Optical Networks - _Chapter 10 WDM Network Design_116 - 10...

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chapter
WDM Network Design
I
n previous chapters, we learned that the optical layer provides high-speed
circuit-switched connections, or lightpaths, between pairs of higher-layer equip-
ment such as SONET/SDH muxes, IP routers, and Ethernet switches. The optical
layer realizes these lightpaths over the physical fiber using elements such as optical
line terminals (OLTs), optical add/drop multiplexers (OADMs), and optical cross-
connects (OXCs). We called a network using such lightpaths a wavelength-routing
network. In this chapter, our goal is to study how to design a wavelength-routing
network. This involves studying not only how to design the optical layer but also
how the higher-layer SONET or IP network is to be designed because the design of
the two layers is closely coupled. We illustrate with an example.
Example 10.1
In Figure 10.1(a), there are three nodes labeled A, B, and C,
connected by WDM fiber links. For simplicity, assume the traffic generated is in
the form of IP packets from routers located at these nodes. Similar examples hold
if the higher layer consists of SONET/SDH muxes. For concreteness, also assume
that all router interfaces operate at 10 Gb/s, which is also the transmission ca-
pacity on each wavelength on the WDM links. Now suppose, based on estimates
of the IP packet traffic, 50 Gb/s of capacity is required between all three pairs of
routers: A–B, B–C, and A–C. The network can be designed to handle this traffic
in two ways.
1. No optical add/drop: In the first method, we set up 10 wavelengths on each
of the links A–B and B–C connecting the routers at the ends of these links.
We observe that the traffic flowing on link A–B is 50 Gb/s (traffic from A–B)
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WDM Network Design
A B C
Router Router Router
Router Router
Router
(a)
(b)
(c)
Figure 10.1
(a) A three-node network. (b) Nodes A–B and B–C are interconnected by
WDM links. All wavelengths are dropped and added at node B. (c) Half the wavelengths
pass through optically at node B, reducing the number of router ports at node B.