This document describes the layers of a Synchronous Transport System (STS) network. It includes the physical photonic layer, section layer, line layer, path layer, and data link path layer. The lower layers like the physical and section layers transmit data without processing while higher layers like the path layer are vulnerable to viruses and mistakes.

4. STS=Synchronous
Transport System
R=Regenerator
ADM=ADD Drop
Multiplexer

5. Data Link
Path Layer
Line Layer
Section Layer
Physical Photonic Layer
Path layer
Line layer

7. vulnerable to viruses or mistakes.