A Pre-Scanning Fast Handoff Scheme for VoIP in WLANs

IEEE 802.11 wireless local area networks (WLAN) have been widely deployed, and many applications are implemented on WLAN. As one of the interesting wireless services, wireless voice over Internet protocol (VoIP) can provide low-cost real-time voice service to mobile users. But the IEEE 802.11 wireless network cannot satisfy the QoS of VoIP. The delay during handoff process can last up to I second. Applications with delay restraint like VoIP will drop the call when the delay is more than 50 ms. Therefore this paper analyzed the delay made by the handoff process, and proposed a pre-scanning fast handoff scheme. This fast handoff scheme is based on selective scanning and caching. It can predict the next AP for STAs and use pre-scanning before the handoff is triggered. Simulations on OPNET show this proposed scheme can reduce the average handoff delay to 23.45 ms and avoid some unnecessary handoffs. The pre-scanning fast handoff scheme is feasible and economical according to its performance in the handoff process.