SONET/SDH OC-3/12 TRANSMITTER AND RECEIVER
S3005/S3006
Figure 2. SONET Structure
SONET OVERVIEW
Functions
Layer Overhead
(Embedded Ops
Channel)
Synchronous Optical Network (SONET) is a stan-
dard for connecting one fiber system to another at
the optical level. SONET, together with the Synchro-
nous Digital Hierarchy (SDH) administered by the
ITU-T, forms a single international standard for fiber
interconnect between telephone networks of differ-
ent countries. SONET is capable of accommodating
a variety of transmission rates and applications.
Payload to
SPE mapping
Path layer
Line layer
Path layer
Maintenance,
protection,
switching
576 Kbps
192 Kbps
0 bps
Line layer
Section layer
Photonic layer
Scrambling,
framing
Section layer
Photonic layer
The SONET standard is a layered protocol with four
separate layers defined. These are:
Optical
transmission
• Photonic
• Section
• Line
Fiber Cable
End Equipment
End Equipment
• Path
Figure 2 shows the layers and their functions. Each
of the layers has overhead bandwidth dedicated to
administration 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 transmis-
sion 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.
Table 1. SONET Signal Hierarchy
Data Rate
(Mbit/s)
Elec.
ITU-T
Optical
STS-1
OC-1
51.84
STS-3
STM-1
STM-4
OC-3
155.52
466.56
622.08
933.12
1244.16
1866.24
2488.32
STS-9
OC-9
STS-12
STS-18
STS-24
STS-36
STS-48
OC-12
OC-18
OC-24
OC-36
OC-48
STM-16
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
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 S3005/S3006 chipset supports OC-3
rates (155.52 Mbit/s) and OC-12 (622.08 Mbit/s) rates.
Figure 3. STS–3/OC–3 Frame Format
A1
B1
D1
H1
B2
D4
D7
D10
Z1
A1
*
A1
*
A2
E1
D2
H2
K1
D5
D8
D11
Z2
A2
*
A2
*
C1
F1
C1
*
C1
*
*
*
*
*
D3
H3
K2
D6
D9
D12
E2
*
*
9 x 261 =
2349 bytes
H1
B2
*
H1
B2
*
H2
*
H2
*
H3
*
H3
*
9
Rows
*
*
*
*
*
*
*
*
*
*
Frame and Byte Boundary Detection
*
*
*
*
*
*
The SONET/SDH fundamental frame format for STS-3
consists of nine transport overhead bytes followed by
Synchronous Payload Envelope (SPE) bytes. This
pattern of 9 overhead and 261 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)
Z1
Z1
Z2
Z2
*
*
Transport Overhead
9 Columns
Synchronous Payload
Envelope
261 Columns
125 µsec
For more details on SONET operations, refer to the
ANSI SONET standard document.
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