ABSTRACT
This chapter focuses on synchronous digital hierarchy (SDH), specifying when needed the differences with synchronous optical network (SONET). Both as an example of the completeness and complexity of the signaling structure of a carrier class network and for practical reasons. As a matter of fact, the SDH/SONET framing is also frequently used in other kinds of equipment for the sake of compatibility and because all the needed hardware is available. Each SDH frame starts with framing bytes, which enable the equipment receiving the SDH data stream to identify the beginning of each frame. The location of the other bytes within this frame structure is determined by its position relative to the framing byte. The regenerator section overhead is terminated and renewed by every SDH equipment that has the role of regenerating the signal. The frame bytes are transmitted bit by bit, sequentially, starting with those in the first row.
