1. AP Scanning Date: 2004-12-18 Authors: January 2005 January 2005
doc.: IEEE yy/xxxxr0
January 2005
AP Scanning
Date:
Authors:
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F.Watanabe et al, DoCoMo USA Labs
F.Watanabe et al, DoCoMo USA Labs

2. Background Channel Scanning is a time-consuming process
January 2005
Background
Channel Scanning is a time-consuming process
Passive scan: several beacon intervals per channel (e.g., >100ms)
Active scan: 15 msec per channel
Direct Probe Request is defined or not defined?
M. Lefkowitz, “Directed probe request clarification,” IEEE /833r0
F.Watanabe et al, DoCoMo USA Labs

3. Channel Scanning Time Scanning time represents
January 2005
doc.: IEEE yy/xxxxr0
January 2005
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
F.Watanabe et al, DoCoMo USA Labs
F.Watanabe et al, DoCoMo USA Labs

4. SSID Overlapping SSID can be set to any value SSID:
January 2005
SSID Overlapping
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
F.Watanabe et al, DoCoMo USA Labs

5. Critical for Voice Applications
January 2005
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
F.Watanabe et al, DoCoMo USA Labs

6. Unicast Probe Request January 2005
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
F.Watanabe et al, DoCoMo USA Labs

7. January 2005
Proposed Changes
Allow STA to send directed probe request during the handoff process only.
AP shall reply with ACK and/or send the probe response later, or if capable, may reply with probe response within SIFS (similar to PS-Poll)
delete
F.Watanabe et al, DoCoMo USA Labs

8. Advantages Short Scanning Time Filter out APs with same SSID
January 2005
Advantages
Short Scanning Time
Filter out APs with same SSID
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
F.Watanabe et al, DoCoMo USA Labs

9. Alternative Scheme 1 January 2005 AP sends ACK after SIFS period Ack
MinChannelTime
Scanning
Station
Probe to APi
APi
Ack
SIFS
F.Watanabe et al, DoCoMo USA Labs

10. Alternative Scheme 2 January 2005 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
F.Watanabe et al, DoCoMo USA Labs

11. January 2005
doc.: IEEE yy/xxxxr0
January 2005
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)
F.Watanabe et al, DoCoMo USA Labs
F.Watanabe et al, DoCoMo USA Labs

12. January 2005
Reference
[1] M. Lefkowitz, “Directed probe request clarification,” IEEE /833r0
[2] M. Jeong et al, “Fast Active Scan for Measurement and Handoff,” IEEE /416r0
[3] M. Jeong et al, “Fast Active Scan Proposal,” IEEE /623r0
[4] J. Spilman et al, “Test Methodology for Measuring BSS Transition Time,” IEEE /0748r1
[5] M. Rudolf et al, “ Discovery Phase and Passive Scanning Mode,” IEEE /0718r0
[6] E. Hepworth et al, “IETF Network Discovery and Selection Overview,” IEEE /1020r0
[7] J. Arkko et al, “Network Discovery and Selection Problem,” draft-ietf-eap-netsel-problem-01.txt
[8] F. Adrangi et al, “Identity Selection Hits for Extensible Authentication Protocol (EAP),” draft-adrangi-eap-network-discovery-03.txt
F.Watanabe et al, DoCoMo USA Labs