A SONET Overview
Issue 3 June 2000
A-3
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
STS-1
P
a
t
h
O
v
e
r
e
a
d
Section
Line
123456 8990
Transport Overhead
3 Columns
STS-1 Synchronous Payload Envelope (STS-1 SPE)
87 Columns
STS-1 Frame Format
90 Columns
9
Overhead
Overhead
h
Rows
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