Introduction to Sdh

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  • Topic: Synchronous optical networking, Multiplexing, Bit
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  • Published : January 9, 2013
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Introduction to the Synchronous Digital
Hierarchy (SDH)

T he History of Digital Transmission

’70s - introduction of PCM into Telecom networks
32 PCM streams are Synchronously Multiplexed to 2.048
Mbit/s (E1)
Multiplexing to higher rates via PDH
1985 Bellcore proposes SONET
1988 SDH standard introduced.

PDH: Plesiochronous Digital Hierarchy

Multiplex levels:
2.048 Mbit/s
8.448 Mbit/s
34.368 Mbit/s
139.264 Mbit/s

Uses Positive justification to adapt frequency differences
Overheads: CRC
Defects: LOS, LOF, AIS

Plesiochronous Multiplexing

Before SDH transmission networks were based on the
PDH hierarchy.
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
Japan

North
America

Europe

274.176 M bit/s

1 39.264Mbit/s

3 97.2 M bit/s
x4
97.728 M bit/s
x3

x6

32.084 M bit/s

44.738 M bit/s

x5

x7

6.312 M bit/s

6.312 Mbit/s
x4

x4
34.368 Mbit/s
x4
8.448 M bit/s

x4
x4

1.544 M bit/s

2.048 M bit/s

PDH Multiplexing

PDH Multiplexing of 2 Mbit/s to 140 Mbit/s requires 22
PDH multiplexers:
16 x 2DME
4 x 3DME
1 x 4DME

Also a total of 106 cables required.
2 Mb it/s

1

8 Mbi t/s

2
D
M
E

3 4 Mb it/s

140 Mbi t/s

3
D
M
E
4
D
M
E

64

2
D
M
E

3
D
M
E

PDH Add/Drop

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 Rates

SDH is a transport hierarchy based on multiples of 155.52
Mbit/s
The basic unit of SDH is STM-1:
STM-1

=

155.52 Mbit/s

STM-4

=

622.08 Mbit/s

STM-16

=

2588.32 Mbit/s

STM-64

=

9953.28 Mbit/s

Each rate is an exact multiple of the lower rate therefore
the hierarchy is synchronous.

SDH Hierarchy

SDH defines a multiplexing hierarchy that allows all
existing PDH rates to be transported synchronously.
The following diagram shows these multiplexing paths:
x1
STM-N

AUG

AU-4

139264 kb/s

VC-4

C-4
x3

xN

x1

x3

TU-3

TUG-3

AU-3

HOP

VC-3

C-3

VC-3

44736 kb/s
(DS3)
34368 kb/s

x7
x7

x1
TUG-2

TU-2

VC-2

C-2

TU-12

VC-12

C12

x3

6312 kb /s
(DS2)
LOP
204 8 kb/ s

x4
TU-11

VC-11

C-11
154 4 kb/ s
(DS1)

Example: Multiplex path for the E1

xN
STM-N

x3
AUG

AU-4

VC-4

x7
TUG-3

x3
TUG-2

TU-12

VC-12

C-12

Section overhead
Pointer processor
HO path overhead
Pointer processor
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
frame.
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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