ing Little's law, the average transfer delay E{T} can be obtained by equation (6.29). with E{W}=j.p(j) (6.29) For network dimensioning, another important QoS is the blocking probability (or called Connection Reject Ratio in this thesis). When there are already N connections over the lub link, any additional new connection requests will be rejected. Then the blocking probability p(N) also means the probability of having N connections in the network. It can be derived from (6.28). When R = N,p(N) reduces to Erlang's first formula (i.e. Erlang-B formula). 6.3.7.3 Extension for case with Multiple RABs and not Applying CAC Typically, UMTS provides a variety of bearers called Radio Access Bearers (RABs) (see section 3.3.4.1) to support a range of services and QoS. This section discusses the scenarios where multiple RABs are used at the same time in UMTS for transmitting different elastic traffic classes. It is assumed that each traffic class is transferred by one RAB type and specifies a different application QoS target. In this case, the dimensioning needs to fulfill the QoS requirements of individual traffic classes on an end-to-end basis. Moreover, in this scenario it is assumed that each elastic flow is assigned to a fixed RAB type (i.e. the RAB is not changed during the transfer of that flow), and no CAC function is applied to any elastic 6 Dimensioning for Single ATM-Based lub traffic class, i.e. the lub link does not restrict the maximum number of simultaneous connections for elastic traffic. Figure 6.21 illustrates system model of a scenario with multiple RABs for different QoS user groups at TCP flow level. In the shown example there are three UE groups. Each UE group defines a specific traffic class and is assigned a different RAB type with a peak data rate ri. The index i indicates the user group with traffic class i. The arrival process of each group is assumed to be a Poisson process with a flow arrival rate Ai, resulting in a mean traffic load Pi. In this scenario, due to the basic property of TCP, proces