on, for simplicity, we will talk about SDH, but all will be valid also for SONET opportunely changing the names. A correspondence between SDH and SONET terminology is reported in Table 3.2 and a comparison between line rates is reported in Table 3.3. TABLE 3.3 SONET and SDH Data Rates Line Signal Line Signal Data Rate Overhead Payload User Data Rate SONET (Mbit/s) Rate (Mbit/s) (Mbit/s) (Mbit/s) 51.84 1.728 50.112 49.536 STM-1 155.52 5.184 150.336 148.608 STM-3 466.56 451.044 445.824 OC-12 STM-4 622.08 20.736 601.344 594.824 OC-18 STM-6 933.12 902.088 891.648 OC-24 STM-8 1,244.16 1,202.784 1,188.864 OC-36 STM-12 1,866.24 1,804.176 1,783.296 OC-48 STM-16 2,488.32 82.944 2,400 2,377.728 OC-192 STM-64 9,953.28 331.776 9,600 9,510.912 OC-768 STM-256 40,000 1,327.104 38,500 Telecommunication Networks Circuit L. Circuit layer networks Lower VC-11 VC-12 order layer Higher order layer Multiple section layer Section layer Trans. Regenerator section layer media layer Physical media layer FIGURE 3.26 SDH layered model. An SDH network is a transport network adopting deterministic TDM multiplexing to optimize the use of transport trunks. The high-level-layered model of an SDH/SONET network is reported in Figure 3.26. The model assumes to receive from the above ser- vice layer a set of circuit requests. A circuit is a permanent or semi-permanent connection between two network clients. The first step is path layer multiplexing: the edge SDH machine consolidates all the circuits requests directed toward the same destination in SDH virtual containers (VCs, virtual tributaries [VTs] in the SONET standard), which will be the elements composing the payload of the SDH TDM frame. The SDH standard defines a set of VCs, depending on the capacity needed, whose name is VC followed by a number, where the number is related to the VC capacity. The path layer is divided into two sublayers, since a double stage multiplexing is fore- seen. This means that low-capacity VCs can be enveloped in a high-capacity VC if needed. The VCs are routed thro