The Synchronous optical network, commonly known as SONET, is a standard for communicating digital information using lasers or light emitting diodes (LEDs) over optical fiber as defined by GR-253-CORE from Telcordia. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic and to allow for interoperability between equipment from different vendors.

The more recent Synchronous Digital Hierarchy (SDH) standard developed by ITU (G.707 and its extension G.708) is built on experience in the development of SONET. Both SDH and SONET are widely used today; SONET in the U.S. and Canada, SDH in the rest of the world. SDH is growing in popularity and is currently the main concern with SONET now being considered as the variation.

SONET differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus an entire network can operate synchronously, though the presence of different timing sources allow for different circuits within a SONET signal to be timed off of different clocks (through the use of pointers and buffers.) SDH was made possible by the existence of atomic clocks.

Both SONET and SDH can be used to encapsulate earlier digital transmission standards, such as the PDH standard, or used directly to support either ATM or so-called Packet over SONET networking. As such, it is inaccurate to think of SONET as a communications protocol in and of itself, but rather as a generic and all-purpose transport container for moving both voice and data.

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Synchronous Optical Network / Synchronous Digital Hierarchy
Optical Networks
► Faster than conventional networks because of photons (low weigh than electrons, no charge hence do not interfere with other)
► WDM/DWDM Networks
► Prior to development of Synchronous Optical Networks (SONET) and Synchronous Digital Hierarchy (SDH), optical network solution in WAN were:
► Vendor specific
► Haphazard in data support
► Without clear migration paths
► With the invention of SONET/SDH,
► all such problems solved
► Vendor interoperability established up to certain extent
► SONET/SDH
► Initially used for voice communication
► But with the arrival of Internet, it play the important role in transmission of IP packets and others
Issue with SONET/SDH
► Single Wavelength Technology
► 1.31 mm
► No Wavelength Division Multiplexing
Hence, at a time only one data stream transmission is possible
► At the time of its standardisation (1988), there was no such high demand for bandwidth
► Most of the network were voice centric
► As present/future networks are going to use multi-wavelength technologies such as WDM or DWDM:
 What will be the future of SONET/SDH?
What is SONET/SDH?
► Defines hierarchical set of transmission rates and transmission formats for optical fiber systems for networking applications
► SONET is
 Synchronous Optical Network
 Developed originally by client and standardized by ANSI
 For North America
► SDH is
 Synchronous Digital Hierarchy
 Developed by ITU-T (previously CCITT)
 For European and other Countries in the world

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Abstract
This paper explains about SONET –Synchronous Digital Network, that has been greeted with unparalleled enthusiasm throughout the world. It also explains how it came into existence and in which way it differs from others. What does synchronous mean?” Bits from one telephone call are always in the same location inside a digital transmission frame”.
This material is assumed to be comfortable to the reader as the basic concepts of a public telecommunications network, with its separate functions of transmission and switching, and is assumed to be aware of the context for the growth of broadband traffic.
In the early 1970’s digital transmission systems began to appear, utilizing a method known as Pulse Code Modulation (PCM), first proposed by STC in 1937. As demand for voice telephony increased, and levels of traffic in the network grew ever higher, it became clear that standard 2 Mbit/s signal was not sufficient. To cope with the traffic loads occurring in the trunk network. As the need arose, further levels multiplexing were added to the standard at much higher speed and thus SONET came into existence. For the first time in telecommunications history there will be a worldwide, uniform and seamless transmission standard for service delivery. SONET provides the capability to send data at multi-gigabit rate over today’s single-mode fiber-optic links
As end-users become ever more dependent on effective communications, there has been an explosion in the demand for sophisticated telecom services. Services such as videoconferencing remote database access, and multimedia file transfer require a flexible network with the availability of virtually unlimited bandwidth. The complexity of the network, means that network operators are unable to meet this demand. At present SONET is being implemented for long-haul traffic, but there is no reason it cannot be used for short distances.
SYNCHRONOUS OPTICAL NETWORK (SONET)
Definition:
Synchronous optical network (SONET) is a standard for optical telecommunications transport formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), which sets industry standards in the U.S. for telecommunications and other industries. The comprehensive SONET standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.
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 reliability.
• 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.
SONET Layers
SONET has four optical interface layers. They are:
• Path Layer,
• Line Layer,
• Section Layer,
• Photonic Layer.
Path Layer : The Path Layer deals with the transport of services between the PTE. The main function of the Path Layer is to map the signals into a format required by the line layer . Its main functions are :
• Reads ,
• Interprets ,
• Modifies the path overhead for the performance and automatic protection switching.
Line Layer : The line layer deals with the transport of the path layer payload and its overhead across the physical medium. The main function of the line layer is to provide synchronization and to perform multiplexing for the path layer . Its main functions are :
• Protecting Switching ,
• Synchronization ,
• Multiplexing ,
• Line maintenance ,
• Error Monitoring .
Section Layer : The section layer deals with the transport of an STS-N frame across the physical medium. Its main functions are :
• Framing ,
• Scrambling ,
• Error Monitoring ,
• Section Maintenance.
Photonic Layer : The Photonic layers mainly deals with the transport of bits across the physical medium. Its main functions are :
• Wavelength ,
• Pulse Shape ,
• Power Levels.