S3031B
SONET/SDH OVERVIEW
Synchronous Optical Network (SONET) is a standard
for connecting one fiber system to another at the op-
tical level. SONET, together with the Synchronous
Digital Hierarchy (SDH) administered by the ITU-T,
form a single international standard for fiber intercon-
nect between telephone networks of different coun-
tries. 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
administration and maintenance. The photonic layer
simply handles the conversion from electrical to opti-
cal and back with no overhead. It is responsible for
E4/STM-1/OC-3 ATM TRANSCEIVER
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 moni-
toring. The line layer is responsible for the reliable
transmission of the path layer information stream
carrying voice, data, and video signals. Its main func-
tions 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 designa-
tions 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 counterpart of each STS-N signal is an optical
carrier level-N signal (OC-N). The S3031B supports
OC-3 rates (155.52 Mbps).
Frame and Byte Boundary Detection
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 (Mbps)
OC-1
OC-3
OC-12
OC-24
OC-48
51.84
155.52
622.08
1244.16
2488.32
The SONET/SDH fundamental frame format for STS-3
consists of nine transport overhead bytes followed by
Synchronous Payload Envelope (SPE) bytes. This pat-
tern 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.)
For more details on SONET operations, refer to the
Bellcore SONET standard document.
Figure 2. SONET Structure
Functions
Payload to
SPE mapping
Maintenance,
protection,
switching
Scrambling,
framing
Optical
transmission
Figure 3. STS-3/OC Frame Format
Layer Overhead
(Embedded Ops
Channel)
A1
B1
D1
H1
A1
*
*
H1
B2
*
*
*
Z1
A1
*
*
H1
B2
*
*
*
Z1
A2
E1
D2
H2
K1
D5
D8
D11
Z2
A2
*
*
H2
*
*
*
*
Z2
A2
*
*
H2
*
*
*
*
M1
J0
F1
D3
H3
K2
D6
D9
D12
E2
Z0
*
*
H3
*
*
*
*
*
Z0
*
*
H3
*
*
*
*
*
Path layer
Line layer
Section layer
Path layer
Line layer
Section layer
576 Kbps
9
Rows
9 x 261 =
2349 bytes
B2
D4
192 Kbps
D7
D10
Photonic layer
Photonic layer
0 bps
S1
Transport Overhead
Fiber Cable
End Equipment
Synchronous Payload
Envelope
261 Columns
End Equipment
9 Columns
v
125
µsec
v
2
August 19, 1999 / Revision D
AMCC [ APPLIED MICRO CIRCUITS CORPORATION ]
