. The 64K CS is the function that provides CN connections between CNUs in dif- ferent terminal systems, by interconnecting channels on the access side with channels on the network side. It also provides CN connections between CNUs in terminal systems and CNUs in the operator network that contain internal tele- service resources. Such resources may be devices that signal over CN connec- tions for digit reception, tone signalling and announcements. Other devices that use CN connections are protocol converters. The 16-kbps channels of the access link are D-channel users (DU). The D-chan- nel is used by LAPD protocol elements. The LAPD system is a logical network that is used for several purposes. The pri- mary purpose is to support message interchange between distributed network process controllers (TE-DNCP) and terminal process controllers (TE-TPC). The other purpose is to support data communication over the 16-kbps channel (this is not part of the model in Fig. 3.39) The LAPD network carries 64 packet transport links, called SAPI 0 to SAPI 63. Only SAPI 0 is used for handling teleservices that are based on CN connections. The LAPD-SAPs are CON-SAPs, i.e. they offer CON connection endpoints. Up to 128 CON connections can be used on each SAP. Different CON connec- tions are used to distinguish between different terminals in a terminal system (ISDN allows up to eight terminals on a basic access). The TE-TPC and the TE-DNCP negotiate teleservice and CN connection needs over the teleservice service access protocol (ITU-TQ.931) This protocol is not standardized as part of ISDN. Instead, ISDN provides corresponding stand- ards for connecting individual terminals within a terminal system to a network termination, which handles the access transport protocol. The transport and transmission network inside a terminal system comprises many variants, see the recommendations ITU-T 1.411, 1.412, 1.430 and 1.431. 14 The model assumes all transport links to be PCM links of type E1. These are 2-Mbps links that carry 32 channels, see