Synchronous Optical Network

Introduction to SONET

Synchronous optical network (SONET) is a standard for optical telecommunications transport. It was formulated by the ECSA for ANSI, which sets industry standards in the United States for telecommunications and other industries. The comprehensive SONET/synchronous digital hierarchy (SDH) standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.

The increased configuration flexibility and bandwidth availability of SONET provides significant advantages over the older telecommunications system. These advantages include the following:

  • reduction in equipment requirements and an increase in network reliabilit

  • provision of overhead and payload bytes—the overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalization

  • definition of a synchronous multiplexing format for carrying lower level digital signals (such as DS–1, DS–3) and a synchronous structure that greatly simplifies the interface to digital switches, digital cross-connect switches, and add-drop multiplexers

  • availability of a set of generic standards that enable products from different vendors to be connected

  • definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates

In brief, SONET defines optical carrier (OC) levels and electrically equivalent synchronous transport signals (STSs) for the fiber-optic–based transmission hierarchy.


Before SONET, the first generations of fiber-optic systems in the public telephone network used proprietary architectures, equipment, line codes, multiplexing formats, and maintenance procedures. The users of this equipment—regional Bell operating companies and interexchange carriers (IXCs) in the United States, Canada, Korea, Taiwan, and Hong Kong—wanted standards so that they could mix and match equipment from different suppliers. The task of creating such a standard was taken up in 1984 by the ECSA to establish a standard for connecting one fiber system to another. This standard is called SONET.

Synchronization of Digital Signals

To understand the concepts and details of SONET correctly, it is important to be clear about the meaning of synchronous, asynchronous, and plesiochronous.

In a set of synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one primary reference clock (PRC). The accuracy of the PRC is better than ±1 in 1011 and is derived from a cesium atomic standard.

If two digital signals are plesiochronous, their transitions occur at almost the same rate, with any variation being constrained within tight limits. For example, if two networks must interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there is a difference between one clock and the other. This is known as a plesiochronous difference.

In the case of asynchronous signals, the transitions of the signals do not necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous.

Basic SONET Signal

SONET defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing and offers end-to-end network management.

The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as synchronous transport signal–level 1, or simply STS–1, which operates at 51.84 Mbps. Higher-level signals are integer multiples of STS–1, creating the family of STS–N signals in Table 1. An STS–N signal is composed of N byte-interleaved STS–1 signals. This table also includes the optical counterpart for each STS–N signal, designated optical carrier level N (OC–N).

Synchronous and nonsynchronous line rates and the relationships between each are shown in Tables 1 and 2.

Table 1. SONET Hierarchy

SignalBit Rate (Mbps)Capacity
STS–1, OC–151.84028 DS–1s or 1 DS–3
STS–3, OC–3155.52084 DS–1s or 3 DS–3s
STS–12, OC–12622.080336 DS–1s or 12 DS–3s
STS–48, OC–482,488.3201,344 DS–1s or 48 DS–3s
STS–192, OC–1929,953.2805,376 DS–1s or 192 DS–3s
Note: STS = synchronous transport signal; OC = optical carrier

Table 2. Nonsynchronous Hierarchy

SignalBit Rate (Mbps)Channels
DS–00.0641 DS–0
DS–11.54424 DS–0s
DS–26.31296 DS–0s
DS–344.73628 DS–1s