Page 299 - Optical Communications Essentials

Optical Networks

Optical Networks 289

Figure 17.7 shows the basic structure of a SONET frame. This is a two-
dimensional structure consisting of 90 columns by 9 rows of bytes, where 1 byte
(1 B) is 8 bits (b). This fundamental SONET frame has a 125-µs duration. Thus
the transmission bit rate of the basic SONET signal is

STS-1 (90 bytes/row)(9 rows/frame)(8 bits/byte)/(125 µs/frame)
51.84Mbps
This is called an STS-1 signal, where STS stands for synchronous transport signal.
Higher-rate SONET signals are obtained by byte-interleaving N STS-1 frames,
which then are scrambled and converted to an optical carrier—level N (OC-N)
signal. Thus the OC-N signal will have a line rate exactly N times that of an
OC-1 signal; that is, OC-N N 51.84 Mbps.
For SDH systems the fundamental building block is the 155.52-Mbps synchro-
nous transport module—level 1 (STM-1). Again, higher-rate information streams
are generated by synchronously multiplexing N different STM-1 signals to form
the STM-N signal. Table 17.1 shows commonly used SDH and SONET signal
levels and the associated OC rates. For practical purposes, the data rates are
abbreviated as shown in the rightmost column.

90 columns of bytes

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

1 column

3 columns 87 columns
Figure 17.7. Basic structure of an STS-1 SONET frame.

TABLE 17.1. Commonly Used SONET and SDH Transmission Rates

SONET level Electrical level SDH level Line rate, Mbps Common rate name
OC-1 STS-1 — 51.84 —
OC-3 STS-3 STM-1 155.52 155Mbps
OC-12 STS-12 STM-4 622.08 622Mbps
OC-48 STS-48 STM-16 2,488.32 2.5Gbps
OC-192 STS-192 STM-64 9,953.28 10Gbps
OC-768 STS-768 STM-256 39,813.12 40Gbps

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