190 kbps 64K CS Teleservice resource control Access and Analogue signal switching node devices Teleservice protocol Teleservice protocol Protocol converter TE-SAP Circuit switch Access transport protocol 2 LAPD, ITU-T Q.920-921 Teleservice Access Protocol, ITU-T Q.931 Transmission link Teleservice Distribution Protocol, ISUP, ITU-T 2.761-766 Teleservice protocols, e.g. ITU-T G.711 External Transport link 6 Teleservice protocols Common Channel Signalling, ITU-T Q.701-716 Fig. 3.39 Logical node models of the ISDN-CSP for teleservices based on circuit-switched con- nections (CN connections) The model is constructed by three types of units: Resource controllers, such as the switch controller in the model. Units that handle messages (of protocols), such as the TPC, DNPC, CCS and all UDTs. These elements are logical nodes or protocol elements (read more about the distinction between logical nodes and protocol elements in Chap. 5). We say that logical nodes and protocol elements terminate protocols, which means that they interpret and act on the contents of messages. Units that transfer data between SAPs, such as links, switches and the ISDN CSP as a whole. Only the ISDN "basic access" (two 64-kbps channels and one 16-kbps channel) is represented in the model on the terminal side. A brief description follows: 3.4 Modelling Telecom Resource Systems Terminal systems are connected to the operator network by specific ISDN access transport links. Such a link consist of a transmission link and two user- data terminations (CN-UDT) that handle the access transport protocol¹³. The 64-kbps channels of the access link are terminated on the terminal side in logical nodes that are called CNU. On the network side, 64-kbps channels are terminated in an access switch (called ATDN) that provides channel concentration. An ATDN can connect thousands of access links. On the other side, the ATDN is connected by one or a few transport links to a 64K CS switch. These transport links carry 64-kbps channels ¹4. The 64K CS can connect many ATDNs