o completely 53.7.1 Basics of SONET/SDH demultiplex the signal to extract a single channel. A standard called Syntran was the result of this work supported primarily by Bellcore The principle behind the standard is to create a synchronous fibre (Bell Communications Research). Syntran, however, as its name optic network that can accept asynchronous electrical tributary suggests, required a synchronous environment in order to permit signals and carry them through the network in payloads, the payload access to any low order signal within the multiplexed high order being of a higher bandwidth than the tributary (See chapter 42). The signal without having to demultiplex the whole thing. tributary is allowed to 'float' around within the payload due to the Needing a synchronous network is not a bad idea, in fact it is the difference in clock rates between the synchronous fibre network and whole basis of the new Synchronous Digital Hierarchy transmission the asynchronous electrical network. As the tributary floats around standards (known as SONET in North America), however, at the it is necessary to know exactly where it is should it need to be time Syntran was introduced the benefits of using it did not over- extracted for termination at its destination in the network. This is come the difficulties of implementing it. Consequently, Syntran saw done through the use of pointer tables that are attached to a payload very little application in the North American network. as overhead bytes. The pointer table identifies where within the At about the same time that Syntran was being developed there payload the first byte of the tributary is located. was an emerging need in the fibre optic network to develop mid span The lowest rate SDH signal is called STM1 (Synchronous Trans- meet standards. That is, a standard that would allow different port Module) and is defined at 155.52mbit/s, including payload and manufacturers products on each end of a fibre to communicate. At overhead. Higher rate signals are exact multiples of the STM