S ISDN- TE-TS TE-TS Access Fig. 3.60 The internal structure of an ISDN-ATD 3.4 Modelling Telecom Resource Systems Figure 3.60 shows that an ATD is normally separated into a number of concen- trating circuit-switching nodes on the network side (called ATDC in the figure). The ATDC is a circuit switch that is specifically designed to terminate access links with two or thirty-two 64-kbps channels on the terminal side, and access links with thirty-two 64-kbps channels on the switching side. Figure 3.61 shows the internal structure of an ATDC. controller 64 kbps 64 kbps 64 kbps 64 kbps Fig. 3.61 The internal structure of a switch in the access transport network The ATDC consists of a number of multiplexers and some internal transmission networks that provide 64-kbps SAPs that terminate in the ATDC core. The ATDC core switches 64-kbps channels according to commands over the CSI of the ATD controller. The ATDC core is the same type of switching element that exists inside the 64K CS switch. The difference between the ATDC and the 64K CS switch is only in the type of links they terminate and how they are used. The CS switch is used to switch channels between any SAPs. The ATDC switch is used to switch channels between access transport links on the terminal side and transport links on the 64K CS side. The internal structure of an ISDN access for two 64-kbps channels (called "basic access") is shown in Fig. 3.62. Transport protocol (frames) 2*64 + 1*16 Fig. 3.62 The transport network for ISDN basic access 3 Logical Network Modelling A basic access integrates both 64- and 16-kbps channels, SO it is actually a small, unswitched transport network that includes one transport link that carries two 64- kbps channels, and another transport link that carries one 16-kbps channel. The 16- kbps channel is used as a carrier for messages between the teleservice terminal process controller and the network process controller (among other things). 3.4.3 Resources in Packet Switched Networks, Introduction The basic resource units in packet-