ll SIP messages, the CN will know that the new participant is logically the same as the previous peer node because they have the same SIP address. Therefore, the CN will only send packets to both interfaces but it will not send packets received from one interface to the other. At this time, the MN and the CN are communicating through C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security AAA server Register SIP registrar Previous media session FIGURE 11.3 S-SIP signaling for mobility support. two connections concurrently, and duplicate packets received by the MN are discarded. After updating the home registration server with the new contact address, the MN will send out a "BYE" message to the CN to terminate the connection via the old interface. 11.3.2 Handoff Period The handoff period is the period between the handoff initiation and the hand- off completion. For hard handoffs, the connections involved will be interrupted during such periods, whereas data packets continue to reach the end nodes for soft handoffs. Let Dhandoff be the handoff delay and DA,B be the delay of messages transmitted between nodes A and B. We have: Dhandoff = 5DMN,CN + 2DMN,AAA + 4D MN, DHCP (11.1) Generally, there are four types of delays: queuing delay, processing delay, transmission delay, and propagation delay. The propagation delay over a wire- less access network is very small and can be neglected. Basically, the delays incurred by inter-domain messages involve: (a) processing and queuing delays in sending terminals, (b) transmission delays over wireless channels, (c) pro- cessing and queuing delays in the BSs or access points (APs), (d) delays in the Internet, and (e) queuing delays in receiving terminals. Usually, the process- ing delay depends on the workload and the serving rate of the node/server. C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks The queuing delay is related to the load of the terminal/server and can be determined based on a queu