Level Level (Mbps) STM-N STS-1 51.84 STS-3 155.52 STM-1 OC-12 STS-12 622.08 STM-4 OC-24 STS-24 1244.16 OC-48 STS-48 2488.32 STM-16 OC-192 STS-192 9953.28 STM-64 Introduction to Telecommunications Network Engineering To network management computer An example of SDH equipment Optical interface: STM-1 Optical interface: 155.52 Mbps Synchronous STM-1 155,52 Mbps STM-4 622.08 Mbps add/drop STM-4 STM-16 622.08 Mbps 2.488 Gbps multiplexer STM-16 STM-64 2.488 Gbps 9.953 Gbps STM-64 9.953 Gbps SDH is used as a tranmission layer in ATM networks 155.52 Mbps electrical 140 Mbps ATM interface cards of work 34 Mbps stations usually use the STM-1 1.5 Mbps, 2 Mbps, and/or electrical interface standard. 64 Kbps interfaces Figure 4.26 The synchronous digital hierarchy of ETSI. Different systems are included in standards, for example, terminal, add/drop, and cross-connection systems. These systems are dis- cussed in Section 4.7 and they make SDH networks more flexible than PDH systems, which include only terminal multiplexer functionality. Access to the tributary data rates is efficient (no step-by-step multi- plexing is required). The system is tolerant against synchronization and other system faults. Standardized redundancy functions allow operators to switch from a faulty line to an operational line. In the future, network management is slated to become vendor inde- pendent, with sophisticated management functions. SDH is replacing PDH systems in the transport network. By transport network we mean the flexible high-capacity transmission network that is used to carry all types of information. By flexible we mean that telecommunica- tions operators are able to easily modify the structure of the transport net- work from the centralized management system. This makes the delivery times for leased lines shorter. Leased lines are needed, for example, for LAN interconnections between the offices of a corporation. Transmission 4.6.4.1 Multiplexing Scheme in SDH The transmission data streams of SDH are called synchronous transport mod- ul