 be transferred through the UTRAN. The FP layer adds its additional header to the TBS and in turn delivers it down to the Transport Network Layer in the form of FP PDUs. In the transport network layer, the AAL2 protocol segments the FP PDUs into small AAL2 packets. These AAL2 layer packets are then packed into ATM cells before being transmitted to the ATM link. 3.3.3.2 ATM Asynchronous Transfer Mode (ATM) is a connection-oriented, packet-switched technology designed for the high-speed transfer of voice, video, and data through public and private networks using cell relay technology. It uses very short fixed-length (53 bytes) packets, called cells, to transfer data traffic. The first 5 bytes of the ATM cell contain header information, such as the connection identifier, while the remaining 48 bytes contain the data or payload (the ATM cell format is given in Appendix 0). For the fixed-size cells the ATM switch does not have to detect the size of a unit of data, therefore switching can be performed efficiently. Moreover, the small size of the ATM cell makes it well suited for the transfer of real-time data, such as voice and video, which is intolerant of delays. ATM defines two levels of virtual connections: Virtual Paths (VP) and Virtual Channels (VC). The connection is identified by two values in the cell header: the Virtual Path Identifier 3.3 UMTS Release 99 (VPI) and the Virtual Channel Identifier (VCI). A virtual path (VP) is a bundle of virtual channels, all of which are switched transparently across the ATM network based on the common VPI. ATM provides several types of logical connections between two end users, which can be set up statically or dynamically. The Permanent Virtual Circuits (PVCs) or Permanent Virtual Paths (PVPs) are usually created long before it is used and remains in place until the connection is deprovisioned. Bandwidth is allocated for them whether it is used or not. In this way they are similar to leased lines. The Switched Virtual Circuits (SVCs) in contrary are dynamic connections. The