Page 307 - Optical Communications Essentials

Optical Networks

Optical Networks 297

have lower-capacity localized OC-12 or OC-3 rings or chains attached to them,
thereby providing the possibility of attaching equipment that has an extremely
wide range of rates and sizes. Each of the individual rings has its own failure-
recovery mechanisms and SONET/SDH network management procedures.
An important SONET/SDH network element is the add/drop multiplexer
(ADM). This piece of equipment is a fully synchronous, byte-oriented multiplexer
that is used to add and drop subchannels within an OC-N signal. Figure 17.14
shows the functional concept of an ADM. Here various OC-12 and OC-3 traffic
streams (shown as solid and dashed lines, respectively) are multiplexed into one
OC-48 stream. Upon entering an ADM, these subchannels can be dropped indi-
vidually by the ADM, and others can be added. For example, in Fig. 17.14, one
OC-12 and two OC-3 channels enter the leftmost ADM as part of an OC-48
channel. The OC-12 is passed through, and the two OC-3 channels are dropped
by the first ADM. Then two more OC-12s and one OC-3 are multiplexed
together with the OC-12 channel that is passing through, and the aggregate
(partially filled) OC-48 is sent to another ADM node downstream.
The SONET/SDH architectures also can be implemented with multiple wave-
lengths. For example, Fig. 17.15 shows a dense WDM deployment on an OC-192

ADM ADM ADM
OC-12
OC-3
OC-3
OC-48 OC-48 OC-48
pipe pipe pipe
Added and dropped channels
Figure 17.14. Functional concept of an add/drop multiplexer for SONET/SDH
applications.

SONET transmitter shelf SONET receiver shelf
WDM WDM
mux demux
VOA Optical amplifiers
OC-192 OC-192
OC-192
OC-192
TX RX
TX
RX
λ 1 λ 1
• •
• •
• •
VOA
OC-192 OC-192
OC-192
OC-192
TX RX
TX
RX
λ n λ n
Figure 17.15. Dense WDM deployment of n wavelengths in an OC-192 trunk
ring.
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