Mobility management for VoIP in IEEE 802.11 based wireless LAN

In an IEEE 802.11 based wireless LAN, when a mobile station (MS) changes its point of attachment to the Internet, it may experience an interruption in service from when it disconnects from one access point (AP) until it registers with a new one. Such an interruption is unacceptable for a real-time service like voice-over-IP; further optimization of mobility management is needed. Several protocols have been proposed to enhance route optimization for intra-domain traffic in a wireless access IP network addressing the handoff issue; Cellular IP has shown good performance and is considered to be a solution for micro-mobility management in 802.11 WLAN. Two handoff schemes in Cellular IP, indirect handoff scheme (IHS) and hard handoff scheme (HHS), can be used in 802.11 WLAN. IHS can provide better performance than HHS on the premise that the MS can obtain the IP address of the new AP before it starts a handoff process. In the current 802.11 based WLAN, the MS cannot acquire this IP address until it has registered with the new AP. We propose an enhanced hard handoff scheme (EHHS) in Cellular IP for supporting VoIP in 802.11 WLAN. In this EHHS, a buffer is deployed in the current AP to achieve zero packet loss during the handoff process. The estimated packet delay introduced by the use of the buffers is analyzed mathematically. Optimization of the EHHS is discussed further. Simulation results show that the EHHS for Cellular IP can not only guarantee the zero packet loss but also provide a tolerable packet delay during the hard handoff procedure. After optimization, it can further reduce 35% of the packet delay.