Page 90 - Optical Switching And Networking Handbook

04_200023_CH03/Batesx 1/17/01 9:42 AM Page 75

SONET 75

Figure 3-17 P
An OC-3c a
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Overhead for an STS-3c v Payload is one contiguous piece
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Add-Drop Multiplexing: A SONET
Benefit

Another major benefit of the SONET specification is the ability to
perform add-drop multiplexing. Even though network elements are
compatible at the OC-n level, they may still differ from vendor to
vendor. SONET does not attempt to restrict all vendors to providing
a single product, nor does it require that they produce one of every
type out there. One vendor may offer an add-drop multiplexer with
access to the DS-1 level only, whereas another may offer access to
DS-1 and DS-3 rates. The benefit of an add-drop multiplexer on a
wide-area network (WAN) is to drop (demultiplex) only those por-
tions of the optical stream required for a location and let the rest
pass through without the demultiplexing process. It would be
extremely inefficient to have to demultiplex an entire OC-12 stream,
for example, only to drop out one DS-1. The ability to extract only
what is necessary helps to prevent errors, loss of data, and other
delays inherent to older technologies. The add-drop multiplexer
makes this attractive for carriers to use in rural areas, where they
may bundle many lower-speed communications channels onto a sin-
gle OC-1 or OC-3 to carry the information back to the central metro-
politan area. Moreover, beyond just dropping a digital signal out of a
higher-speed OC-n, the carrier can fill in what has been vacated (for
example, if a DS-1 is dropped off along the optical path, a new DS-1
can be multiplexed back into the OC-3 in its place). This enables the
carriers considerable flexibility. Figure 3-18 shows an add-drop mul-
tiplexer. Here, portions of the bandwidth can be dropped off and