delay requirements. The lower the size, the lower is the delay. Nevertheless, lower sizes lead to an increase of the relative overhead as each packet has a dedicated header. (2) AAL 2 provides for multiplexing several AAL2 connections over a single Virtual Channel Connection (VCC). That means, instead of having user data partially filling an ATM cell, 3 UMTS Terrestrial Radio Access Network (UTRAN) AAL2 will fill the cell with data from other several active AAL2 user connections. Thereby, high bandwidth efficiency is gained by efficient use of transport resources. With the above features, the AAL2 protocol is suitable for bandwidth efficient transmission of low-rate, short and variable packet length in delay sensitive applications such as telephony in UMTS. The exact function of the AAL2 protocol is illustrated in Figure 3.9. The first step is generating an AAL2 Common Part Sublayer (CPS) packet flow from each user data flow (i.e. FP stream). Each AAL2 CPS packet contains a 3-octet header and a payload of up to 45 (or 64) octets. The header of the AAL2 CPS packet consists of an 8-bit Channel Identifier (CID), 6-bit Length Indicator (LI), 5-bit User-to-User Indication (UUI) and 5-bit Header Error Control (HEC). The CID field uniquely identifies the individual user channels within the AAL2, and allows up to 248 individual AAL2 user connections within each AAL2 structure. The LI field is used to indicate the actual length of the CPS packet payload associated with each individual user. Next, AAL2 CPS packets belonging to different flows are multiplexed and encapsulated into a 47-octet block. Each block is preceded with a 1-octet Start Field containing a pointer (useful for resynchronizing after a cell loss), to form a 48-octet AAL2 CPS PDU. Each CPS PDU is exactly an ATM cell payload of 48 octets, to which a 5-octet ATM header will be attached. The format of an AAL CPS packet and AAL2 CPS-PDU is given in Appendix A.2. User 1 User 2 User 3 Data flows 3 bytes 1- 64 bytes AAL 2 CPS-Packets AAL 2 CPS PDUs 1 byte 47 bytes