SONET defines 4 layers, namely photonic layer, Section layer, Line layer and Path layer. The photonic layer is the lowest and performs the physical layer activities while all other 3 layers correspond to Data link layer of OSI model. The photonic layer includes physical specifications for the optical fiber channel, the sensitivity of the receiver, multiplexing functions and so on. It uses NRZ encoding.

1) Path: The path layer is responsible for the movement of a signal from its optical source to its optical destination. At the optical source, the signal is changed from an electronic form into an optical form, multiplexed with other signals, and encapsulated in a frame. At the optical destination, the received frame is DE multiplexed, and the individual optical signals are changed back into their electronic forms. Path layer overhead is added at this layer. STS multiplexers provide path layer functions.

2) Line Layer: The line layer is responsible for the movement of a signal across a physical line. Line layer overhead is added to the frame at this layer. STS multiplexers and add/drop multiplexers provide line layer functions.

3) Section Layer: The section layer is responsible for the movement of a signal across a physical section. It handles framing, scrambling, and error control. Section layer overhead is added to the frame at this layer.

4) Photonic Layer: The photonic layer corresponds to the physical layer of the OSI model. It includes physical specifications for the optical fiber channel, the sensitivity of the receiver, multiplexing functions, and so on. SONET uses NRZ encoding with the presence of light representing 1 and the absence of light representing 0