 injected HSPA traffic. In this case, the traffic which is above the MDCR has no bandwidth guarantee at all and therefore can be discarded in favor of Rel99 traffic. While on the Rel99 path, there is no cell loss on both directions as the Rel99 traffic is using a separate CBR VP which has a guaranteed bandwidth between the Node B and RNC. Downlink Total ATM Cell Discards Uplink Total ATM Cell Discards Scenario Scenario Figure A.25: Total ATM cell discards on downlink and uplink When comparing the cell losses of using 2 and 3 VPs solution, it can be found that the 2 VPs scenario has more cell losses than the 3 VPs scenario on both uplink and downlink directions. This is because, in the configured 2 VPs scenario the HSDPA and HSUPA traffic are sharing one UBR+ VP but without separated VCs, i.e. HSDPA data and HSUPA signaling traffic are mixed on the downlink while HSUPA user data and HSDPA signaling traffic are combined on the uplink. Thus, there is no way to distinguish the signaling traffic from the user data traffic and therefore the signaling traffic can be dropped irrespective of its importance. Usually the signaling traffic consists of control messages sent by HSPA flow control or congestion control function, which are used to adapt the user data rate according to the available bandwidth in order to protect the lub interface from heavy congestions. If the signaling traffic is dropped, the system will not react properly on the congestion situations and therefore the transport path keeps overloaded. But with 3 VPs solution, the signaling traffic is protected from the user data traffic as different VPs are used for transmitting HSDPA and HSUPA traffic separately. For each individual UBR+ VP to transmit either HSDPA or HSUPA traffic, the configured MDCR will guarantee the bandwidth for the Appendix signaling traffic. Therefore, the signaling traffic is with a high probability not dropped in this case and hence the system can respond more correctly to the congestion situations. As a consequence, the resultant loss