s TR-SAP. 3 Logical Network Modelling Transport network CSP 2M-frames TR-UDT TR-UDT 2M-frames TR-UDT TR-UDT local 2M 8*32*64K transmission network Transmission link ~8 Mbps TR-UDT TR-UDT 8M-frames The multiplexed transport network CSP-symbol Type: Instance: N Number of links C Bitrate per link 2*(N-3) Fig. 3.47 A small multiplexed transport network This is a suitable solution when the TRUs on each side of the transmission link exist in close proximity, SO that they can be interconnected to the multiplexer over a local transmission network. Note that the network has two spare links. Multiplexers are relaying units. The purpose of a multiplexer is to concentrate traffic over (expensive) transmission lines. A multiplexer is a unit that receives SDUs from many transmission SAPs of a lower bitrate, sequences them and sends the resulting sequence as an SDU over a single transmission SAP with a higher bitrate, and vice versa, as shown in the example in Fig. 3.48. In multiplexed transport network, the network becomes layered, since on each multiplexing layer there exists a transport protocol that handles the frames of the actual layer and provides channel synchronization (not shown in Fig. 3.47) to the next lower level. Modern multiplexing networks (based on PDH, SDH, etc. 16) are built as multiplexing hierarchies which concentrate 2-Mbps streams up to 140 16 PDH, Plesiochronous Digital Hierarchy, was the first digital transport concept, introduced by CCITT (former ITU-T) in the 1970s. Synchronous Digital Hierarchy (SDH) is a modern, more flexible multiplexing and transport network concept, specified in ITU-T G.707-709. It evolved from the North American SONET (Synchronous Optical Network). 3.4 Modelling Telecom Resource Systems Mbps and beyond (higher levels are normally used over optical transmission links). A number of such digital hierarchies have been standardized over the years. for example, the PDH standard used mostly in Europe is based on 2 Mbps and four times multiplexing in four successive steps (2.048-8.448-3