by up to 500 000 Terminal Systems. If data traffic is significant, traffic parame- ters are quite different, and the same CS switch will only be able to support a fraction of those 500 000 Terminal Systems. This is the main reason that one tries to use packet switches instead of circuit switches for data traffic. Since the ATD-SAPs in terminal systems terminate CN connections, we call the users of these SAPs CN users (CNU). In ISDN, a CNU may connect to a basic access (two 64-kbps channels) or a primary access (thirty-two 64-kbps chan- nels), depending on how much capacity is required for the actual teleservices: One 64-kbps connection can support uncompressed voice service. Two 64-kbps (128 kbps) connections can be used for high-speed data communication. Six connections (384 kbps) are required for some video services, and 32 connec- tions (2.048 Mbps) are typical when the CNU is a PABX (Private Automatic Branch Exchange). All teleservices that can cope with one or two 64-kbps chan- nels can be supported over a basic access. Teleservices that require more capac- ity must be used over primary accesses. The transport network depicted in Fig. 3.56 is not part of the teleservice net- work. It only provides resources to that network in terms of transport links. The route concept is a way to remove transport networks from models, see Fig. 3.57. A route defines a transport path between two users that may comprise several 2-Mbps transport links. A route is specified with respect to its capacity and which type of transport service it depends on (particularly in existing public networks, since there are many different types of transport networks that are used). The route also speci- fies which transport SAPs of the actual transport network it uses. The route symbol is neutral to whether the modelled network is circuit or packet switched. (However, the specifications of a route in packet-switching networks look slightly different, see Sect. 3.4.3.) Route specifications concerning a particular 64K CS switch must be available