13.2 Common Network Topologies
519
Drill Problem 13.2 Consider a star network that operates at 1 Gb/s and uses
a 16 × 16 star coupler with an excess loss of 1.35 dB. Assume each station is
located 5 km from the star coupler and that the fiber attenuation is 0.25 dB/km
at 1550 nm. Let the connector loss at each end of the link be 1.0 dB. Suppose
the optical source emits 0 dBm into the fiber and that the pin photodetector has
a −27-dBm sensitivity at 1 Gb/s. Show that the system margin is 9.11 dB.
13.3 Basic SONET/SDH Concepts
With the advent of fiber optic transmission lines, the next step in the evolution of the
digital time-division-multiplexing (TDM) scheme was a standard signal format called
synchronous optical network (SONET) in North America and synchronous digital
hierarchy (SDH) in other parts of the world. This section addresses the basic concepts
of SONET/SDH, its optical interfaces, and fundamental network implementations.
The aim here is to discuss only the physical-layer aspects of SONET/SDH as they
relate to optical transmission lines and optical networks. Topics such as the detailed
data format structure, SONET/SDH operating specifications, and the relationships of
switching methodologies such as carrier Ethernet service aggregation with SONET/
SDH are beyond the scope of this text. These can be found in numerous sources
[9–12].
13.3.1 SONET/SDH Frame Formats and Speeds
Figure 13.7 shows the basic structure of a SONET frame. This is a two-dimensional
structure consisting of 90 columns by 9 rows of bytes, where one byte is eight
bits. Here, in standard SONET terminology, a section connects adjacent pieces of
equipment, a line is a longer link that connects two SONET devices, and a path is
a complete end-to-end connection. The fundamental SONET frame has a 125-μs
duration. Thus, the transmission bit rate of the basic SONET signal is
STS-1 = (90 bytes/row)(9 rows/frame)
(8 bits/byte)/(125 μs/frame) = 51.84 Mb/s
This is called an STS-1 signal, where STS stands for synchronous transport signal.
All other SONET signals are integer multiples of this rate, so that an STS-N signal
has a bit rate equal to N times 51.84 Mb/s. When an STS-N signal is used to modulate
an optical source, the logical STS-N signal is first scrambled to avoid long strings of
ones and zeros and to allow easier clock recovery at the receiver. After undergoing
519
Drill Problem 13.2 Consider a star network that operates at 1 Gb/s and uses
a 16 × 16 star coupler with an excess loss of 1.35 dB. Assume each station is
located 5 km from the star coupler and that the fiber attenuation is 0.25 dB/km
at 1550 nm. Let the connector loss at each end of the link be 1.0 dB. Suppose
the optical source emits 0 dBm into the fiber and that the pin photodetector has
a −27-dBm sensitivity at 1 Gb/s. Show that the system margin is 9.11 dB.
13.3 Basic SONET/SDH Concepts
With the advent of fiber optic transmission lines, the next step in the evolution of the
digital time-division-multiplexing (TDM) scheme was a standard signal format called
synchronous optical network (SONET) in North America and synchronous digital
hierarchy (SDH) in other parts of the world. This section addresses the basic concepts
of SONET/SDH, its optical interfaces, and fundamental network implementations.
The aim here is to discuss only the physical-layer aspects of SONET/SDH as they
relate to optical transmission lines and optical networks. Topics such as the detailed
data format structure, SONET/SDH operating specifications, and the relationships of
switching methodologies such as carrier Ethernet service aggregation with SONET/
SDH are beyond the scope of this text. These can be found in numerous sources
[9–12].
13.3.1 SONET/SDH Frame Formats and Speeds
Figure 13.7 shows the basic structure of a SONET frame. This is a two-dimensional
structure consisting of 90 columns by 9 rows of bytes, where one byte is eight
bits. Here, in standard SONET terminology, a section connects adjacent pieces of
equipment, a line is a longer link that connects two SONET devices, and a path is
a complete end-to-end connection. The fundamental SONET frame has a 125-μs
duration. Thus, the transmission bit rate of the basic SONET signal is
STS-1 = (90 bytes/row)(9 rows/frame)
(8 bits/byte)/(125 μs/frame) = 51.84 Mb/s
This is called an STS-1 signal, where STS stands for synchronous transport signal.
All other SONET signals are integer multiples of this rate, so that an STS-N signal
has a bit rate equal to N times 51.84 Mb/s. When an STS-N signal is used to modulate
an optical source, the logical STS-N signal is first scrambled to avoid long strings of
ones and zeros and to allow easier clock recovery at the receiver. After undergoing