SONET Transport Hierarchy
Each level of the hierarchy terminates its corresponding fields in the SONET payload, as such:
A section is a single fiber run that can be terminated by a network element (Line or Path) or an optical regenerator.
The main function of the section layer is to properly format the SONET frames, and to convert the electrical signals to optical signals. Section Terminating Equipment (STE) can originate, access, modify, or terminate the section header overhead. (A standard STS-1 frame is nine rows by 90 bytes. The first three bytes of each row comprise the Section and Line header overhead.)
Line-Terminating Equipment (LTE) originates or terminates one or more sections of a line signal. The LTE does the synchronization and multiplexing of information on SONET frames. Multiple lower-level SONET signals can be mixed together to form higher-level SONET signals. An Add/Drop Multiplexer (ADM) is an example of LTE.
Path-Terminating Equipment (PTE) interfaces non-SONET equipment to the SONET network. At this layer, the payload is mapped and demapped into the SONET frame. For example, an STS PTE can assemble 25 1.544 Mbps DS1 signals and insert path overhead to form an STS-1 signal.
This layer is concerned with end-to-end transport of data.