09-04-2012, 05:00 PM
Introduction to the Synchronous Digital Hierarchy
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The History of Digital Transmission
1) ’70s - introduction of PCM into Telecom networks.
2) 32 PCM streams are Synchronously Multiplexed to 2.048 Mbit/s (E1).
3) Multiplexing to higher rates via PDH
4) 1985 Bellcore proposes SONET
1988 SDH standard introduced.
What is SDH?
The basis of Synchronous Digital Hierarchy (SDH) is
synchronous multiplexing - data from multiple tributary
sources is byte interleaved.
In SDH the multiplexed channels are in fixed locations
relative to the framing byte.
Demultiplexing is achieved by gating out the required
bytes from the digital stream.
This allows a single channel to be ‘dropped’ from the data
stream without demultiplexing intermediate rates as is
required in PDH.
SDH Hierarchy
SDH defines a multiplexing hierarchy that allows all
existing PDH rates to be transported synchronously.
The following diagram shows these multiplexing paths:
Basic SDH frame structure
STM-N frame structure is shown in the Figure below. The
three main areas of the STM-N frame are indicated:
SOH;
Administrative Unit pointer(s);
Information payload.
SDH layers
The following diagram shows 2Mbit/s multiplexed to STM-
.
The transmission path passes through five layers in this
connection.