h UE sends its Internet request randomly and independently, it can be further assumed that the flow arrival process is modeled as a Poisson process. As according to Palm's theorem (Theorem 6.1), the arrival process can be assumed as a Poisson process if traffic is generated by a large number of independent sources. In summary, the analytical model for dimensioning the single lub link is built on the above two assumptions that data transfers are according to a Poisson arrival process and the size of the transferred data is generally distributed. R=C/rp NodeB Figure 6.16: Flow Level Model for lub Interface For data transfer within the UMTS radio access network, each user connection is assigned a certain Radio Access Bearer (RAB) (refer to section 3.3.4.1), which defines the peak data rate for this user to transfer data between the UE and the UMTS core network. That means the attainable transfer rate for each individual elastic traffic flow (TCP flow) is restricted to a limited bandwidth (i.e. the maximum data rate defined by the RAB). If all users in the network are assigned to the same RAB type (e.g. RAB 128kbps), i.e. all active flows have the same peak data rate r peak, as illustrated in Figure 6.17, then the lub link with capacity C with C > rpeak can be modeled as a Processor Sharing queuing system with R servers where C/rpeak. With the previous assumptions of Poisson arrival process and general 6 Dimensioning for Single ATM-Based lub service time distribution, the resultant analytical model is M/G/R-Processor Sharing (M/G/R-PS) model. According to this model, only up to R flows can be served simultaneously without rate reduction imposed by the shared lub link, i.e. each of such flow can fully utilize the capacity of the assigned RAB peak rate. Processor sharing comes only into play when more than R flows are active in the system, SO that multiple flows start sharing one server capacity, i.e. the peak bandwidth for a RAB. For less than R active flows, each one can be served without rate reduction imposed by th