ation Internet delay Destination M/M/1 M/M/1 M/G/1 SIP message arrival rate at a Mobile Terminal - SIP message arrival rate at the Base Station - SIP message arrival rate at the Server or CH - Other application level messages arrival rate at the Server or CH - Constant Internet Delay FIGURE 17.10 Queuing model for analyzing delay in SIP-based handoff to WLAN. where n = 3 is the maximum number of RLP retransmission trials, and Nm is the number of TCP retransmissions. The parameters T, p, Pf, and Cij are the same as defined earlier, and q 1-(1-p)k - - is the packet loss rate. To derive the value of k, we have assumed that a TCP segment is carried in one packet. We assume that the air link frame duration is 20 ms. As derived earlier, a 9.6 Kbps radio channel can afford 24 bytes in each frame. Also, we assume that the size of one SIP message is 500 bytes. Therefore, the number of air link frames in a SIP message is k=500 22 21. For 19.2 Kbps and 128 Kbps, channels the number of frames are k=11 and k = 2, respectively. 2) Handoff delay to a WLAN network from another WLAN or UMTS net- work: For the WLAN network, the queueing delays are shown in Figure 17.10. Different parameters used are also listed in Table 17.1. The corresponding transmission delay for a SIP message, DSIP-WLAN, can be calculated in the same manner as Dsip-umts and is given as follows: DSIP-WLAN = DMH + D" + DGW + 1 + DDest Here DGW is essentially the same as the queueing delay at any of the CSCF servers and is given as follows. For both 2 Mbps and 11 Mbps WLAN networks, the number of frames cor- responding to a SIP message (500 bytes) is k = 1. Although, typically in WLAN, the SIP-based control messages and the data use the same channel, we have assumed that the control messages would have higher preemptive priority than the data frames and would not wait for pending data frame transmission. Once we have the estimates for all the components, we can determine the total handoff delay for UMTS networks as DHandoff = DAttach + DPDP + DSIP-UMTS (17.9) C 2