 Pointer 5 bytes 48 bytes ATM cells CPS PDU header packet header (incl. pointer to 1st. CPS packet) Figure 3.9: AAL2 multiplexing 3.3 UMTS Release 99 3.3.3.4 Transport Requirements on the Iub Interface The lub interface is responsible for providing an efficient, seamless transport between the Node B and the RNC in order to meet the requirements of WCDMA radio interface. For fulfilling this task, the transport of the lub interface needs to satisfy the following requirements: 1) Low delay and delay variations across the lub 2) Low packet losses 3) Bandwidth efficient transport As already addressed in Chapter 1, delay is an issue of importance for the UTRAN lub interface. In order to support WCDMA radio functions such as soft handover, power control, packet scheduling, radio channel allocation, etc., it is necessary to ensure that the Node B receives downlink frames within an exact time window to assure this radio frame to be able to be delivered to the UE according to the time requirements of the air interface. Whenever a radio frame arrives too late at the Node B, i.e. excessively delayed frames which are behind the scheduled transmission time, then this radio frame is no longer valid and is immediately discarded by the Node B. This leads to strict delay and delay variation requirements on the lub interface for the transport of all types of user traffic: real-time as well as non real-time. The lub delay mainly consists of the AAL2 multiplexing/demultiplexing delay, ATM transmission delay, propagation delay over cabled networks, switching delay due to intermediate switching nodes along UTRAN terrestrial interfaces. 3GPP TR 25.853 [3GP01c] provides the detailed delay budgets within the access stratum. In addition to a low delay and delay variation, there should be a moderately low packet losses rate on the lub interface, e.g. less than 1%. Because the discarded packets result in a degraded user performance, and moreover it triggers RLC retransmissions if with RLC Acknowledged Mode (AM). The retransmissions of the ra