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AP Scanning

AP Scanning. Fujio Watanabe, Moo Ryong Jeong, Toshiro Kawahara DoCoMo USA Labs fwatanabe@ieee.org. Background. Channel scanning is a time-consuming process Passive scan: several beacon intervals per channel (e.g., >100ms) Active scan: 15 msec per channel

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AP Scanning

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  1. AP Scanning Fujio Watanabe, Moo Ryong Jeong, Toshiro Kawahara DoCoMo USA Labs fwatanabe@ieee.org Fujio Watanabe, DoCoMo USA Labs

  2. 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) Presence Server Business Application Server Intra Server AS SIP Server PSTN Router AP AP Fixed Phone Fast Roaming Reference: http://www.nttdocomo.co.jp/new/contents/04/whatnew0713-1.html Fujio Watanabe, DoCoMo USA Labs

  4. 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 802.11-04/0748r1 Fujio Watanabe, DoCoMo USA Labs

  5. 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. 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 • 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

  8. 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 Fujio Watanabe, DoCoMo USA Labs

  9. 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 • 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 Fujio Watanabe, DoCoMo USA Labs

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

  12. 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. 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

  14. Reference [1] M. Jeong et al, “Fast Active Scan for Measurement and Handoff,” IEEE 802.11-03/416r0 [2] M. Jeong et al, “Fast Active Scan Proposal,” IEEE 802.11-03/623r0 [3] J. Spilman et al, “Test Methodology for Measuring BSS Transition Time,” IEEE 802.11-04/0748r1 [4] M. Rudolf et al, “802.11 Discovery Phase and Passive Scanning Mode,” IEEE 802.11-04/0718r0 [5] E. Hepworth et al, “IETF Network Discovery and Selection Overview,” IEEE 802.11-04/1020r0 [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

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