Optimization of VoIP session setup delay over wireless links using SIP

Wireless networks beyond 2G aim at supporting real-time applications such as VoIP. Signalling protocols such as the session initiation protocol (SIP) are used to set up VoIP calls. The session setup time can be affected by the quality of the wireless link, measured in terms of frame error rate (FER), which can result in retransmission of lost packets and can lengthen the session setup time. Therefore, such protocols should have a session setup time optimized against loss. One way to do so is by choosing the appropriate retransmission timer and the underlying protocols. In this paper, we focus on the SIP protocol and evaluate its session setup delay with various underlying protocols (transport control protocol (TCP), user datagram protocol (UDP), radio link protocol (RLP)) as a function of the FER. For a 9.6 Kbps channel, the SIP session setup time can be up to 84.5 s with UDP and 105 s with TCP when the FER is up to 10%. The use of RLP(1,2,3) puts the session setup time down to 2.4 s under UDP and 3.5 s under TCP for the same FER and the same channel bandwidth.