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A SONET Overview
A-2 Issue 3 June 2001
Basic Purpose 10
The basic purpose of SONET is to provide a standard synchronous optical
hierarchy with sufficient flexibility to accommodate digital signals that currently
exist in today's network as well as those planned for the future.
SONET currently defines standard rates and formats and optical interfaces.
These and other related issues continue to evolve through the ANSI committees.
SONET ultimately will permit an optical midspan meet in a multivendor
environment.
The American National Standard defines the following:
â– Optical parameters (
ANSI
* T1.106-1988)
â– Electrical parameters (
ANSI
T1.102-1993 Draft)
â– Multiplexing schemes to map existing digital signals (for example, DS1,
DS2, and DS3) into SONET payload signals (
ANSI
T1.105-1991)
â– Criteria for optical line automatic protection switch (APS)
(
ANSI
T1.105-1991)
â– Overhead channels to support standard operation, administration,
maintenance, and provisioning (OAM&P) functions (
ANSI
T1.105-1991).
*
Technical Overview 10
SONET Signal Hierarchy 10
The SONET signal hierarchy is based on a basic "building block" frame called the
synchronous transport signal - level 1 (STS-1), as shown in Figure A-4 on the
following page. The STS-1 frame has a reoccurring rate of 8000 frames per
second. Each frame is 125 microseconds.
The STS-1 frame consists of:
â– 90 columns (each column is an 8-bit byte)
â– 9rows.
The STS-1 frame is transmitted serially starting from the left with row 1 column 1
on through column 90, then row 2 column 1 through 90, continuing on, row-by-
row, until all 810 bytes (9 X 90) of the STS-1 frame have been transmitted.
* Registered trademark of America National Standards Institute
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