802.11 Discovery Phase and Passive Scanning Mode

1. 802.11 Discovery Phase and Passive Scanning Mode Marian Rudolf, Juan-Carlos Zuniga InterDigital Marian.Rudolf@InterDigital.com JuanCarlos.Zuniga@InterDigital.com Marian Rudolf, Juan-Carlos Zuniga, InterDigital

2. Two distinct phases of fast roaming • Discovery phase • Involves scanning from the STAs • The STAs build a database of which AP are in the vicinity • Within these, STA identifies most suitables for eventual handoff • Handoff phase • also referred to as transition or commit phase • Reassociation of the STA towards new AP is triggered • Service transition from old AP to new AP • This contribution focuses on the first one: The Discovery phase Marian Rudolf, Juan-Carlos Zuniga, InterDigital

3. Tighter requirements on Discovery • With increasing demand for support of delay-sensitive applications, roaming has to be fast and efficient • The STA can no longer wait until the service quality degrades to start scanning for handoff candidates. • The STA has to know at all times what AP to handoff to in case service quality would start degrading or link with current AP would be interrupted. • At this point it is probably too late for discovery • The discovery phase has to: • be on-going at all times • keep a close-to real-time database of the candidates Marian Rudolf, Juan-Carlos Zuniga, InterDigital

4. Scanning: a key component of Discovery • Passive scanning Pros • No added traffic/load on Radio Network • Battery savings Cons • Slow – each channel typically requires > 100ms • These delays are not tolerable for delay-sensitive applications. • Active scanning Pros • Faster than passive scanning Cons • Large number of STA and APs and need to continuous scanning (delay-sensitive applications) will lead to Probe requests/responses pollution  congestion • Forces APs to send information redundant to Beacon channel Marian Rudolf, Juan-Carlos Zuniga, InterDigital

5. How can we make discovery faster? • Serving AP to send candidates lists to STAs • This limits the span of the scan STAs have to perform • This relieves the STA for scanning empty channels • Attributes have been proposed already (e.g. load, capabilies, security, etc.) • Helps both passive and active scanning • Candidate lists do not relieve STAs from the need to scan • Some statistics have have to be perceived by the STA itself (e.g. SNR, etc.) • Candidate lists have been proposed to limit number of scans • They could also be used to increase the speed of each scan • Passive scans are typically slow (>100ms) because the STA does not know when beacon is sent, has to wait the whole beacon interv. • If the STA knew, even coarsely, the timing of candidates’ beacon scan time would be dramatically reduced from >100ms to few ms. • This could be easily achieved simply by adding beacons’ timing to candidate list info. Marian Rudolf, Juan-Carlos Zuniga, InterDigital

6. Benefits from providing timing of beacons to STA • Satisfies need for on-going and continuous scanning without • Probe requests/responses pollution • Sacrificing capacity and system performance from added load • forcing APs to send redundant information • Dramatically reduces passive scanning time from >100ms per channel to few ms • Battery savings • Compatible with pre-802.11r WLAN nodes Marian Rudolf, Juan-Carlos Zuniga, InterDigital

7. Timing diagram for scanning one candidate AP Marian Rudolf, Juan-Carlos Zuniga, InterDigital

8. How can the timing info be obtained? • From Inter-AP communication • STAs reporting “scan reports” to their AP • AP listening to neighboring AP’s beacons • O&M Marian Rudolf, Juan-Carlos Zuniga, InterDigital