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Copyright 2007

S.D.H.

SYNCHRONOUS DIGITAL HIERARCHY

In the earlier years the development of multiplexing especially of optical transmissions used a technique known as Piesiouchronus Digital Hierarchy (P.D.H.). Piesiouchronus meaning nearly synchronous and has the major disadvantage that to access a traffic path, the whole stream must be de-multiplexed. This results in a situation with multiplexers being used in large quantities at drop off and collection points in a ring. Even with this large handicap, P.D.H. is still in major use all over the word.

To overcome this disadvantage a number of designs to the multiplexing technique where implemented along with general improvements to come up with the S.D.H. standards. With this a single multiplexer can be inserted into a ring to access any quantity of the traffic paths in the main stream.

Within the standard, a number of multiplexing rates where defined. These being STM-1, STM-4, STM-16, STM-64, giving speeds of 155Mbit/s, 622Mbit/s, 2.5Gbit/s and 10Gbit/s. Each STM-1 stream can be accessed as 63 x VC-12 (giving 63 x 2Mbit/s) access, 3 x VC3 (giving 3 x 45Mbit/s (34Mbit/s) ) access. Or more commonly as a mixture of VC-12 and VC3.

Also within the STM-1 stream are the overhead data paths, these are used for a multitude of reasons, including :

Mux and ring management, mux to mux messaging to controller the worker and protection paths, synchronisation parameters to ensure no traffic is lost, transparent ports used for any point to point management (typically third party), etc.

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