1. Cooperative AP Discovery
November 2011
doc.: IEEE /1414r2
December 2011
Cooperative AP Discovery
Date:
Authors:
Wang and Yee, MediaTek
Katsuo Yunoki, KDDI R&D Laboratories

2. November 2011
doc.: IEEE /1414r2
December 2011
Abstract
This document describes a technical proposal for TGai which addresses the following phase.
AP Discovery
Wang and Yee, MediaTek
Katsuo Yunoki, KDDI R&D Laboratories

3. Conformance w/ TGai PAR & 5C
November 2011
doc.: IEEE /1414r2
December 2011
Conformance w/ TGai PAR & 5C
Conformance Question
Response
Does the proposal degrade the security offered by Robust Security Network Association (RSNA) already defined in ? No
Does the proposal change the MAC SAP interface?
Does the proposal require or introduce a change to the architecture?
Does the proposal introduce a change in the channel access mechanism?
Does the proposal introduce a change in the PHY?
Which of the following link set-up phases is addressed by the proposal?
(1) AP Discovery (2) Network Discovery (3) Link (re-)establishment / exchange of security related messages (4) Higher layer aspects, e.g. IP address assignment 1
Wang and Yee, MediaTek
Katsuo Yunoki, KDDI R&D Laboratories

4. November 2011
doc.: IEEE /1414r2
December 2011
Background
We assume a scenario with multiple APs in a channel and a large number of STAs concurrently attempting to find an appropriate AP for association in that channel.
Other submissions have presented results on the amount of channel time needlessly spent on Probe Request / Response and waiting for Beacons (see /1413, for example.).
STAs are assumed to only support legacy protocols. Reduced FILS latency through modification of AP protocol behavior is the goal of this work.
Wang and Yee, MediaTek
Katsuo Yunoki, KDDI R&D Laboratories

5. December 2011
Observations
Scanning across multiple channels is very time consuming with STA implementation-specific delays. We assume the operating channel is fixed.
STAs typically first scan for Beacons from known APs. Some STAs perform Probe Request by default.
Operating independently, APs transmit Beacons at random time intervals. Most APs set their BI = 100ms.
Operating independently, APs will respond to a received Probe Request without considering if another AP is more suitable.
Wang and Yee, MediaTek

6. Coordinated AP Behavior
December 2011
Coordinated AP Behavior
If APs coordinate their Beacon intervals in order to uniformly distribute the beacon transmission times of all APs, the average beacon wait time of a STA can be reduced.
If APs exchange BSS load information and defer to other suitable AP with lighter load to respond to Probe Requests, channel time can be better utilized.
Wang and Yee, MediaTek

7. Proposed Protocol Mechanisms - Summary
December 2011
Proposed Protocol Mechanisms - Summary
Coordinated Beaconing among Neighboring APs
Goal: The APs of a Hot-Spot take turns to transmit beacon frames in a deterministic order
AP Self-Censorship based on system loading
Goal: Only one AP, the active responder, responds to broadcast probe request in order to avoid probe response storm.
Deferred Beacon Transmission for next scheduled beaconing AP
Goal: The active responder AP updates its loading information in the next scheduling time. The next scheduled beaconing AP defers transmitting of its beacon frame to avoid collision.
Wang and Yee, MediaTek

8. Proposed Protocol Mechanisms (1)
December 2011
Proposed Protocol Mechanisms (1)
Coordinated Beaconing among Neighboring APs
Option 1: re-use MBSS mechanisms such as Mesh Beacon Collision Avoidance (MBCA).
Option 2: a distributed beaconing synchronization algorithm (No need to define new protocol element, only new behavior. May re-use some of the MBSS protocol element)
Wang and Yee, MediaTek

9. Proposed Protocol Mechanisms (2)
December 2011
Proposed Protocol Mechanisms (2)
AP self-censorship by using system loading
Every AP sends its loading information by carrying BSS Load and multiple BSSID information element in beacon frames
All AP learns the system loading of other APs
Only the AP with the lightest load, active responder AP, responds to broadcast probe request. Other APs remain silent.
The active responder AP will be the one responding to probe requests till another AP resumes the role of active responder.
(No need to define new protocol element, need to define new behavior)
Wang and Yee, MediaTek

10. Proposed Protocol Mechanisms (3)
December 2011
Proposed Protocol Mechanisms (3)
Deferred Beacon Transmission
The active responder sends a beacon frame at the next scheduled beaconing time
The scheduled beaconing AP defers transmitting its beacon frame by using random backoff.
If no beacon frame from active responder AP is received at the scheduled beaconing time, the scheduled beaconing AP transmits a beacon frame after backoff timer expired (No need to define new protocol element if using the new distributed beaconing algorithm. May need new protocol element if using MBSS protocol )
Wang and Yee, MediaTek

11. December 2011
FILS latency reduction and efficiency improvement using these mechanisms
Assuming there are n APs with BI= 100 ms in a channel
Baseline Link Setup Delay
Passive scanning: Worst case delay is 100ms regardless of n.
Active scanning:
Average delay = (Probe_Req Tx Time)+ n*((ProbeResp Tx time)/2 + AP system and channel access delay)
Expected FILS latency
Beacon coordination time
A generic distributed algorithm should complete one-time beacon coordination log (n) BIs,
Scanning time
Passive scanning: A STA will receive a beacon frame in 100/n ms
Active scanning: Avoid unicast probe responses storm
Delay after Load Balancing = (Probe_Req Tx Time)+(ProbeResp Tx time) + AP system and reduced channel access delay
Load balancing overhead
Periodic BSS Load update time
Wang and Yee, MediaTek

12. Further Improvement of these mechanisms
December 2011
Further Improvement of these mechanisms
Information which will further improve the FILS performance
Beacon Transmit Power
The receiving STA can check SNR against Tx power to estimate the channel
Neighbor Report IE
Additional information complicates the algorithm for selecting the best AP. The cost and effect should be evaluated carefully
Wang and Yee, MediaTek

13. Simulation Results - Work in Progress December 2011
Wang and Yee, MediaTek

14. December 2011
Motions
Motion 1: Move to add the following into section 5 “Fast Network Discovery” of the TGai spec framework document:
5.x AP Discovery
A mechanism which reduces AP Discovery latency via changes only in AP protocol behavior instead of STA protocol behavior shall be defined.
Wang and Yee, MediaTek

15. December 2011
Motions (Continued)
Motion 2: Move to add the following into section 5 “Fast Network Discovery” of the TGai spec framework document:
Insert under 5.x AP Discovery:
5.x.x Beacon Coordination
In the presence of overlapping BSS, the transmission of beacons shall be coordinated among neighboring APs in order to reduce the beacon wait time.
Wang and Yee, MediaTek

16. December 2011
Motions (Continued)
Motion 3: Move to insert the following into section 5 “Fast Network Discovery” of the TGai spec framework document:
Insert under 5.x AP Discovery:
5.x.x Probe Response Latency Management
In the presence of overlapping BSS, in order to reduce delay due to unnecessary Probe Response frames, neighboring APs shall coordinate to select a suitable AP for responding to Probe Request frames.
Wang and Yee, MediaTek