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Explain SONET/SDH – a communication protocol?

Today we have two standardized protocols to govern the transfer of a number of digital bit streams namely:
1. SONET or synchronous optical networking
2. SDH or synchronous digital hierarchy

– The bit stream is transferred over the optical fibers using either light emitting diodes (LEDs) or lasers i.e., the sources that emit highly coherent light.
– However, an electrical interface can also serve as a transmission medium at low level of the transmission rates.
– This method eventually replaced the PDH (plesiochronous digital hierarchy) system that was once used for the transmission of large amounts of data traffic and phone calls over a single fiber avoiding the problems with synchronization.
– SONET and SDH are actually the same and have been designed for circuit mode communications.
– However, their primary purpose is to support the uncompressed and real time circuit – switched voice.
– Earlier when the SONET/ SDH did not exist, the main problem was posed by the different synchronization sources of the various circuits; also this was the reason why the rate of operation as well as the phase of operation of every circuit was different.
– This problem was easily overcome using SONET/ SDH since it made possible to simultaneously transport the different circuits having different synchronization sources lying in the same framing protocol.
– Thus SONET/SDH is not just a communication protocol but also a transport protocol.
– Due to its two essential qualities it became the much favored option for the transport of fixed length cells:
1. Transport – oriented features and
2. Protocol neutrality
– These cells are nothing but the ATM (asynchronous transfer mode) frames.
– It saw quick evolution of the concatenated payload containers and mapping structures to make ATM connection transportation possible.
– SONET/ SDH are used all over the world.
– While SDH has found its use all over the world, SONET is confined to United States and Canada.
– Even though the SONET came in to existence before SDH, still SONET is considered to be a varied form of SDH.
– This is so because the SDH has got a great share of the world market.
– ETSI or the European telecommunications standards institute holds the credit for initially defining the SDH standard known as the ITU (international telecommunication union) standards.
– However ANSI (American national standards institute) and Telecordia together defined the SONET standards.
– The same functions are defined using different terms in SONET and SDH.
– This sometimes causes confusion and exaggerates the differences between them.
– So after setting out few exceptions, SONET began to be considered as a subset of SDH.
– SONET consists of a number of transport containers for delivering a number of things such as:
1. Variety of protocols
2. Telephony
3. Ethernet
4. ATM
5. TCP/ IP traffic

– SONET is not ideally connection – oriented and so is not a native communications protocol.
– However, the HTTP traffic void of connection is indeed transported within its frames.
– Both these protocols by structure are heavy in nature.
– The way in which the header is interleaved between data is quite complex but this only provides a unique data rate to data and ability to float around relative to the frame structure as well as rate.
– However, this causes the protocol to have very low latency.
– Data passing may experience a delay of 32 microseconds.
– Protocol standards allow for extra padding at various stages of the structure.
– Even though this increases the complexity further, it is still recommended since it improves the overall performance.
– Basic unit of transmission for both the protocols are as follows:
1. For SDH it is Synchronous transport module, level 1 (STM – 1). Operating speed is 155.520 MBPS.
2. For SONET the same unit is referred to as OC – 3c (if the signal is carried optically) or STS – 3c (if the signal is carried electronically.). Operating speed is same as that of the SDH.

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