S3017/S3018
SONET OVERVIEW
SONET/SDH/ATM OC-12 TRANSMITTER AND RECEIVER
N
byte-interleaved STS-1 signals. The optical counter-
part of each STS-N signal is an optical carrier level-N
signal (OC-N). The S3017/S3018 chipset supports OC-
12 rates (622.08 Mbit/s).
Frame and Byte Boundary Detection
The SONET/SDH fundamental frame format for STS-12
consists of 36 transport overhead bytes followed by
Synchronous Payload Envelope (SPE) bytes. This pat-
tern of 36 overhead and 1044 SPE bytes is repeated
nine times in each frame. Frame and byte boundaries
are detected using the A1 and A2 bytes found in the
transport overhead. (See Figure 3.)
For more details on SONET operations, refer to the
ANSI SONET standard document.
Synchronous Optical Network (SONET) is a standard
for connecting one fiber system to another at the optical
level. SONET, together with the Synchronous Digital
Hierarchy (SDH) administered by the ITU-T, forms a
single international standard for fiber interconnect be-
tween telephone networks of different countries. SONET
is capable of accommodating a variety of transmission
rates and applications.
The SONET standard is a layered protocol with four
separate layers defined. These are:
• Photonic
• Section
• Line
• Path
Figure 2 shows the layers and their functions. Each of
the layers has overhead bandwidth dedicated to admin-
istration and maintenance. The photonic layer simply
handles the conversion from electrical to optical and
back with no overhead. It is responsible for transmitting
the electrical signals in optical form over the physical
media. The section layer handles the transport of the
framed electrical signals across the optical cable from
one end to the next. Key functions of this layer are
framing, scrambling, and error monitoring. The line
layer is responsible for the reliable transmission of the
path layer information stream carrying voice, data, and
video signals. Its main functions are synchronization,
multiplexing, and reliable transport. The path layer is
responsible for the actual transport of services at the
appropriate signaling rates.
Data Rates and Signal Hierarchy
Table 1 contains the data rates and signal designations
of the SONET hierarchy. The lowest level is the basic
SONET signal referred to as the synchronous transport
signal level-1 (STS-1). An STS-N signal is made up of
Figure 2. SONET Structure
Functions
Payload to
SPE mapping
Maintenance,
protection,
switching
Scrambling,
framing
Optical
transmission
Layer Overhead
(Embedded Ops
Channel)
Path layer
Line layer
Section layer
Path layer
Line layer
Section layer
576 Kbps
192 Kbps
Photonic layer
Photonic layer
0 bps
Fiber Cable
End Equipment
End Equipment
Table 1. SONET Signal Hierarchy
Elec.
STS-1
STS-3
STS-12
STS-24
STS-48
ITU-T
STM-1
STM-4
STM-16
Optical Data Rate (Mbit/s)
OC-1
OC-3
OC-12
OC-24
OC-48
51.84
155.52
622.08
1244.16
2488.32
Figure 3. STS–12/OC–12 Frame Format
A1 A1
9 Rows
A1 A1
12 A1
Bytes
A2 A2
A2 A2
12 A2
Bytes
Transport Overhead 36 Columns
36 x 9 = 324 bytes
Synchronous Payload Envelope 1044 Columns
1044 x 9 = 9396
2
v
125
µsec
December 10, 1999 / Revision B
v
AMCC [ APPLIED MICRO CIRCUITS CORPORATION ]
