optical fibres, cables, radio links, satellite channels) and it might be a mix of a number of different media types. Physical media connections are created by physically connecting physical nodes to wires, cables and antennas of the spatial network. These are the entities that integrate physical networks in spatial networks. Figure 3.44 shows how physical nodes can be mapped on a transmission net- work. 15 Physical media network represents requirements on a spatial network. Spatial network is a con- cept of great importance for network planning and design. It is, however, not discussed in this book. 3 Logical Network Modelling TA-TS TA-OM TA-OM TA-TS Transmission protocol Physical media network (PM) PM-TS TAU x TA-OM TA-OM TA-TS Fig. 3.44 Mapping physical nodes on a transmission network The transport link, like the transmission link, does not provide a switched con- nectivity service to its users (called TRU). A transport link is a permanent link. It is constructed by combining a transmission link with two logical nodes that handle a transport protocol. The messages of that protocol are usually called frames. Fig- ure 3.45 explains the transport link model. OM bits frame synch channel identity ........... 11111111 frames TR-UDT TR-UDT 11111111 Transmission link 3 ......... Transmission SDUs Chan. synch The transport link CSP symbol Type: An E1 instance: 32*64 kbps N Number of channels per link C Bitrate per channel Fig. 3.45 The transport link The transport link defines two user-SAPs (TR-SAP), similar to the transmission link. The important difference, however, is that the transport SAP defines subflows of the bitstream over the SAP. These subflows have some similarities to the CON connections of packet-switched networks. The subflows are often called channels or timeslots. The structure of subflows is defined by fragmenting the continuous bitstream from/to the transmission link SAPs into frames, and within each frame, 3.4 Modelling Telecom Resource Systems by defining groups of bits as channels. For example, the