 in the LAPD protocol³ of the ATC (not shown in Fig. 3.1). The flow over a SAPI 0 is transported between the terminal process controller and the distributed network process controller. It supports the teleservice access protocol ITU-T Q.931 between these logical nodes. The data flow over SAPI 16 is transported between a user-data termination and some packet handling switch in the operator network. This flow supports the X.25 layer 3 protocol (and can also support IP, the Internet network protocol). We will denote packet handlers as X.25 PH and IP PH respectively. The units that order teleservices are called teleservice controllers (TEC), and the users of teleservices are called teleservice users (TEU). TEUs access teleserv- ices over TE-SAPs. An ISDN system offers many types of TE-SAPs. The basic teleservice is teleph- ony connectivity (3.1-kHz analogue voice signals). Signals over such TE-SAPs are pulse code modulated by a function of the user data termination that is an analogue-digital signal converter (A/D converter). Conversion from analogue to digital signals generates a 64-kbps flow that is sent over a B-channel to a 64K 3 LAPD stands for Link Access Procedure on the D-channel. It is described in the recommendations ITU-T Q.920-Q.921. 3.2 The Communication Service Provider CS switch. Encoding is done either according to A- or to u-law rules, which define the teleservice protocol between user-data terminations. Conversion between A- and u-coded analogue signals (when required) is performed by spe- cial resources (A/ju converter) that are connected to a 64K CS somewhere in the ISDN resource system on the operator network side (see Fig. 3.6). The two B-channels can also be used for both circuit-switched and packet- switched data communication up to 128 kbps, if suitable user-data terminations (X.21, X.25 or IP) exist in the terminal system. A broader type of access offers up to 2-Mbps data communication (none of these services are shown in Fig. 3.1). The ISDN system is an example of a CSP. Figure 3.1 demonstr