 Physical medium 64K CS Fig. 3.55 The PSTN-switching node model The PSTN-UDT handles a single -kHz analogue channel, both for user data and for control flows. PCM coding is done by the PSTN-UDT, SO that the internal part of the network can use the same resources as in ISDN. The PSTN-UDT also performs discrimination on the access side by identifying special analogue signals from voice signals. There is no discrimination of different types of process control flows, as provided by LAPD in the ISDN-case. On the network side, the ISDN and the PSTN networks may look similar from a structural point of view. Note, how- 3.4 Modelling Telecom Resource Systems ever, that in PSTN networks around the world, there still exist many networks that use older analogue signalling systems instead of CCS. The distributed service con- trol protocol used over CCS is called TUP (Telephony User Part, see ITU-T Q.721-725) and is simpler than the corresponding protocol in ISDN (the ISUP), since TUP must only support one service-telephony. If we focus only on the resource system part of a network that provides CN con- nections, the model might look as in Fig. 3.56. controller CS switch switched transport controller network Access 2 Mbps Transport Network ATD part controller Fig. 3.56 Resources in a network that provides CN connections The model shows a network comprising four 64K CS switches. These switches are controlled from the network process control system of the teleservice net- The switches are interconnected over a switched transport network (a CSP) by 2-Mbps links. The links are set up semipermanently by means that are not shown in this model. Two of the CS switches are supported by access transport networks. The two switches to the right do not connect access transport networks. The upper of these two is a transit switch that can relay traffic between the switches to the left, in case the links between the left switches should fail. The fourth switch is assumed to be a gateway switch that relays traffic to and between other net- wo