r transporting the uplink and downlink transport channel frames between RNC and Node B. This is performed by a set 3.3 UMTS Release 99 of data streams provided by the lub interface: lub DCH data stream (a bi-directional transport channel); lub RACH data stream; lub FACH data stream; lub DSCH data stream; lub CPCH data stream; and the lub PCH data stream. As shown in Figure 3.8, each of such lub Data Streams is conveyed by the Frame Protocol (FP) and thus a set of FPs reside within the lub user plane to provide the means to deliver the actual traffic on the transport channels. The Frame Protocol is in charge of putting the data stream to be transported on the corresponding transport channel over the lub interface. Thus for each type of transport channel there is a certain type of FP. They all provide a basic service: the transport of Transport Block Set (TBS) between the Node B and RNC across the lub interface. Transport Block (TB) is defined as the basic unit passed down to L1 (physical layer) from the MAC layer. An equivalent term for the Transport Block is MAC PDU (Protocol Data Unit). The Transport Block Set in turn is defined as a set of Transport Blocks which is passed to L1 from MAC at the same time instance using the transport channel. An equivalent term for Transport Block Set is MAC PDU Set. According to the UTRAN protocol structure, the MAC protocol is terminated at the RNC. That means MAC PDUs are not carried explicitly over the lub interface. Instead, the information contained in the MAC PDUs is conveyed by the Frame Protocols from the RNC to the Node B in order to pass it further towards the UE across the air interface. Frame Protocols are divided into two groups: FP for Dedicated Transport Channel data streams and FP for Common Transport Channels data streams. DCH is the single type of dedicated transport channel, SO there is only one FP representing that group, DCH FP. For common transport channel data streams the following Frame Protocols exist: RACH FP, CPCH FP, FACH FP, and DSCH FP. lub Interfac