SONET/SDH/ATM OC-48 1:8 RECEIVER
SONET OVERVIEW
Synchronous Optical Network (SONET) is a standard
for connecting one fiber system to another at the opti-
cal level. SONET, together with the Synchronous
Digital Hierarchy (SDH) administered by the ITU-T,
forms a single international standard for fiber inter-
connect between 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 3 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 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
functions are synchronization, multiplexing, and reli-
able 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
N
byte-interleaved STS-1 signals. The optical counter-
S3042
part of each STS-N signal is an optical carrier level-N
signal (OC-N). The S3042 chip supports OC-48 rate
(2.488 Gbps).
Frame and Byte Boundary Detection
The SONET/SDH fundamental frame format for
STS-48 consists of 144 transport overhead bytes
followed by Synchronous Payload Envelope (SPE)
bytes. This pattern of 144 overhead and 4176 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 4.)
For more details on SONET operations, refer to the
Bellcore SONET standard document.
Figure 3. 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
End Equipment
Fiber Cable
End Equipment
Table 1. SONET Signal Hierarchy
Elec.
STS-1
STS-3
STS-12
STS-24
STS-48
CCITT
STM-1
STM-4
STM-8
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
Figure 4. STS–48/OC–48 Frame Format
A1 A1
9 Rows
A1 A1
48 A1
Bytes
A2 A2
A2 A2
48 A2
Bytes
Transport Overhead 144 Columns
144 x 9 = 1296 bytes
Synchronous Payload Envelope 4176 Columns
4176 x 9 = 37,584 bytes
s
125
µsec
June 24, 1999 / Revision E
s
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AMCC [ APPLIED MICRO CIRCUITS CORPORATION ]
