Introduction to SONET and SDH
The well-known author Andrew Tanenbaum (well, at least he is well-known in the computer communications field) of a well-known computer communications textbook is attributed with saying "the nice things about standards is that there are so many of them to choose from." The SONET and SDH standards are a perfect example. Why two? - especially when they are so similar to one another? I suppose politics has a lot to do with it, but this pair of standards, from a compatibility point of view, is actually an improvement over the previous generation's pair of wide area physical infrastructure standards, E1-E3 and T1-T3. The latter set of standards do not interoperate, which is why RAD sells the DXC-2 T1/E1 Converter and Timeslot Cross-Connect. (I never advertise on RAD University - it is against one of my founding principles with which I conceived of RAD University - non-commercial freely available information on the Internet. But this time I just couldn't resist. I hope management is pleased.)
SDH (Synchronous Digital Hierarchy) is an international standard defined by the ITU (International Telecommunications Union) for high speed telecommunication over optical/electical networks. SDH is capable of transporting digital signals in variable capacities. It is a synchronous system defined to provide a flexible, yet simple network infrastructure. SDH (and its American variant- SONET) emerged from standard bodies somewhere around 1990. SONET (The Synchronous Optical NETwork), first proposed by Bell Communications Research, was standardized by the American National Standards Institute and then was adopted by the ITU-T. These two standards are the dominant standards for wide area transmission over optical networks. They can both be used to encapsulate previously defined digital transmission standards, as the physical layer transport for ATM, or as a point-to-point infrastructure to support IP, using the PPP over SONET/SDH standard, which is called Packet over SONET/SDH and abbreviated as POS.
The full specificatons of SONET and SDH are huge. The intent of this tutorial is to provide an overview and to provide some links for further information. The full specifications may be obtained from the standards organizations responsible for each of the standards (ITU-T and ANSI). In this tutorial, the terms SONET and SDH are used almost interchangeably. The differences between them are few - SONET has two basic framing units while SDH has one, SDH has more mapping options, and the internal format of the overhead and payload are slightly different, but not different enough to prevent interoperability. We can justify the flexible use of terms in this tutorial by saying that in the name of political correctness we did not want to give preference to one or the other (or to ANSI or to the ITU-T) - on one hand, SONET was the original standard, but on the other, in the worldwide market, SDH has a greater penetration.
Among the various aspects of communication over fiber that the SDH and SONET standards specify, the standards address framing, multiplexing, and network topology, which will each be treated in the following pages.
