4.368-139.264 Mbps). Another PDH standard is the one used primarily in USA, Canada and Asia, which is based on 1.544 Mbps, and ends at 44.736 Mbps. The SDH standard combines both PDH standards in a common standard that goes up to 2488 Mbps (~ 2.5 Gbps). 8 bits every 125 us 32 bits every 125 us Bitrate=256 kbps Bitrate/SAP=64 kbps The multiplexer relay symbol Type: Instance for M=4: M Number of SAPs C Bitrate per SAP Fig. 3.48 The function of a multiplexer A physical network solution to the logical network in Fig. 3.47 can, for example, be three switches in a teleservice network (that provide switching of 64-kbps chan- nels), interconnected by transport links in a star structure, as shown in Fig. 3.49. TR-TS TR-TS 32*64K transport link 32*64K transport link TR-TS Fig. 3.49 Mapping physical nodes on a transport network Multiplexed transport links are permanent links as well. A more efficient trans- port network is constructed by connecting transmission links to circuit switches inside the transport network that can switch large groups of channels. Such a net- work is a switched transport network. Switching is provided by special transport 3 Logical Network Modelling switches that are controlled by transport network operators (over their transport process control system and transport switch controllers). In switched transport net- works each transport SAP is assigned a TR-SAPI. All TR-SAPIs are owned by the transport network system, i.e., transport network users (TRU) cannot set up trans- port links themselves. An example of the resources in a switched transport network is given in Fig. 3.50. Transport protocol (frames) Switch Controller Transmission link 2 Mbps frames The switched transport network CSP symbol Type: Instance: N Number of links C Bitrate per link 2* (N-3) Fig. 3.50 A small switched transport network Switches are also relaying units. Transport switches do not switch individual channels. They are normally called digital cross-connect switches (DCC). The DCC in Fig. 3.50 switches 2-Mbps bitstreams. By i