17.1
Chapter 17
SONET/SDH
17.2
SONET was developed by ANSI;�SDH was developed by ITU-T.
Note
17.3
17-1 ARCHITECTURE
Let us first introduce the architecture of a SONET system: signals, devices, and connections.
Signals�SONET Devices�Connections
Topics discussed in this section:
17.4
Table 17.1 SONET/SDH rates
17.5
Figure 17.1 A simple network using SONET equipment
17.6
17-2 SONET LAYERS
The SONET standard includes four functional layers: the photonic, the section, the line, and the path layer. They correspond to both the physical and the data link layers.
Path Layer
Line Layer�Section Layer
Photonic Layer
Device–Layer Relationships
Topics discussed in this section:
17.7
SONET defines four layers:�path, line, section, and photonic.
Note
17.8
Figure 17.2 SONET layers compared with OSI or the Internet layers
17.9
Figure 17.3 Device–layer relationship in SONET
17.10
17-3 SONET FRAMES
Each synchronous transfer signal STS-n is composed of 8000 frames. Each frame is a two-dimensional matrix of bytes with 9 rows by 90 × n columns.
Frame, Byte, and Bit Transmission�STS-1 Frame Format�Encapsulation
Topics discussed in this section:
17.11
Figure 17.4 An STS-1 and an STS-n frame
17.12
Figure 17.5 STS-1 frames in transmission
17.13
A SONET STS-n
signal is transmitted at �8000 frames per second.
Note
17.14
Each byte in a SONET frame can carry a digitized voice channel.
Note
17.15
Find the data rate of an STS-1 signal.
Solution
STS-1, like other STS signals, sends 8000 frames per second. Each STS-1 frame is made of 9 by (1 × 90) bytes. Each byte is made of 8 bits. The data rate is
Example 17.1
17.16
Find the data rate of an STS-3 signal.
Solution
STS-3, like other STS signals, sends 8000 frames per second. Each STS-3 frame is made of 9 by (3 × 90) bytes. Each byte is made of 8 bits. The data rate is
Example 17.2
17.17
In SONET, the data rate of an STS-n
signal is n times the data rate
of an STS-1 signal.
Note
17.18
What is the duration of an STS-1 frame? STS-3 frame? STS-n frame?
Solution
In SONET, 8000 frames are sent per second. This means that the duration of an STS-1, STS-3, or STS-n frame is the same and equal to 1/8000 s, or 125 μs.
Example 17.3
17.19
In SONET, �the duration of any frame is 125 μs.
Note
17.20
Figure 17.6 STS-1 frame overheads
17.21
Figure 17.7 STS-1 frame: section overhead
17.22
Section overhead is recalculated for each SONET device
(regenerators and multiplexers).
Note
17.23
Figure 17.8 STS-1 frame: line overhead
17.24
Figure 17.9 STS-1 frame: path overhead
17.25
Path overhead is only calculated for end-to-end (at STS multiplexers).
Note
17.26
Table 17.2 Overhead bytes
17.27
What is the user data rate of an STS-1 frame (without considering the overheads)?
Solution
The user data part in an STS-1 frame is made of 9 rows and 86 columns. So we have
Example 17.4
17.28
Figure 17.10 Offsetting of SPE related to frame boundary
17.29
Figure 17.11 The use of H1 and H2 pointers to show the start of � an SPE in a frame
17.30
What are the values of H1 and H2 if an SPE starts at byte number 650?
Solution
The number 650 can be expressed in four hexadecimal digits as 0x028A. This means the value of H1 is 0x02 and the value of H2 is 0x8A.
Example 17.5
17.31
17-4 STS MULTIPLEXING
In SONET, frames of lower rate can be synchronously time-division multiplexed into a higher-rate frame. For example, three STS-1 signals (channels) can be combined into one STS-3 signal (channel), four �STS-3s can be multiplexed into one STS-12, and so on.
Byte Interleaving�Concatenated Signal�Add/Drop Multiplexer
Topics discussed in this section:
17.32
Figure 17.12 STS multiplexing/demultiplexing
17.33
In SONET, all clocks in the network are locked to a master clock.
Note
17.34
Figure 17.13 Byte interleaving
17.35
Figure 17.14 An STS-3 frame
17.36
Figure 17.15 A concatenated STS-3c signal
17.37
An STS-3c signal can carry �44 ATM cells as its SPE.
Note
17.38
Figure 17.16 Dropping and adding STS-1 frames in an add/drop multiplexer
17.39
17-5 SONET NETWORKS
Using SONET equipment, we can create a SONET network that can be used as a high-speed backbone carrying loads from other networks. We can roughly divide SONET networks into three categories: linear, ring, and mesh networks.
Linear Networks�Ring Networks�Mesh Networks
Topics discussed in this section:
17.40
Figure 17.17 Taxonomy of SONET networks
17.41
Figure 17.18 A point-to-point SONET network
17.42
Figure 17.19 A multipoint SONET network
17.43
Figure 17.20 Automatic protection switching in linear networks
17.44
Figure 17.21 A unidirectional path switching ring
17.45
Figure 17.22 A bidirectional line switching ring
17.46
Figure 17.23 A combination of rings in a SONET network
17.47
Figure 17.24 A mesh SONET network
17.48
17-6 VIRTUAL TRIBUTARIES
SONET is designed to carry broadband payloads. Current digital hierarchy data rates, however, are lower than STS-1. To make SONET backward-compatible with the current hierarchy, its frame design includes a system of virtual tributaries (VTs). A virtual tributary is a partial payload that can be inserted into an STS-1.
Types of VTs
Topics discussed in this section:
17.49
Figure 17.25 Virtual tributaries
17.50
Figure 17.26 Virtual tributary types