Page 300 - Optical Communications Essentials

Optical Networks

290 Chapter Seventeen

90 × N columns of bytes

Section
and Synchronous
9 rows Path overhead payload
line
of bytes envelope (SPE)
overhead
N columns

3 × N columns 87 × N columns
Figure 17.8. Basic format of an STS-N SONET frame.

Referring to Fig. 17.7, the first three columns comprise transport overhead
bytes that carry network management information. The remaining field of 87
columns is called the synchronous payload envelope (SPE) and carries user data
plus 9 bytes of path overhead (POH). The POH supports performance monitor-
ing by the end equipment, status, signal labeling, a tracing function, and a user
channel. The 9 path-overhead bytes are always in a column and can be located
anywhere in the SPE. An important point to note is that the synchronous byte-
interleaved multiplexing in SONET/SDH (unlike the asynchronous bit inter-
leaving used in earlier TDM standards) facilitates add/drop multiplexing of
information channels in optical networks.
For values of N greater than 1, the columns of the frame become N times
wider, with the number of rows remaining at 9, as shown in Fig. 17.8. Thus, an
STS-3 (or STM-1) frame is 270 columns wide with the first 9 columns contain-
ing overhead information and the next 261 columns being payload data. The
line and section overhead bytes differ somewhat between SONET and SDH, so
that a translation mechanism is needed to interconnect them. To obtain further
details on the contents of the frame structure and the population schemes for
the payload field, the reader is referred to the SONET and SDH specifications.

17.2.2. Optical interfaces
To ensure interconnection compatibility between equipment from different
manufacturers, the SONET and SDH specifications provide details for the
optical source characteristics, the receiver sensitivity, and transmission distances
for various types of fibers. Six transmission ranges and the associated fiber
types are defined with different terminology for SONET and SDH, as Table 17.2
indicates. The transmission distances are specified for G.652, G.653, and G.655
fibers. The ITU-T Recommendation G.957 also designates the SDH categories
by codes such as I-1, S-x.1, L-x.1, and so on, as indicated in the table.
The optical fibers specified in ANSI T1.105.06 and ITU-T G.957 fall into the
following three categories and operational windows:

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