Synchronous digital hierarchy (SDH), more commonly known in America as synchronous optical networking (SONET), is a standard in telecommunications that deals with how data are transferred on optical fibers. This data stream is used with computers, networks and phones to transfer data from one device to another. The SDH standard improved on two previous standards, pulse cord modulation (PCM) and plesiochronous digital hierarchy (PDH). Major features of SDH are that it can transfer a large number of bits and the packets are synchronous.
Whenever a computer needs data, or a server collects information, it requires a stream of data. The SDH standard covers how these data are streamed and collected by sending them out in packets. As the name suggests, the packets are synchronous in size, and each packet is made of an overhead and a payload. The overhead, known simply as "head" in other data transmission standards, is supplementary data, while the payload is the major data being streamed. This opens a channel with the device that needs the data and continues to stream the data until all the bits have reached their destination.
The SDH transmission stream meets many network operators’ needs. It can easily expand to meet new transmission needs and sizes, and the stream is flexible and quickly responds to change. The data stream also drops less often than other streaming methods, so users can continue using their devices without being impeded. It also is less costly than the previous data stream methods, which helps with financial considerations.
SDH was created after two other major data stream transmission systems. PCM was the first, and the cost of bandwidth in this system was very high. The amount of data it could move eventually became unable to meet the needs of users any longer, so PCM was removed and PDH took over.
PDH was a large improvement, laying the groundwork for SDH, but it had its own problems. The data stream was non-synchronous, which would sometimes lead to unexpected dropouts. The network structure also was strict, so administrators had difficulty responding to change, and no world standard existed.
The synchronous system took over as the first transmission format with a world standard. It also uses optical wires, which are more efficient for transmitting data, and includes backward compatibility with PDH streams. Management with PDH was tough, so SDH includes many new managerial functions that let administrators easily work with the data stream.