SONET

The uses of optical fiber cables for data transfer allow us to send data with high data rate. There exist two globally accepted standards for this type of high rate data transfer that are,

Synchronous Optical Network (SONET) developed by ANSI and

Synchronized Digital Hierarchy (SDH) developed by ITU-T

SONET defines a technology for carrying many signals different capacities through a synchronous, optical and flexible hierarchy. SONET/SDH is a synchronous network using synchronous TDM multiplexing. All clocks in the system are locked to a master clock. 

The following diagram depict a simple network using SONET,

A simple network using SONET equipment

R: Regenerator

STS MUX

Synchronous transmission signal multiplexer or de-multiplexer: It either multiplexes signal from multiple sources into a STS signal or de-multiplexes an STS signal into different destination signals. SONET defines a group of electrical signaling levels called synchronous transport signals (STSs). Each STS level (STS-l to STS-192) supports a certain data rate, specified in megabits per second. The corresponding optical signals are called optical carriers (OCs). SDH specifies a similar system called a synchronous transport module (STM). Optical carriers (OCs) are transmitting over the fiber cable. 

Regenerator

It is a repeater that takes a received optical signal and regenerates it.

Add/Drop Multiplexer

Can add signals coming from different sources into a given path or remove a desired signal from a path and redirect it.

Terminals

A terminal is a device that uses the services of a SONET network.

                                                                                                      

SONET Layers

SONET defines four layers: path, line, section, and photonic.

SONET layers compared with OSI layers

Path Layer

The path layer is responsible for the movement of a signal from its optical source to its optical destinations multiplexers provide path layer functions.

Line Layer

Deals with the movement of a signal across a physical line. STS multiplexers and add/drop multiplexers provide line layer functions

Section Layer

This layer responsible for the movement of a signal across a physical section.

It deals with framing, and error control. Scrambling functions are also performing in this layer.

Photonic Layer

The photonic layer is responsible for physical specifications for the optical fiber channel. SONET uses NRZ encoding.

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