aving to disturb any of the other traffic. This reduces the overhead associated with multiplexers by a factor of 10. Chapter 4 The PSTN Table 4.4 Virtual Container/Virtual Tributary Line Rates and Standards Virtual Container/ Virtual Tributary Level Line Rate Standard VC-11/VT-1.5 1.728Mbps DS-1/E-1 VC-2/VT-2 2.304Mbps 3.456Mbps DS-1C VC-2/VT-6 6.912Mbps VT-6-N n x 6.9Mbps (Future) Async DS-3/VC-3 44.736/34.368Mbps DS-3/E-3 139.264Mbps DS-4/E-4 SDH/SONET Muxes and Cross-Connects SDH/SONET was built for and largely relies on fiber-optic transmission media. It also includes a variety of multiplexers and cross-connects, as well as equipment that could be placed at the customer's premises. There are two main categories of SDH/SONET multiplexers (see Figure 4.9): Terminal muxes-Terminal muxes enable signals to move through the hierarchy of optical carrier levels. They act as access nodes and support current services by accepting electrical interfaces and lower-level signals, including DS-1/E-1, DS-2, and DS-3/E-3. They concentrate one or more optical carrier signals and represent one of the optical carrier levels. Add/drop muxes (ADMs)-ADMs facilitate easy dropping and adding of payload and are therefore the building blocks of the SDH/SONET network. An ADM converts one or more lower-level signals, such as T-1 or E-1 sig- nals, into and from one of the optical carrier levels. It can drop lower-rate signals to be transported on different facilities, or it can add lower-rate sig- nals into the higher-rate optical carrier levels, and basically it allows telcos to add and drop traffic easily and conveniently all along the network. There are also two categories of SDH/SONET cross-connects: Wideband DCSs-These terminate SDH/SONET and DS-3/E-3 signals. Switching occurs at the DS-0, DS-1/E-1, and VT/VC levels. Signaling Systems Low Speed High Speed DS-1 (1.544Mbps) CEPT-1 (2.048Mbps) OC-48 Terminal 2.5Gbps OC-3 (155Mbps) OC-48 OC-48 High Speed High Speed 2.5Gbps 2.5Gbps OC-12 Low Speed Figure 4.9 Terminal muxes versus ADMs Bro