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A SONET Overview
A-4 Issue 3 June 2001
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-5 shows these defined layers in a signal path. They include:
â– 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.
â– 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.
â– 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-5. Section, Line, and Path Definitions
DS1s
DS3
Digital
Multiplexer
Lightwave
Terminating
Equipment
Sections
Line
Path
DS1s
DS3
Digital
Multiplexer
Lightwave
Terminating
Equipment
Lightwave Repeaters
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