Page 324 - Optical Communications Essentials

Network Management

314 Chapter Eighteen

Optical Optical Optical
mux amplifiers demux

Optical Optical
channels channels

OTS OTS OTS

OMS

OCh
Figure 18.7. The OMS layer represents a link carrying wavelengths between
multiplexers or OADMs. The OTS layer relates to a link between two optical
amplifiers.

multiplexers or OADMs. The optical transmission section (OTS) layer relates to
a link between two optical amplifiers.
The OCh is divided further into three sublayers, as shown in Fig. 18.6: the
optical channel transport unit (OTU), the optical channel data unit (ODU), and
the optical channel payload unit (OPU). Each of these sublayers has its own
functions and associated overhead, which are as follows:

■ The OPU frame structure contains the client signal payload and the overhead
necessary for mapping any client signal into the OPU. Mapping of client signals
may include rate adaptation of the client signal to a constant-bit-rate signal.
Examples of common signals are IP, various forms of Ethernet, ATM, Fibre
Channel, and SONET/SDH. The three payload rates associated with the OPU
sublayer are 2.5, 10, and 40Gbps. These correspond to standard SONET/SDH
data rates (OC-48/STM-16, OC-192/STM-64, and OC-768/STM-256, respec-
tively), but may be used for any client signal.
■ The ODU is the structure used to transport the OPU. The ODU consists of the
OPU and the associated ODU overhead and provides path-layer-connection
monitoring functions. The ODU overhead contains information that enables
maintenance and operation of optical channels. Among these are maintenance
signals, path monitoring, tandem connection monitoring, automatic protec-
tion switching, and designation of fault type and location.

■ The optical channel transport unit (OTU) contains the ODU frame structure,
the OTU overhead, and appended forward error correction (FEC). The OTU
changes the digital format of the ODU into a light signal for transport over an
optical channel. It also provides error detection and correction and section
layer connection monitoring functions.

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