 video or data) is carried by a Dedicated Transport Channel (DCH) between RNC and Node B. The packet flows for each DCH from the upper layer down to the Frame Protocol (FP) layer has been explained in section 3.3.3.1. The DCH traffic stream is handed over from the FP layer to the Transport Network Layer (TNL) in the form of FP PDUs. At the lub transport network layer, FP PDUs from each DCH are sent to a common AAL2 buffer every TTI. Thus, the FP PDUs streams of all DCHs are aggregated at the AAL2 queue. In the UTRAN the lub delay is in fact the FP PDU delay, i.e. the delay of a radio frame across the lub. From the system point of view, the FP PDU delay comprises all delays accumulated on the lub interface (see Figure 6.42): (i) the segmentation and reassembly delay; (ii) the queuing delay in the AAL2 buffer; (iii) the queuing delay in the ATM buffer; and (iv) the transmission delay or processing delay over the lub link. (i) The segmentation and reassembly delay includes the delay of segmenting FP PDUs into AAL2 Packets and AAL2 packets to ATM cells, and the corresponding reassembly for the opposite task. This delay is usually neglected due to a very low value. 6.4 Dimensioning for Transport Network Performance (ii) As shown in Figure 6.42, the DCH traffic from each user is aggregated at the AAL2 layer, the arrival traffic of the AAL2 queue can be seen as the superposition of the arriving FP PDU streams of all users. In order to avoid overload on the ATM link, usually the aggregated AAL2 traffic is limited by the available Peak Cell Rate (PCR) of the ATM (section 3.3.3.2), known as AAL2 shaping. With this shaping at the AAL2 layer, the service rate of the AAL2 queue is deterministic. (iii) For the ATM buffer, the service rate is the link rate, which is also deterministic. Due to traffic shaping on the AAL2 layer, the data rate on the ATM layer is below the configured peak cell rate. Therefore there will be no congestion or overload situation on the ATM link in general. As a result, the queuing delay in the ATM buf