Module 14 : SONET
Lecture : SONET
 

North America

Europe

0.064 (DS0)

0.064

1.544 (DS1)

2.048 (E1)

6.312 (DS2)

8.448 (E2)

44.736 (DS3)

34.368 (E3)

139.264 (DS4)

139.264 (E4)

   
  Principle of SONET/SDH
In a synchronous device the clock transitions occur precisely at the same rate. All signal transitions are fixed with
  reference to a very accurate atomic clock (such as Cs clock), called Primary Reference Clock (PRC). The accuracy of such a clock is one part in $ 10^{11}$or better. The advantage of a synchronous system is that multiple signals can be stacked without any need for bit stuffing.
In SDH, data from different sources are multiplexed in a way that the channels have fixed locations with respect to the
  framing byte.
As the location is fixed, it is not necessary to demultiplex while dropping a single channel from the stream.
 

The basic signal of SONET is synchronous transport signal, called STS-1 which operates at 51.84 Mbit/sec. After conversion to optical signals, STS- 1 is known as optical carrier, or OC-1 . The higher level signals are multiples of STS-1 signal and operate at multiples of base frequency. Thus STS-3 (or its optical equivalent OC-3) operates at a bit rate of 155.52 Mbps interleaving frames from three STS-1 signals. This STS-3/OC-3 is the base signal for SDH and is known as the synchronous transport module or STM-1 . The hierarchy for SONET/SDH hiherarchy is shown in the following table.

Signal Name

Data Rate

SONET

SDH

Optical

(Mbps)

STS-1

-

OC-1

51.84

STS-3

STM-1

OC-3

155.52

STS-12

STM-4

OC-12

622.08

STS-48

STM-16

OC-48

2488.32

STS-192

STM-64

OC-192

9953.28

STS-768

STM-256

OC-768

$ \sim$40 Gbps

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