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Understanding SONET/ SDH
By
K Surya Prakash
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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
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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
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Line Rate
(Mbps)
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SONET
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SDH
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Optical
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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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