 of the frames and the basic inband control procedures for each type of transport channel such as the Dedicated Channel, etc. 3.3 UMTS Release 99 Application UP-PS TCP/UDP GTP-U Node B AAL 2 AAL 2 AAL 5 WCDMA L1 WCDMA L1 Physical Physical Physical Layer Layer Layer Figure 3.4: User plane protocol stack in UTRAN (Packet-Switched Domain) Node B UP-CS Application AAL 2 AAL 2 AAL 2 WCDMA L1 WCDMA L1 Physical Physical Physical Layer Layer Layer Figure 3.5: User plane protocol stack in UTRAN (Circuit-Switched Domain) The transport network of UTRAN in Re199 is built with Asynchronous Transfer Mode (ATM) transport technology. Since the ATM is suitable to carry traffic of different natures, it needs the adaptation layers for particular type of information flow. Therefore, above the ATM layer the ATM adaptation layer (AAL) protocol is employed. In the UTRAN transport network, AAL Type 2 (AAL2) is chosen for the user plane of the lub interface. The AAL Type 2 signaling protocol is defined by the International Telecommunication Union Telecommunication Sector (ITU-T) in recommendation Q.2630.1 (Capability Set 1) [ITU99] and recent enhancements in Q.2630.2 (Capability Set 2) [ITU00]. The reason for choosing AAL2 for the user plane at the lub interface is that it allows the efficient multiplexing of 3 UMTS Terrestrial Radio Access Network (UTRAN) several data flows over a single ATM virtual channel and is designed for efficient transmission of low-bit-rate services with stringent delay requirements. The major difference between the packet-switched domain and the circuit-switched domain lies in the Transport Network Layers used for the Iu interface. In the Packet-Switched domain, the Iu interface employs the GPRS Tunneling Protocol (GTP), which is a transport network protocol that provides connectionless data transfer. GTP-U is one of the forms of GTP used in the core network for transfer of user data in separated tunnels. It is carried by UDP/IP and in turn transported over ATM. The AAL Type 5 (AAL5) is selected since it is the