Synchronous Optical Network (SONET)

Synchronous Optical Network (SONET) is a standard for synchronous data transmission on optical fibers. In other words, SONET is a standardized digital communication protocol. SONET can be utilized to transmit and multiplex multiple data streams across a fiber optic cable.

SONET was developed by the American National Standards Institute (ANSI) in the 1980s and was originally made for public telephone networks. Today SONET acts as a standard so that digital networks can interconnect and that existing conventional transmission systems can take advantage of optical media through tributary attachments. Backbone carrier networks will typically utilize SONET. Typically, telecommunication companies will share data over the line of a fiber optic cable instead of going through the more expensive process of digging trenches to bury new cables. Data is multiplexed by separating the cable into separate channels. The speed of data transmission is comparable to Gigabit Ethernet speeds.

The network elements defined in SONET include the STS multiplexer, STS demultiplexer, regenerator and the add/drop multiplexer. The STS multiplexer is the process that multiplexes signals and converts electrical signals to optical ones. STS demultiplexer condenses signals and converts optical signals back to electrical signals. Regenerators increase incoming optical signals, allowing them to travel farther. The add/drop multiplexer enables a signal to be added or removed from a source.

SONET connections are broken down between sections, lines and paths. A section is the part of a network which connects two devices. The line connects two multiplexers, and the path is the network end-to-end.  

SONET also defines four different layers, the path, line, section and photonic layers. The path layer moves signals from its source to its destination. The line layer is where the signal moves across the cable. The section layer defines the movement of signals across cables. The photonic layer is the specification for 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 approaching in the 30 gigabits per second range are possible.

Base units of SONET are defined as Optical Carrier level-1, or OC-1. OC-1 supports up to 51.84 megabytes per second (Mbps). The next level up, OC-3, supports up to triple the bandwidth. Each level increases by multiples of four. OC-3, OC-12, OC-24, OC-48 can be used as examples. The set of multiples of the base rate known as "Optical Carrier levels (OCx)."

SONET standards are specified in ANSI T1.105 and T1.117.


SONET is seen to have multiple characteristics that are considered advantageous, such as:

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

The one main disadvantage of utilizing SONET, however, is that it is high in cost.


Synchronous digital hierarchy (SDH) is the international equivalent of SONET. SONET and SDH are very similar standards made for the same reason. However, the basic unit for SDH is called the synchronous transmission module level-1(STM-1), as compared to SONET’s Optical Carrier level.

SDH is an International Telecommunications Union (ITU) standard and can work with SONET line rates. However, SONET and SDH have differing structures when restructuring data. SDH frames are made out of 2,430 bytes and use Synchronous Transport Module (STM), while SONET frames are made up of 6,480 and use Synchronous Transport Signal (STS).

This was last updated in June 2019

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