ent layer. There are several AAL classes to support the different service classes. Connections falling under a service class have similar quality-of-service (QoS) requirements. The network can handle each service class separately by providing 3G TRANSMISSION NETWORK PLANNING AND OPTIMISATION separate virtual paths. The following service classes are defined by the ATM forum: Constant bit rate (CBR): It required a static bandwidth that is available for the connection's lifetime. Traffic is fixed and synchronous. A typical example is real-time voice or video services. Real-time variable bit rate (RT-VBR): This is for services requiring a variable bit rate at the time of use. When the application is real-time, service falls under this category. A typical example is compressed video. Non-real-time variable bit rate (NRT-VBR): Another form to VBR, this is for applications that are not delay-critical. A typical example is e-mail. Available bit rate (ABR): Bursty traffic with a known bandwidth requirement falls under this category. Basically the traffic will follow whatever bandwidth the network can give to this. A typical example is Web browsing. Unspecified bit rate (UBR): Applications not having strict requirements for delay and delay variations fall under this category. It is also known as 'best-effort traffic', and there is no QoS guaranteed for this service class. Typical examples are e-mail and FTP. Different types of AAL layers can handle different types of traffic. There are five AAL layers present, numbered from AAL1 to AAL5. They are described briefly below. ATM Adaptation Layer 1 (AALI) This is used for traffic requiring a constant bit rate and a permanent connection during its execution (e.g. voice). The data should be transferred with minimum delay and high quality. Usually when data transfer is from one layer to another, some 'overheads' are added to the data stream. These overheads are basically some bits that are added to make the data stream 'layer' compatible. The larger the overhead (i.e. the higher t