujio Watanabe, DoCoMo USA Labs

10. Alternative Scheme 1 AP sends ACK after SIFS period
November 2004
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
Fujio Watanabe, DoCoMo USA Labs

11. Alternative Scheme 2 AP sends ACK after SIFS period Ack Scanning
November 2004
Alternative Scheme 2
AP sends ACK after SIFS period
MinChannelTime
Scanning
Station
Probe to APi
APi
Ack
SIFS
Fujio Watanabe, DoCoMo USA Labs

12. November 2004
Proposed changes
Allow STA to send directed probe request (unicast probe request frame)
AP shall reply with ACK and may send the probe response later
Fujio Watanabe, DoCoMo USA Labs

13. Month 2002
doc.: IEEE /xxxr0
November 2004
Conclusion
We considered improvement of active scanning for real time application (e.g., VoIP)
Our schemes accelerate active scanning process when there is prior knowledge on neighbor APs information (defined TGk)
Proposed active scanning schemes are effective for:
Reducing handoff time
Filter out APs with the same SSID (non-intentional)
Fujio Watanabe, DoCoMo USA Labs
John Doe, His Company

14. November 2004
Reference
[1] M. Jeong et al, “Fast Active Scan for Measurement and Handoff,” IEEE /416r0
[2] M. Jeong et al, “Fast Active Scan Proposal,” IEEE /623r0
[3] J. Spilman et al, “Test Methodology for Measuring BSS Transition Time,” IEEE /0748r1
[4] M. Rudolf et al, “ Discovery Phase and Passive Scanning Mode,” IEEE /0718r0
[5] E. Hepworth et al, “IETF Network Discovery and Selection Overview,” IEEE /1020r0
[6] J. Arkko et al, “Network Discovery and Selection Problem,” draft-ietf-eap-netsel-problem-01.txt
[7] F. Adrangi et al, “Identity Selection Hits for Extensible Authentication Protocol (EAP),” draft-adrangi-eap-network-discovery-03.txt
Fujio Watanabe, DoCoMo USA Labs