The SONET/SDH architecture identifies three different layers, each of which corresponds to one level of communication between SONET/SDH equipment. The layers are as follows, starting with the lowest:
The regenerator section, or section layer
The multiplex section, or line layer
The path layer
Figure A-3 shows the three SONET/SDH layers.
As shown in Figures A-1 and A-2, each of these layers in turn has its own overhead (header). The transport overhead (TOH) of a SONET/SDH frame is mainly subdivided into two parts that contain the section overhead (SOH) and the line overhead ...