just by cross-connecting virtual paths between offices. These paths provide the permanent connections between offices. The user may then configure his private network, with connections provided by the virtual channels, inside the virtual paths he has leased from a network operator. To establish an individual call through a dial-up ATM network, the signaling cells with a specific VPI and VCI (reserved for signaling purposes at a UNI) are exchanged between the network and the user. Then the network defines a route with a certain VPI and VCI for this call in each link. For the routing of cells along an established virtual connection, the ATM switches look at both the VPI and VCI in each cell. In Figure 6.62 virtual paths 1, 2, and 3 are cross-connected at the path level and virtual channel identifications remain unchanged. The figure also shows an example where virtual channels 41 and 42 of virtual path 4 are switched to the virtual channels 25 and 27 of virtual paths 4 and 5. In this case the switch updates both the VPI and VCI fields in the header of ATM cells before it transmits them to the next switch. 6.8.5 Service Classes and Adaptation Layer ATM is designed to support different services and the purpose of the adapta- tion layer (AAL) is to make the cell transport of ATM suitable for different applications. The service classes are defined to support various types of appli- cations. The corresponding adaptation layer protocols define how each class of service is implemented. The role of the AALs is to provide the mapping of particular types of traffic onto the underlying ATM cell layer. This requires that the AAL header containing the protocol information be added to the user data before transmission in the payload of ATM cells as shown in Figure 6.60. Depending on the adaptation layer in use, this reduces the user information to 48 44 octets in a cell [1]. We can divide services into four basic classes depending on whether the required service is constant or variable bit rate, isochronous or synchronous, or co