Study of Best-Effort VoIP Handovers between WLAN and EVDO Networks

The IEEE 802.11 based Wireless LANs (WLANs) have emerged as a viable technology for supporting real-time applications such as voice over IP (VoIP). Even the personal digital assistants and Smartphones are being equipped with WLAN, leading such devices to operate in dual-mode with the WLAN and WAN (wide area network) radios capable of supporting IP communication. This development enables applications such as VoIP to roam or handover freely across the two networks. Seamless handover of VoIP between such networks poses two challenges: the call must persist in spite of mobility across networks, and the LAN and WAN must support the delay and packet loss requirement bounds for VoIP calls. In this paper, we present an experimental study of best-effort VoIP handovers across WLAN and the CDMA EVDO networks, using Skype as the VoIP application. The objectives of our study were: to understand and evaluate the mobility events and actions taking place in a handover; to evaluate the feasibility of running VoIP over WLAN and CDMA EVDO in the process of understanding mobility; and to provide insight into further research in this area. Our study shows that EVDO link acquisition latency and downlink scheduling, and IP mobility procedures contribute towards VoIP call failures. These results provide crucial input to the design of best-effort real-time applications as well as mobility protocols.