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Fast L2 Handoff in IEEE 802.11a/b/g Networks

Introduction
With the growth of the IEEE 802.11-based wireless LANs, VoIP and similar applications are now being used over wireless networks too. Since the station is now mobile, it performs a handoff, whenever it moves out of the range of one Access Point (AP) and tries to connect to a different one. Handoff takes a few hundred milli-seconds, which causes interruptions in VoIP sessions.
We developed a new handoff algorithm which reduces the MAC layer handoff latency, in most cases, to a level where VoIP communication becomes seamless.
Handoff procedure
  • Discovery phase
    1. Using the normal channel access procedure, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), gain control of the wireless medium.
    2. Transmit a probe request frame which contains the broadcast address as destination.
    3. Start a probe timer.
    4. Listen for probe responses.
    5. If no response has been received by minChannelTime, scan next channel.
    6. If one or more responses are received by minChannel-Time, stop accepting probe responses at maxChannelTime and process all received responses.
    7. Move to next channel and repeat the above steps.
    After all channels have been scanned, all information received from probe responses are processed so that the STA can select which AP to join next.
  • Reauthentication phase
    The reauthentication process involves authentication and re-association to the new AP as well as transfer of the STA's credentials from the old AP to the new AP. Authentication is a process by which the AP either accepts or rejects the identity of the STA. The STA begins the process by sending the authentication frame, authentication request, informing the AP of its identity; the AP responds with an authentication response, indicating acceptance or rejection. After authentication is successful, the STA sends the reassociation frame, reassociation request, to the new AP which will then send a reassociation response, back to the STA, containing an acceptance or rejection notice.

Our approach (MOBIWAC'04)
As we show in our paper, the Probe Delay is the biggest delay in the whole process (more than 90% of the total handoff delay), therefore our approach focused on reducing such delay. To implement our new algorithm we modified the HostAP wireless driver. For more details on our algorithm please refer to our paper Reducing MAC Layer Handoff Latency in IEEE 802.11 Wireless LANs (pdf).
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