dbook: Services, Technologies, and Security communication delay between nodes m and N. For example, in Figure 10.6, rt(cdomain,hdomain) is the round-trip delay between the cdomain SIP server and that of hdomain. If we estimate the server's average processing time of the INVITE request by f3, and the UA's average processing time of the INVITE request by 7, then again from Figure 10.6a, + B++ (10.3a) ~ Ntcdomain,hdomain) + B+H (10.3b) ~ Th(hdomain,r_user) +r (10.3c) and therefore, rt(c_user,cdomain) + rt(cdomain,hdomain) + rt(hdomain,r_user) +2,3 + r (10.3d) where l is the average call establishment time (the INVITE/RINGING round- trip) as seen by c_user, is the average INVITE/RINGING round-trip time between the cdomain and hdomain SIP servers and u is the average INVITE/ RINGING round-trip time between the hdomain SIP server and ruser. Now, if we consider H-SIP, then from Figure 10.6c, >rt(c_user,cdomain) + B+x+0' (10.4a) rt(cdomain.fdomain) + B+H (10.4b) ii' ~rt(fdomain,r_user) +r (10.4c) and therefore, + rt(cdomain.hdomain) + rt(fdomain,r_user) +23+x+r (10.4d) where X' is again the average call establishment time as seen by c_user, is the average INVITE/RINGNG round-trip time between the cdomain and fdomain SIP servers, u' is the average INVITE/RINGING round-trip communication time between the fdomain SIP server and r_user, and X is the average handle resolution delay. Note here that each INVITE request will require the server to issue one fresh handle resolution. Both l and X' are measured by c_user as the time between sending the INVITE request and receiving the RINGING response. 10.4.3 Performance Measurements The measurements of the registration times, call establishment times and communication delays in this section were all averaged from 10,000 samples dispersed over a 10-day period, i.e., n = 1,000 samples a day. 10.4.3.1 Registration Examining Equation 10.1a and Equation 10.2a, we deduce that tc < tA for sufficiently large N. In the general case of a roaming user, the round-trip delay C 2009 by Taylor & 