363-206-295
A SONET Overview
Issue 1 December 1997 A-3
The STS-1 frame is transmitted serially starting from the left with row 1 column 1
on through column 90, then row 2 column 1 through 90, continuing on, row-by-
row, until all 810 bytes (9 X 90) of the STS-1 frame have been transmitted.
Since each STS-1 frame consists of 810 bytes and each byte has 8 bits, the frame
contains 6480 bits a frame. There are 8000 STS-1 frames per second, at the
STS-1 signal rate of 51,840,000 (6480 X 8000) bits a second.
The first three columns in each of the nine rows carry the SECTION and LINE
overhead bytes. Collectively, these 27 bytes are referred to as transport overhead.
The remainder of the frame, columns 4 through 90, is reserved for payload signals
(for example, DS1, DS3, and path overhead) and is referred to as the STS-1
synchronous payload envelope (STS-1 SPE). The optical counterpart of the
STS-1 is the optical carrier level 1 signal (OC-1), which is the result of a direct
optical conversion. The electrical counterpart of the STS-1 is the electrical carrier
level 1 signal (EC-1).
Figure A-1. SONET STS-1 Frame — Simplified Version
h
Overhead
Overhead
9
90 Columns
STS-1 Frame Format
87 Columns
STS-1 Synchronous Payload Envelope (STS-1 SPE)
3 Columns
Transport Overhead
9089654321
Rows
Line
Section
d
a
e
r
e
v
O
h
t
a
P
STS-1
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