Networks Transmission network Transport network Concentration network Teleservice network Fig. 3.40 Resources in circuit-switched networks The basic circuit-oriented CSP unit in a logical network is the transmission (log- ical) link, see Fig. 3.41. Transmission protocol Physical media Transmission-SDUs The transmission link CSP symbol (OM omitted) Type: Instance: C Bitrate in bits/second (bps) Fig. 3.41 The transmission link 3 Logical Network Modelling The transmission link comprises a physical medium and two nodes that we will call transmission user-data terminations (TA-UDT). The TA-UDTs handle the transmission protocol and provide transmission services to users that are called TAU. The transmission link is a CSP without any CSI for connection control. The connectivity service is therefore not switched, and there are no SAPIs to define. A transmission link is a permanent link. Note, however, that electronically control- led switching of physical media connection endpoints may exist in a network. The transmission-SAP (TA-SAP) provides a stream of bits, i.e. the TA-SDU is a bit. The important property of a transmission link is its QoS, including the trans- mission capacity (C, in bits/second). The transmission protocol comprises bit syn- chronization, line signal coding (how the bits of TA-SDUs are mapped on electrical parameters), modulation (whenever applicable) and transmission over the medium. The nodes of the transmission link perform discrimination between bits that are useful for transmission users (TAU), and bits that are used for operation and maintenance (OM) of the transmission link. A transmission link is the most primitive form of a CSP, since it has only two SAPs (for users), there is no structure implied in the bitstream and there is no con- nectivity structure to control (thus, no CSIs are needed). The SAPs of the transmis- sion link may exist in physical nodes that are the transmission terminal systems (TA-TS) of the link, or they may exist between a TA-TS and a physical node inside the transmiss