Module 14 : SONET
Lecture : SONET
 

Each rate is an exact multiple of the lower rate ensuring that the hierarchy is synchronous. The network hierarchy is organized in a master-slave relationship with the lower level nodes receiving time signal from the higher level nodes. All clock level can be traced back to the primary reference clock (PRC), which is very stable.

  FRAMES IN SONET
 

As the bit rate of information is moving at very fast rate, a convention has to be applied distinct digital channels that have been multiplexed together can be distinguished. SONET uses the concept of framing to achieve this.
A framing bit can be thought of as a pointer or an address. As the line is moving fast, it would be easy to skew it a little to left or to right and the information would then get out of sequence. The extra bit of information creates a locator for the system.
SONET organizes data into 810-byte blocks called frames . The bytes are arranged as a two dimensional array of 9 rows and 90 columns. The data transmission is serial, i.e. starting with the extreme left byte ( first row, first column) we proceed to right, byte by byte, and reach the 90th element on the first row. The next byte assessesd is the first column of the second row and so on. Frames are sampled at the rate of 8000 frames per second, i.e., one frame is transmitted every 125 micro second. This determines the speed to be as follows :

$\displaystyle 8000 \times 810$

$\displaystyle =$

$\displaystyle 6,480, 000\ {\rm bytes/sec}$

 

 

$\displaystyle =$

$\displaystyle 6.48\times 10^6\times 8\ {\rm bits/sec}$

 

 

$\displaystyle =$

$\displaystyle 51.84\ {\rm Mb/sec}$

 


If one compares the above data rates with that given earlier for the telephone hierarchy, one observes that the SONET/SDH rates are about 10% higher. This is because of overheads required to administer and monitor signal transmission.
The frame has two main constituents. Each row has 3 bytes of transport overheads and and 87 bytes of payload. Thus there are $ 9\times 3 = 27$bytes of transport overheads and $ 9\times 87 = 783$payload bytes. Payload is the traffic that is transported through the SONET network.

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