hat a pipe with guaranteed bandwidth is established between the RNC and the Node B. So the intermediate ATM switches should not discard any ATM cells (neither of Rel99 nor of HSPA) as long as the RNC and the Node B comply with ATM service level agreement for CBR traffic. Thus, to use this high quality ATM service category CBR for the transport of HSPA and Rel99 traffic, the required bandwidth is overspecified and this causes unnecessary high costs in terms of leased ATM bandwidths. In order to save the transport costs for the fixed lines, applying traffic separation techniques is a very popular and cost-efficient solution for the network operators or service providers. Instead of paying for overestimated CBR VPs to transmit all traffic types, cheap UBR (Unspecified Bit Rate) or UBR+ (UBR with a minimum guaranteed rate) VPs can be used separately to transport HSPA traffic whereas the CBR VPs are only used to transmit Rel99 traffic. II. Concept of Traffic Separation The basic idea of traffic separation technique is to apply separate ATM Virtual Paths (VPs) or Virtual Circuits (VCs) with different ATM QoS categories to transmit different traffic types. One example of using traffic separation to transmit Rel99, HSDPA and HSUPA traffic at the lub interface is depicted in Figure A.22. In this example, each traffic type is carried by one individual ATM VP. Rel99 traffic is transported with ATM CBR (Constant Bit Rate) service category. It is defined as high priority traffic class, where bandwidth is reserved up to requested Peak Cell Rate (PCR) with guaranteed cell loss ratio and cell transfer delay. This also means high transport costs. While the transport of HSDPA and HSUPA traffic uses ATM traffic class UBR (Unspecified Bit Rate) or UBR+. UBR is a best effort service and is the lowest class of service in ATM. It is defined as low priority traffic class, which utilizes all bandwidth unused by the high priority traffic. Therefore it does not provide any guarantees for bandwidth, cell loss ratio and cell transfer delay. 