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Synchronous Optical Network (SONET)

By Alexander S. Gillis

What is Synchronous Optical Network (SONET)?

Synchronous Optical Network (SONET) is a North American standard for synchronous data transmission over optical fibers. In other words, SONET is a standardized digital communication protocol. SONET transmits and multiplexes multiple data streams across a fiber optic cable.

The American National Standards Institute (ANSI) originally developed SONET for public telephone networks in the 1980s. Today, SONET acts as a standard for digital networks to enable and interconnect with existing conventional transmission systems and enable them to take advantage of optical media through tributary attachments.

Backbone carrier networks typically use SONET. Telecom operators share data over the line of a fiber optic cable instead of digging trenches to bury new cables, which is typically a more expensive process. To multiplex the data, the network separates the cable into separate channels. The speed of data transmission is comparable to Gigabit Ethernet speeds.

SONET components

The network elements defined in SONET include the following:

• Synchronous Transport Signal (STS) multiplexer. Multiplexes signals and converts electrical signals to optical ones.
• STS demultiplexer. Condenses signals and converts optical signals back to electrical signals.
• Regenerator. Increases incoming optical signals, which enables them to travel farther.
• Add-drop multiplexer. Adds or removes signals from sources.

SONET connections break down among the following:

• Sections. Connect two devices.
• Lines. Connect two multiplexers.
• Paths. Connect networks from end to end.

SONET also defines the following four layers:

1. Path layer. Moves signals from sources to destinations.
2. Line layer. Moves across cables.
3. Section layer. Defines the movement of signals across cables.
4. Photonic layer. Specifies optical fiber channels.

SONET standards

SONET provides standards for a number of line rates up to the maximum line rate of 10 gigabits per second (Gbps). Actual line rates can approach up to the 30 Gbps range. ANSI T1.105 and T1.117 specify SONET standards.

The basic unit of SONET is optical carrier level 1 (OC-1). OC-1 supports up to 51.84 megabits per second (Mbps). The next level up, OC-3, supports up to triple the bandwidth. Each optical carrier level increases by multiples of four, e.g., OC-3, OC-12, OC-48 and so on -- OC-24 is the only exception.

SONET benefits

Benefits of SONET include the following:

• High data rates.
• Large transmit distances.
• Support of multiple data types, such as data, voice and video.
• Can carry high-level protocols, such as IP.
• Defines interoperability standards for organizations.

One disadvantage of SONET, however, is its high cost.

SONET vs. SDH

Synchronous Digital Hierarchy (SDH) is the international equivalent of SONET. SONET and SDH are similar standards both used for the same reason. However, the basic unit for SDH is Synchronous Transport Module level 1 (STM-1), instead of SONET's optical carrier levels.

SDH is an International Telecommunications Union standard, and it can work with SONET line rates. However, SONET and SDH have different structures that restructure data. SDH frames comprise 19,440 bits and use STM, while SONET frames comprise 6,480 bits and use STS.

Editor's note: This article was revised to improve the reader experience.