Understanding SONET/ SDH
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
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
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.
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.
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.
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.
1. Introduction to SONET (Top
of this page)
2. SONET frame structure
3. Transport Overhead
4. Scrambling and
5. Pointer Processing