he number of bits added), the longer is the delay. Overheads are kept to a minimum SO as to reduce the delay. ATM Adaptation Layer 2 (AAL2) This is for variable bit-rate information that requires a strict relationship between the transmission and reception clocks. It provides bandwidth-efficient transmission for short, variable-length packets. It is able to multiplex the short packets from multiple users to one ATM connection. AAL2 is designed for applications such as compressed video (i.e. the RT-VBR type of traffic class). This layer can also do error-checking and sequencing of the data. Sometimes, when data are flowing (i.e. payload is empty), this layer also pads the payload SO as to maintain the real-time aspect of the traffic. ATM adaptation Layers 3 and 4 (AAL3/4) Both these layers are used for data transfer. Another feature of these layers is multiplexing, i.e. allowing the same virtual channel to carry traffic from multiple sessions. This enables effective usage of the VC in applications such as X.25. ATM Adaptation Layer 5 (AAL 5) This is similar to layers 3 and 4 but more efficient. It can be used for both the message and stream mode of data transfer. DIMENSIONING 8.3.3 Multiplexing and Switching in the ATM As ATM cells traverse the network, VCI and VPI values may change depending on the routing and addressing needs of the network and ATM elements. VCI and VPI have local significance only to each interface. The cells may enter a particular ATM switch with certain VPI/VCI values and leave with different values. VPI/VCI identify the next ATM pair of segments that the cells have to travel to reach the final destination. Depending on the level of the switching performance at these cells, the cells can be switched on either the VC or the VP level. If a cell is switched on the VC level, it is switched from one VP to another. If the switching is done on the VP level, all the VCs are included in a particular VP through the transmission path. 8.4 DIMENSIONING We saw in Chapter 7 how the radio link budget change