Synchronous Digital Hierarchy (SDH)| What is SDH in Telecommunication Engineering?

  Synchronous Digital Hierarchy (SDH)| What is SDH in Telecommunication Engineering?

Synchronous Digital Hierarchy (SDH)



1. The objectives of SDH.

 2. The relative advantages of SDH when compared with PDH. 

3. Other features of SDH hierarchy.

Synchronous Digital Hierarchy (SDH), which can be termed as transport backbone of the broadband era. 

The Network Node Interface (NNI) operating in Synchronous Digital Hierarchy (SDH) recommended by ITU (T)/CCIR offers a clear solution and sets an international standard for high speed digital transmission. 

Evolution Current public telecommunication networks are based on the long established ITU (T) Plesiochronous Digital Hierarchy (PDH) of transmission which is not well suited to support the advanced signal processing and control techniques now possible and increasingly in demand. 

The SDH is designed to overcome the limitations of PDH. ITU (T) adopted SDH in 1988. 

The original version was known as SONET -– Synchronous Optical Network that was in use in North America 2 or 3 years prior to ITU (T) adopted it and changed the name as SDH. 

1984 : T1X1 American Standardization agency was made responsible for defining interfaces of optical lines for interconnection of high bit rates between operators. 

1985 : Feb 1985, Bell core introduced SONET (Sync Optical Network) concept of T1X1. It defined a new transmission hierarchy, which permits direct access to the lower level tributaries of a multiplex system. The activities of SONET are divided into 3 parts, Frame format, Optical interface and Measurement.

 1988 : Feb 1988. The ITU (T)’s G.707, G.708, G.709 recommendations for SDH were approved. Bit rate 155.520 mbps was standardized as basic frame of SDH and the structure remained compatible with utilization of the SONET frame at 51.840 mbps in USA

 1989 : The recommendations of ITU (T) working group XVIII were published in Blue Book 1989.

 1990 : New versions of the recommendations G.707, G.708, G.709 were approved basing on the decisions of European Telecommunication Standards Institute (ETSI) regarding multiplexing structure. Other recommendations concerning multiplexing equipment, optical interfaces, subscriber loop equipment, network management were approved. 

The recommendations for cross connection equipment, architecture of synchronous network, and performance data of the network were being drawn up. 

 The maximum band that can be provided by PDH is 565 mbps and the number of speech channels is 7680, and not in a position to meet any further demand. 

The stages of multiplexing and demultiplexing required for higher orders are more which contributes for higher costs, more maintenance where as in SDH system the whole job of multiplexing to higher orders is done by a single multiplexer, there by the cost is less, space occupied is less, maintenance is less. 

Bit interleaving multiplexing system adopted in PDH, scatters the bits at every stage, making the identification of the location of each bit very difficult and at the same time to bring them back to the original sample shape, as many stages of demultiplexing are to be carried out to return to its primary mux stage 


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