Introduction to the Synchronous Digital
T he History of Digital Transmission
’70s - introduction of PCM into Telecom networks
32 PCM streams are Synchronously Multiplexed to 2.048
Multiplexing to higher rates via PDH
1985 Bellcore proposes SONET
1988 SDH standard introduced.
PDH: Plesiochronous Digital Hierarchy
Uses Positive justification to adapt frequency differences
Defects: LOS, LOF, AIS
Before SDH transmission networks were based on the
Plesiochronous means nearly synchronous.
2 Mbit/s service signals are multiplexed to 140 Mbit/s for
transmission over optical fiber or radio.
Multiplexing of 2 Mbit/s to 140 Mbit/s requires two
intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s.
Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex
equipment known as 2, 3 and 4 DME.
Alarm and performance management requires separate
equipment in PDH.
PDH vs. SDH Hierarchy
PDH transmission rates:
SDH is designed to unify all transmission rates into a
single Mapping hierarchy
274.176 M bit/s
3 97.2 M bit/s
97.728 M bit/s
32.084 M bit/s
44.738 M bit/s
6.312 M bit/s
8.448 M bit/s
1.544 M bit/s
2.048 M bit/s
PDH Multiplexing of 2 Mbit/s to 140 Mbit/s requires 22
16 x 2DME
4 x 3DME
1 x 4DME
Also a total of 106 cables required.
2 Mb it/s
8 Mbi t/s
3 4 Mb it/s
140 Mbi t/s
If a small number of 2 Mbit/s streams passing through a
site need to be dropped then in PDH this requires large
amount of equipment to multiplex down to 2Mbit/s.
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 is a transport hierarchy based on multiples of 155.52
The basic unit of SDH is STM-1:
Each rate is an exact multiple of the lower rate therefore
the hierarchy is synchronous.
SDH defines a multiplexing hierarchy that allows all
existing PDH rates to be transported synchronously.
The following diagram shows these multiplexing paths:
6312 kb /s
204 8 kb/ s
154 4 kb/ s
Example: Multiplex path for the E1
HO path overhead
LO path overhead
T ransport of PDH payloads
SDH is essentially a transport mechanism for carrying a
large number of PDH payloads.
A mechanism is required to map PDH rates into the STM
This function is performed by the container (C).
A PDH channel must be synchronised before it can be
mapped into a container.
The synchroniser adapts the rate of an incoming PDH
signal to SDH rate.
SDH and non-synchronous signals
At the PDH/SDH boundary Bit stuffing is performed...
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