ow has tf replacing th. Hence shadow reg- istration has decreased the disruption time in both cases of MIP and SIP. Therefore shadow registration is useful when the mobile node is far from the home network. 22.2.3.2 Performance of SIP in Fax over IP The performance of Fax over IP (FoIP) needs to rival its performance in legacy fixed line at user premises. Fax over the PSTN is very robust and reliable. However, the reach and the popularity of the Internet combined with the fact that its use is almost free, is a major driver for VoIP networks and for similar data applications that exist on the PSTN. Fax transmission has special requirements. Firstly while the loss of a packet during a human conversation C 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance does not affect a voice call a great deal, it can easily affect a fax call. This is because fax transmission requires far more signaling and handshaking than a regular telephone call. This includes negotiating details such as speed, paper size, and delivery confirmation. Apart from the signaling in a fax call, the sending and receiving of fax documents are mostly done and interpreted by automated fax machines. Therefore errors in either signaling or the actual transmission result in lengthy recovery times. A study was performed by Choudhary [11] on the performance of FoIP net- works. The performance parameters for FoIP are significantly different com- pared with VoIP. In fax transmissions, no roaming or handoffs are required. However, end-to-end delays, link utilization and the performance of the wire- less links are essential. 22.2.3.2.1 Experimental network models With three network mod- els, Choudhary [11] measured the link utilization of the inter-proxy-server link (IPSL), the link utilization of the auxiliary link (AUXL), the average end-to-end delay of the SIP signaling packets, the average end-to-end delay of fax data packets, the average SIP call setup time, and the average fax call setup time. 1. In Network Model 1 calls a