ly transported around a telecommunications network. SONET is both the forerunner and the North American equivalent of SDH. SONET and SDH have many similarities, but they use different terminologies. In the following sections, the main differences between the two standards are first described and then, to avoid confusion, SDH is used to explain the principles of synchronous networking. Chapter 10 Synchronous Optical Networks and Synchronous Digital Hierarchy Table 10.1 SONET and SDH Bitrates SONET Optical SONET Synchronous SDH Synchronous Carrier level Transport Signal level Transport Module Payload Rate (Mbit/s) Line Rate (Mbit/s) STS-1 (STM-0) 48,960 51,840 STS-3 STM-1 150,336 155,520 OC-12 STS-12 STM-4 601,344 622,080 OC-48 STS-48 STM-16 2,405,376 2,488,320 OC-192 STS-192 STM-64 9,621,504 9,953,280 OC-768 STS-768 STM-256 38,486,016 39,813,120 SONET and SDH: Similarities and Differences SONET was developed through the American National Standards Institute (ANSI) for use in North America, while SDH was developed through the International Telecommunications Union T (ITU-T) for use elsewhere in the world. Both standards are based on byte- interleaved multiplexing and share common bitrates as shown in Table 10.1. However, they use different terminology. SONET describes its signals as Synchronous Transport Signals, while SDH describes its signals as Synchronous Transport Modules. A number is associated with each of these signal types to designate the level in the multiplexing hier- archy that is related to the bitrate. SDH does not have an STS-1 equivalent, although this rate is sometimes referred to as STM-0. Instead the SDH hierarchy starts at STM-1, which is the equivalent of SONET's STS-3. The highest rate in use today is STS-768 (STM-256), although higher rates have been defined in readiness for further developments. Other terminology within SONET also differs from SDH. For example, SONET refers to Virtual Tributaries (VTs) whereas SDH refers to Virtual Containers (VCs), and the equivalent of the VC-3, referred 