Introduction to Sdh

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?