Saturday, 16 March 2013

SDH Principles and Interview questions on SDH Multiplexing structure


Overview

     The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.Each container then has some control information known as the 'Path Overhead' (POH) and stuffing bits added to it. The path overhead bytes allow the system operator to achieve end to end monitoring of areas such as error indication, alarm indication and performance monitoring data. The container and the path overhead together form a 'Virtual Container' (VC).

    Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'. The VC and its relevant pointer together form a 'Tributary Unit' (TU).

       Tributary units are then multiplexed together in stages (Tributary User Group 2 (TUG-2) - Tributary User Group 3 (TUG-3) - Virtual Container 4 (VC-4)), to form an Administrative Unit 4 (AU-4). Additional stuffing, pointers and overheads are added during this procedure.This AU-4 in effect contains 63 x 2 Mbit/s channels and all the control information that is required.

    Finally, Section Overheads (SOH) are added to the AU-4.These SOH's contain the control bytes for the STM-1 section comprising of framing, section performance monitoring, maintenance and operational control information.An AU-4 plus its SOH's together form an STM-1 transport frame.

 Graphical SDH Multiplexing Structure


Diagram below shows full SDH Multiplexing structure. PDH signals enter on the right into the relevant container and progress across to the left through the various processes to form the STM frame.



  2 Mbit/s Multiplexing Structure


Let us see the multiplexing stages of  2 Mbit/s circuit. The relative bit rate and process is shown for each stage.



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7 comments:

  1. Kudos man, keep the good work up!

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  2. By far the best SDH principles description I've seen.

    ReplyDelete
  3. No other document on the internet has explained it this simple.

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  4. The best explanation ever. thanx a million

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  5. its great explanation but its not proper questions and Answers......

    ReplyDelete