SONET (standing for Synchronous Optical Networking) is a standard for communicating digital information over optical fibre. It was developed to replace the PDH system for transporting large amounts of telephone and data traffic. The more recent Synchronous Digital Hierarchy (SDH) standard built on the experience of 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. (Berthold, Joseph E, 2007)
Synchronous networking differs from PDH in that the exact rates that are used to transport the data are tightly synchronized to network based clocks. Thus the entire network operates synchronously. 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. The basic SONET signal operates at 51.840 Mbit/s and is designated STS-1 (synchronous transport signal one). The STS-1 frame is the basic unit of transmission in SONET.
The two major components of the STS-1 frame are the transport overhead and the synchronous payload envelope (SPE). The transport overhead (27 bytes) comprises the section overhead and line overhead. These bytes are used for signalling and measuring transmission error rates. The SPE comprises the payload overhead (9 bytes, used for end to end signalling and error measurement) and the payload of 774 bytes. The STS-1 payload is designed to carry a full DS-3 frame. The entire STS-1 frame is 810 bytes. The STS-1 frame is transmitted in exactly 125 microseconds on a fibre-optic circuit designated OC-1 (optical carrier one). In practice the terms STS-1 and OC-1 are used interchangeably.
Before SONET, fiber optic systems in the public telephone network employed many different proprietary architectures, equipment, line codes, multiplexing formats, and maintenance procedures. Users of fiber optic systems--including the Regional Bell Operating Companies and interchange carriers (IXCs) in the US, Canada, Korea, Taiwan and Hong Kong--needed a standard that would connect these proprietary systems' equipment to one another. (Berthold, Joseph E, 2007)
In 1984, the Exchange Carriers Standards Association (ECSA) formulated the required standard named SONET for the American National Standards Institute (ANSI), which is responsible for setting telecommunications industry standards in the US. In Europe, SONET is referred to as SDH (Synchronous Digital Hierarchy). The first level in the SDH (Synchronous Digital Hierarchy) is STM-1 (Synchronous Transport Mode 1) having a line data rate of 155 Mb/s approximately which is equivalent to SONET's STS-3c. The SDH or SONET are the communications protocols, as well as the generic all-purpose transport containers, for transmission of all types of digital data including voice, text, image and video. Unlike a typical frame-oriented data transmission, like in the ethernet networks where the header of the frame, payload and trailer (CRC data) is transmitted in a sequence, SONET is the part of header and the payload is interleaved on ...