 PDH digital transport standard in Europe is based on a transport link that has channels of 8 bits, 32 chan- nels per frame and a frame length of 125 microseconds. This corresponds to 64 kbps per channel and 2048 kbps (~ 2 Mbps) per frame, which also is the bitrate required over the transmission link. Besides the channels, the TR-SAP also includes a synchronization flow that identifies the different channels in the bitstream. The transport protocol performs frame synchronization and discrimination between transport end-users (TRU), channels and other channels. Normally, only 30 of the 32 channels are used for user-data (TR-SDUs); the other two are used for synchronization, for OM and as transport channels for process control systems. (How frames are actually used is more complex than this; see relevant standards, e.g. ITU-T G.731-739.) The important property of a transport link is its QoS, which includes defining the number of channels (N) and the transmission capacity per channel (c). The SAPs of the transport link may exist in physical nodes that are the transport terminal systems (TR-TS) of the link, or they may exist between a TR-TS and a physical node that implements the TR-UDT inside the transport link (as, e.g. the ISDN physical layer termination in ISDN terminal systems), see Fig. 3.46. TR-TS Transport protocol (frames) TR-TS TR-UDT TR-UDT TA-TS Transmission protocol PM-TS (bitstream) Fig. 3.46 Mapping physical nodes on the transport link A transport network for 64-kbps channels can be built by simply connecting all TR-TSs that need to communicate with each other over 2-Mbps individual trans- mission links. Each link becomes a transport system of its own. This is, however, an expensive transmission solution. In order to minimize transmission costs, many transport links are multiplexed over higher-rate transmission links. Figure 3.47 shows a small multiplexed transport network that uses an 8-Mbps transmission link to interconnect four users (TRU). Each of the TRUs has access to 32 channels of 64 kbps in it