PDH is an integrated digital network, so it can carry a range of traffic, as long as that traffic is presented in a digital manner. Therefore, PDH represented the first opportunity for users and carriers to combine voice and data traffic over the same pipes. In addition, it specifies the different transmission levels or data rates, some of which are available for customers to subscribe to and others of which are used by operators internally within the backbones. Finally, it defines within each of the transmission levels how many channels can be made available. The Transport Network Infrastructure The T-, E-, and J-Carrier Standards Various PDH standards are followed in different regions of the world: J-carrier (Japanese-carrier) in Japan, T-carrier (Terrestrial-carrier) in North America, and E-carrier (European-carrier) in Europe and most of Asia, Latin America, the Mid- dle East, and Africa. Figure 4.4 compares these three standards. They all share one increment as a common denominator: A single channel carries 64Kbps. But each of the three standards multiplexes together a different number of these 64Kbps chan- nels to derive higher transmission rates. Having three separate standards-T-, E-, and J-carrier-necessitates crossing between systems that use different standards, which causes additional overhead. Elements of the PDH Infrastructure As shown in Figure 4.5 and described in the following sections, the following are the key elements of the PDH infrastructure: Transmission media Repeaters Channel service units (CSUs)/data service units (DSUs) Multiplexers Digital loop carriers (DLCs) Digital cross-connect systems (DCSs) J-Carrier T-Carrier E-Carrier Level 5 397.200Mbps 400.352Mbps 565.148Mbps Level 4 97.728Mbps 274.176Mbps 139.264Mbps Level 3 32.064Mbps 44.736Mbps 34.368Mbps (T-3/US-3, E-3/CEPT-3) Level 2 6.312Mbps 6.312Mbps 8.448Mbps Intermediate Level (NA only, DS-1C) 3.162Mbps Level 1 (T-1/DS-1, E-1/CEPT-1) 1.544Mbps 1.544Mbps 2.048Mbps Single Channel 64Kbps 64Kbps 64Kbps (DS-0, 64Kbps) Figure 4.4 T-carrie