Solutions to Chapter 4 - Communication Networks

Communication Networks (2 nd Edition)
Chapter 4 Solutions
The spacing in Hertz is:
Frequency Bandwidth =
8
(3∗10
)(0.75 ∗10
−9
2
(1550 ∗10
)
−9
)
=
93.65GHz
Thus, the maximum achievable bit rate that each wavelength can achieve using on-off modulation
is 2 × 93.65 = 187 Gbps. Each signal carries 2.5 Gbps of traffic, so we see that because of the
relatively large guard bands needed, WDM wavelengths have a very large spacing compared to
their width.
23. How does WDM technology affect the hierarchical SONET ring topology in Figure 4.29 In other
words, what are the consequences of a single fiber providing large number of high-bandwidth
channels
Solution:
WDM allows for much more aggregate bandwidth on each fiber. Thus, in the context of the
hierarchical SONET ring topology, the high-tier ring can use WDM in order to increase its
bandwidth and the number of channels it can support and, in turn, the number of lower-tier rings
that it can service. WDM allows it to achieve this increased service ability without the need to lay
down more physical fibers over its links. On the other hand, the use SONET ring topology mean
that each wavelength is associated with a separate SONET ADM, and this creates a problem in
terms of interconnecting traffic across rings.
24. WDM and SONET can be used to create various logical topologies over a given physical
topology. Discuss how WDM and SONET differ and explain what impact these differences have in
the way logical topologies can be defined.
Solution:
Both WDM and SONET define logical topologies by setting up semi-permanent paths between
nodes in the network. WDM creates these paths by assigning lightpaths between nodes. WDM
without wavelength conversion has an inherent restriction imposed on the logical topologies that it
can create, because no two lightpaths that share the same link can use the same wavelength. In
SONET, since tributaries are electrically defined, the only restriction imposed on the topologies is
the total bandwidth utilized on each link.
25. Compare the operation of a multiplexer, an add-drop multiplexer, a switch, and a digital crossconnect.
Solution:
A multiplexer is a 1:N device. It takes N separate signals on N different inputs and combines
them into one higher rate signal on one output port.
An add-drop multiplexer takes in N signals on one input port and replaces one of them with a new
signal from a separate input port. The new aggregate signal is routed to an output port and the
signal that was replaced is “dropped” to a separate output port.
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