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                                                 Understanding SONET/ SDH

By  K Surya Prakash

 

          The evolution of optical fiber as high-speed, low-cost transmission medium led to the Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH).

           In this article, you will learn basics of SONET/ SDH,  differences between them and structure of SONET frame.


Next: ( SONET frame structure ) >

Table of Contents

Introduction to SONET/ SDH          

SONET was developed in the United States through ANSI T1X1.5 committee. ANSI work commenced in 1985 with the CCITT (now ITU) initiating a standardization effort in 1986. The US wanted a data rate close to 50Mbps. But the Europeans wanted the data rate to be around 150 Mbps. A compromise was reached and the US data rates were made subset of ITU specification, known formally as Synchronous Digital Hierarchy (SDH).

          SONET/SDH networks are configured as linear networks, where SONET/SDH nodes knows as Add Drop Multiplexers (ADMs) are hooked together in a line as shown in figure-1. There may be two or four fibers between the two consecutive ADMs with one set serving as “protection” or “back up”.

           Add/drop multiplexers (ADMs) are places where traffic enters and leaves. The traffic can be at various levels in the SONET/ SDH hierarchy (see Table-1).  We will learn more about ADMs later.

    
Figure-1

           Also SONET network elements can receive signals from a variety of facilities such as DS1, DS3, ATM, Internet, and LAN/MAN/WAN. They can also receive signals from a variety of network topologies. We will study how all this is done in subsequent sections.  In addition SDH signals my also be connected with a SONET and vice versa. In this case, circuitry translates specific SDH information into its SONET equivalent, and vice versa.

SONET/SDH Rates:

            The SONET frame in its electrical nature is called Synchronous Transport Signal-level N (STS-N). The SDH equivalent is called Synchronous Transport Module level N (STM-N).  After conversion into optical pulses it is known as Optical Carrier level N. The line rates for different levels of SONET and SDH signals are shown in Table-1 below.

 Table-1:

Signal Designation

Line Rate
(Mbps)

SONET

SDH

Optical

STS-1

STS-3

STS-12

STS-48

STS-192

STM-0

STM-1

STM-4

STM-16

STM-64

OC-1

OC-3

OC-12

OC-18

OC-192

51.85

155.52

622.08

2488.32

9953.28

           You need not worry about the different levels of SONET /SDH at this stage. I had given detailed explanation of these levels later. I feel, to understand SDH easily, it is better to have knowledge of SONET initially. This is the reason I devoted major portion of this article to SONET. Except in terms of terminology there are no major differences between the two. But wherever there are differences I had pointed them out.

Next Chapters:

1. Introduction to SONET (Top of this page)

2. SONET frame structure

3. Transport Overhead

4. Scrambling and Descrambling

5. Pointer Processing

 

 

 

 

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