y are established and released on demand and remain in use only as long as data is being transferred. One of the main advantages of ATM is that it provides a sophisticated QoS support for the transfer of various types of services with individual QoS requirements. The ATM forum defines five different service categories. Each class is designed to accommodate data bursts according to customer needs and provide the appropriate quality of service (QoS) for each service class. The five service categories are Constant Bit Rate (CBR), Real-Time Variable Bit Rate (rt-VBR), Non-Real-Time Variable Bit Rate (nrt-VBR), Available Bit Rate (ABR), and Unspecified Bit Rate (UBR). More detailed definitions for these service categories can be found in Appendix A.3. Each ATM connection contains a set of parameters that describe the traffic characteristics of the source (see Appendix A.3). The ATM network checks with the source to determine the specific traffic requirements according to the traffic characteristics. When each connection is set up, traffic parameters are determined and a traffic contract is made. Then the ATM network shall provide the QoS that falls in the agreed traffic contract. 3.3.3.3 AAL2 Protocol The ATM Adaptation Layer (AAL) is designed to support different types of traffic and applications. The AAL performs functions required by the user, control and management planes and supports the mapping between the ATM layer and the next higher layer. The main services provided by the AAL are segmentation and reassembly, handling of transmission errors and losses, timing and flow control. AAL2 (AAL type 2), specified in ITU-T recommendation I.363.2, has two main advantages: (1) AAL2 allows variable packet length within cells and across cells. The packet size is controlled by a system defined maximum AAL2 packet length and the duration of the AAL2 multiplexing timer (called CU Timer). Thus it makes an easy control of the packetization delay by letting the application choose the most convenient maximum packet size for its 