Synchronous Optical Network (SONET)

Specific Instructional Objectives

At the end of this lesson the student will be able to:

·         Explain the operation of a SONET network

·         Explain the function of different SONET layers

·         Specify SONET frame format


To satisfy the requirements of ever increasing data rate for diverse applications, ANSI developed a standard known as Synchronous Optical Network (SONET) by utilizing the enormous bandwidth of optical fibre. Another very similar standard developed by ITU-T is known as Synchronous Digital Hierarchy (SDH). It is a synchronous TDM system controlled by a master clock, which adds predictability. 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 reliability.

·         Provision of overhead and payload bytes – the overhead bytes permit management of the payload bytes on an individual basis and facilitate centralized fault sectionalisation

·         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.

Synchronization of Digital Signals

To understand the concepts and details of SONET correctly, it is important to follow 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 delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one primary reference clock (PRC).

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 primary reference clocks (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 transport signals and optical carriers

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