 layer protocol overheads TNL_OH, the previously introduced lub congestion factor Q and the QoS target Qs. (4) By knowing the dimensioning factor N, the dimensioned lub bandwidth can be calculated as Iub_BW = p.n. A.16 Use of Traffic Separation for Transport of HSPA and Rel99 So far, HSPA services have been already supported in the existing ATM-based UMTS networks to enhance data transmissions. Besides, the UMTS system still accommodates a significant amount of Re199 traffic such as voice telephony. However, Rel99 and HSPA services have rather different QoS requirements: Re199 mainly carries delay sensitive traffic like voice or streaming services; while HSPA traffic is primarily interactive and background traffic which is insensitive to the delay. This chapter presents traffic separation approach to transmit HSPA traffic in the existing ATM-based UMTS network, together with Rel99 traffic in the same radio access network. The traffic separation technique enables QoS differentiations of HSPA and Rel99 traffic, providing a differentiated QoS support for each type of traffic according to its individual QoS requirements while at the same time aims to achieve a maximum utilization of the transport resources in the radio access network. The potential benefit of applying traffic separation and its impact on the performance of the transport network as well as the end users are explored in this chapter. The quantitative evaluations are provided by means of simulations. A.16 Use of Traffic Separation for Transport of HSPA and Rel99 I. Problem without Traffic Separation Figure A.21 illustrates the evolved UMTS system with integrated REL99 and HSPA services. It is seen that one UMTS cell supports (1) normal UMTS Rel99 users like traditional voice users; (2) HSDPA users who require HSDPA service for high-speed data transfer on the downlink, e.g. Internet access; (3) HSUPA users who only uses HSUPA service for uplink data transmissions, e.g. ftp upload; (4) or HSPA users who use HSUPA on the uplink and HSDPA on the downlink si