Synchronous Optical Network (SONET)

Definition of Synchronous Optical Network (SONET) in The Network Encyclopedia.

What is Synchronous Optical Network (SONET)?

A physical layer specification for broadband synchronous transmission of voice, video, and data over long distances of fiber-optic cabling at speeds of more than 1 Gbps. Synchronous Optical Network (SONET) networks can carry voice, video, and data simultaneously and are often used by telecommunications providers to provide the underlying transport mechanism for Asynchronous Transfer Mode (ATM) networking in internal telco and long-distance carrier networks. SONET can also be used as the underlying transport for Fiber Distributed Data Interface (FDDI), Integrated Services Digital Network (ISDN), and Switched Multimegabit Data Services (SMDS) communication.

SONET was developed by Bellcore in the mid-1980s and has been standardized by the American National Standards Institute (ANSI). A European version called Synchronous Digital Hierarchy (SDH), which has been standardized by the International Telecommunication Union (ITU), is essentially equivalent to SONET.

How It Works

SONET is built from multiplexed DS0, DS1, or DS3 digital signal channels using optical time-division multiplexing (TDM) to form a single Synchronous Transport Signal (STS) link for communication. The basic SONET transmission rate is 810 bytes transmitted every 125 microseconds, and frames are transmitted whether or not a payload (data) is present. A standard STS-1 SONET data path thus consists of 810 DS0 channels, of which 783 are used for data transmission and 27 are used for framing, error correction, format identification, and other forms of overhead.

SONET is usually implemented as a dual-ring topology to provide redundancy and fault tolerance. These rings are usually self-healing within 50 milliseconds of a break. SONET speeds are classified by one of the following:

  • Electrical signal speeds called Synchronous Transport Signal (STS)
  • Optical carrier line speeds called optical carrier (OC)

The following table shows the currently defined SONET speeds.

SONET Speeds

Electrical Signal Optical Carrier Speed
51.48 Mbps
155.52 Mbps
622.08 Mbps
1.24 Gbps
2.48 Gbps
9.95 Gbps