363-206-295
A SONET Overview
A-4 Issue 1 December 1997
SONET Layers 1
SONET divides its processing functions into three layers. These three layers are
associated with equipment that reflects the natural divisions in network spans.
Figure A-2 shows these defined layers in a signal path. They include:
■ SECTION and Section Terminating Equipment - the transmission
spans between lightwave terminating equipment and the regenerators.
The spans between the regenerators are also considered sections.
Section terminating equipment provides regenerator functions and
terminates the section overhead to provide single-ended operations and
section performance monitoring.
■ LINE and Line Terminating Equipment - the transmission span between
terminating equipment (STS-1 cross-connections) that provides line
performance monitoring. If there are no intervening repeaters, the line
terminating equipment also functions as section terminating equipment.
■ STS-1 and VT Path and Path Terminating Equipment - the transmission
span for an end-to-end tributary (DS1 or DS3) signal that provides
functions including signal labeling and path performance monitoring for
signals as they are transported through a SONET network. STS-1 path
terminating equipment can also provide cross-connections for lower rate
(that is, DS1) signals. A virtual tributary (VT) is a sub-DS3 payload and is
described later in more detail.
Figure A-2. Section, Line, and Path Definitions
Lightwave Repeaters
DS1s
Equipment
Terminating
Lightwave
Multiplexer
Digital
DS3
DS1s
Path
Line
Sections
Equipment
Terminating
Lightwave
Multiplexer
Digital
DS3
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