Introduction to SONET

Short for Synchronous Optical Network, a standard for connecting fiber-optic transmission systems. SONET was proposed by Bellcore in the middle 1980s and is now an ANSI standard. It remains in widespread use today. In a nutshell, SONET allows multiple technologies and vendor products to interoperate by defining standard physical network interfaces.

SONET defines interface standards at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems.

The international equivalent of SONET, standardized by the ITU, is called SDH.

The SONET/SDH Digital Hierarchy

The basic foundation of SONET consists of groups of DS-0 signals (64Kbits/sec) that are multiplexed to create a 51.84Mbit/sec signal, which is also known as STS-1 (Synchronous Transport Signal). STS-1 is an electrical signal rate that corresponds to the Optical Carrier line rate of OC-1, SONET's building block. Subsequent SONET rates are created by interleaving (at the byte level) STS-1 signals to create a concatenated, or linked, signal. For example, three STS-1 frames can form an STS-3 frame (155Mbits/sec). Rates above STS-3 can be created by either directly multiplexing STS-1 signals or by byte-interleaving STS-3 signals. The following table lists the hierarchy of the most common SONET/SDH data rates:

Optical LevelElectrical LevelLine Rate (Mbps)Payload Rate (Mbps)Overhead Rate (Mbps)SDH Equivalent
OC-1 STS-1 51.840 50.112 1.728 -
OC-3 STS-3 155.520 150.336 5.184 STM-1
OC-12 STS-12 622.080 601.344 20.736 STM-4
OC-48 STS-48 2488.3202405.37682.944 STM-16

The "line rate" refers to the raw bit rate carried over the optical fiber. A portion of the bits transferred over the line are designated as "overhead". The overhead carries information that provides OAM&P (Operations, Administration, Maintenance, and Provisioning) capabilities such as framing, multiplexing, status, trace, and performance monitoring. The "line rate" minus the "overhead rate" yields the "payload rate" which is the bandwidth available for transferring user data such as packets or ATM cells.

The SONET/SDH level designations sometimes include a "c" suffix (such as "OC-48c"). The "c" suffix indicates a "concatenated" or "clear" channel. This implies that the entire payload rate is available as a single channel of communications (i.e. the entire payload rate may be used by a single flow of cells or packets). The opposite of concatenated or clear channel is "channelized". In a channelized link the payload rate is subdivided into multiple fixed rate channels. For example, the payload of an OC-48 link may be subdivided into four OC-12 channels. In this case the data rate of a single cell or packet flow is limited by the bandwidth of an individual channel.

Protocol Layers

The SONET/SDH standard includes a definition of a transmission protocol stack which solves the operation and maintenance problems often found when dealing with networks that have component streams lacking a common clock.

The photonic layer is the electrical and optical interface for transporting information over fiber optic cabling. It converts STS electrical signals into optical light pulses (and vice versa, at the receiving end). The section layer transports STS frames over optical cabling. This layer is commonly compared with the Data-Link layer of the OSI model, which also handles framing and physical transfer.

The line layer takes care of a number of functions, including synchronization and multiplexing for the path layer above it. It also provides automatic protection switching, which uses provisioned spare capacity in the event of a failure on the primary circuit.

The highest level, the path layer, takes services such as DS-3, T1, or ISDN and maps them into the SONET/SDH format. This layer, which can be accessed only by equipment like an add/drop multiplexer (a device that breaks down a SONET/SDH line into its component parts), takes care of all end-to-end communications, maintenance, and control.


SONET/SDH supports several topologies, including point to point, a hub and spoke star configuration, and the ring topology. The ring topology, which is by far the most popular, has been used for years by such network technologies as FDDI and Token Ring and has proven quite robust and fault-tolerant. A SONET/SDH ring can contain two pairs of transmit and receive fibers. One pair can be designated as active with the other one functioning as a secondary in case of failure. SONET/SDH rings have a "self-healing" feature that makes them even more appealing for long distance connections from one end of the country to another.

One of SONET/SDH's most interesting characteristics is its support for a ring topology. Normally, one piece of fiber -- the working ring -- handles all data traffic, but a second piece of fiber -- the protection ring remains on standby. Should the working ring fail, SONET/SDH includes the capability to automatically detect the failure and transfer control to the protection ring in a very short period of time, often in a fraction of a second. For this reason, SONET/SDH can be described as a self-healing network technology.

Rings normally will help SONET/SDH service to reach the "five nines" availability level. However, the usefulness of rings also depends on their physical location. If the rings are located next to each other, then if a back-hoe from a construction company takes out one of your fibers, it is quite likely that the second one will go as well. Thus, your rings should be physically separated from each other as much as possible in order to achieve high uptime.

Further Reading

The SONET Pocket Reference