Page 294 - Optical Communications Essentials

Optical Networks

284 Chapter Seventeen

Network 1 Network 2
Stations Stations
Node Node
Trunks

Router Trunk Router
Figure 17.2. Definitions of various elements of a network.

■ Topology. The topology is the logical manner in which nodes are linked
together by information transmission channels to form a network.
■ Switching and routing. The transfer of information from source to destin-
ation through a series of intermediate nodes is called switching, and the selec-
tion of a suitable path through a network is referred to as routing. Thus a
switched communication network consists of an interconnected collection of
nodes, in which information streams that enter the network from a station
are routed to the destination by being switched from one transmission path to
another at a node.
■ Router. When two networks that use different information-exchange rules
(protocols) are interconnected, a device called a router is used at the inter-
connection point to translate the control information from one protocol to
another.
To get a better understanding of optical networks, we need to define some
terms used in a public network, such as that shown in Fig. 17.3.

■ Central office. A node in a public network is called a central office (CO) or a
point of presence (POP). The CO houses a series of large switches that estab-
lish temporary connections for the duration of a requested connection time
between subscriber lines which terminate at the switch.
■ Access network. The access network encompasses connections that extend
from the CO to individual businesses, organizations, and homes. Its function
is to concentrate the information flows which originate in the access network
prior to their entering a long-haul or backbone network.
■ Metro interoffice network. A metropolitan (typically abbreviated metro) net-
work connects groups of central offices within a city or a city-size geographic
region. The distances between central offices for this type of network typically
range from a few to several tens of kilometers.
■ Backbone network. The term backbone means a network that connects mul-
tiple LAN, MAN, or WAN segments. Thus a backbone handles internetwork

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