Page 203 - Optical Switching And Networking Handbook

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188 Chapter 8

Add-Drop Multiplexing: A
SONET/SDH Application

A major application for using SONET/SDH is the ability to perform
add-drop multiplexing. Although network elements are compatible
at the OC-n/STM-n level, they may still differ from vendor to vendor.
SONET/SDH 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/E-1 level only, whereas another may offer access
to DS-1/E-1 and DS-3/E-3 rates. The benefit of an add-drop multi-
plexer on a wide-area network (WAN) is to drop (demultiplex) only
the portions 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/STM-4,
for example, only to drop out one DS-1/E-1.The ability to extract only
what is necessary helps to prevent errors, loss of data, and other
delays inherent in 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/STM-1 to carry the information back to a central
metropolitan area. Moreover, beyond just dropping a digital signal
out of a higher-speed OC-n/STM-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 provides the carriers with considerable flexibility. Figure 8-1
shows an add-drop multiplexer. Here, portions of the bandwidth can
be dropped off and additional new signals can be added in place of
the data stream dropped out of the higher-speed signal. A single-
stage add-drop multiplexing function can multiplex various inputs
into an OC-n signal. At an add-drop site, only those signals which
need to be accessed are dropped and inserted. The remaining traffic
continues through the network switching system without requiring
special processing. The figure shows a local traffic convention
whereby the necessary traffic is added or dropped as appropriate.