is in their basic digital line rates. In North America it is at the DS1 or DS3 lines rates; in SDH countries it is at the 2.048-, 34-, or 139-Mbps rates (see Chapter 6). This has been resolved in the SONET/SDH environment through the SDH administrative unit (AU) at a 34-Mbps rate. Four such 34-Mbps AUs are "nested" (i.e., joined) to form the SDH STM-1, the 155-Mbps basic building block. There is an AU3 used with SDH to carry a SONET STS-1 or a DS3 signal. In such a way, a nominal 50-Mbps AU3 can be transported on an STM-1 SDH signal. 19.3.2 SDH Standard Bit Rates The standard SDH bits rates are shown in Table 19.2. ITU-T Rec. G.707 (Ref. 5) states "that the first level digital hierarchy shall be 155,520 kbps higher syn- chronous digital hierarchy rates shall be obtained as integer multiples of the first level bit rate." This implies a DS3 configuration. It contains 28 DS1s. ADVANCED BROADBAND DIGITAL TRANSPORT FORMATS Table 19.2 SDH Bit Rates with SONET Equivalents SONET Equivalent SDH Level SDH Bit Rate (kbps) Line Rate 155,520 STS-3/OC-3 622,080 STS-12/OC-12 2,488,320 STS-48/OC48 Two other SDH hierarchical levels are under consideration by CCITT (Ref. 5): Level 8 1,244,160 kbps; Level 12 1,866,240 kbps. 19.3.3 Interface and Frame Structure of SDH Figure 19.16 illustrates the relationship between various multiplexing elements of SDH and shows generic multiplexing structures. Figure 19.17 illustrates one multiplexing example for SDH, where there is direct multiplexing from container-1 using AU-3. 19.3.3.1 Definitions Synchronous Transport Module (STM). An STM is the information structure used to support section layer connections in the SDH. It is analogous to STS in the SONET regime. STM consists of information payload and section overhead (SOH) information fields organized in a block frame structure that repeats every 125 usec. The infor- mation is suitably conditioned for serial transmission on selected media at a rate that is synchronized to the network. A basic STM (STM-1) is defined at 155,520 kbps. Higher-c