is low, which is the case between a SIP UA and a SIP server. The two other traffic sources are provided as network background traffic. The traffic between SIP UA and SIP server has an exponential distribution in its inter-arrival time. Background traffic is generated according to a deter- ministic rate. In the scenarios of low background traffic, this data rate is set at 32 Kb/s, while in the scenarios of high background traffic, it is increased to 4,000 Kb/s. In the simulations considered in this section, the link capacity between two routers is set to 4.5 Mb/s. we can also see that our proposed dynamic adjustment methods for T1 improve the network performance as the link error rate increases. We dynamically adjust T1 to improve the network performance as the link error rate increases using one of the following meth- ods: method 1 adjusts T1 according to the data packet loss ratio; method 2 adjusts T1 according to the RTT. Since the transmission delay between two endpoints is a constant (set to 45 ms) that cannot be reduced, we compared the average delays, excluding the 45 ms transmission delay, as shown in Figure 11.6. From this Figure, we can see that, compared to the transport method using SCTP, this average delay is reduced by 57% using PR-SCTP when the link error is 0.5%, and by 83% when the link error is 1%. If the proposed dynamic T1 adjustment methods are applied, the average packet delay is further reduced. Comparing the two dynamic T1 adjustment methods, we can see that the method based on the RTT is more effective than the method based on the packet loss ratio when SIP traffic is low. In the case of low SIP traffic, the real RTT is likely to be less than 500 ms. In the method based on the RTT, T1 can be adjusted to be PR-SCTP (Constant T1) PR-SCTP (Dynamic T1, method 1) PR-SCTP (Dynamic T1, method 2) 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 Link error rate FIGURE 11.6 Comparison of average packet delay in different schemes. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Tec