Routing planning refers to the procedures that determine which path in a network are assigned to particular connections. The switching centres may use fixed routes to each destination. Adaptive routing may be employed in which each exchange may use different routes for the same destination, depending upon traffic conditions. For effective routing of a call, some form of interconnection of switching exchanges are required. In the following pharagraph variuos forms of networking and its related routing are discussed.
Three basic topologies are adopted for interconnecting exchanges. Exchanges are interconnected by group of trunk lines referred as trunck groups. Two trunk groups are required between any two exchanges. Mesh, star and mixed or hierarchical are the three basic topologies. Determination at the total number of trunk circuits in any network is necessarily a function of the amount of traffic between each pair of stations or exchanges.
Mesh-connected network. This is also called fully connected topology. The advantage of mesh network is that each station has a dedicated connection to other stations. Therefore, this topology offers the highest reliability and security. If one link in the mesh topology breaks, the network remains active. A major or disadvantage of this topology is that it uses too many connections and therefore requires great deal of wiring, espeically when the number of station increases. The mesh topoly requires N(N - 1)/2 connections. For 100 stations, 4950 links required. Fig. 9.2 depicts a full connected mesh structure.
As an example, assume that each exchange generates 1 erlang traffic to each of the other centres. From the appendix B, it can be found that the number of trunks required is approximately four with around 1% grade of service. This is for oneway traffic. It is not necessary to have equal number of trunks for two way communication with respect to a particular station.
In this case, six trunks are sufficient for grade of service of just over 1%. Thus, for low traffic levels, (below about 10 erlangs) the use of two way, provides a significant reduction in the number of trunk circuits required.
For high traffic networks, the cost of network increases. As a compromise, generally, the circuits are divided into three groups. Two groups are used for one way trunks and third group is for both way trunks. An exchange first finds its outgoing trunk before trying one in common group.
This arrangement is practicable if number of exchanges in the network are limited and placed nearby (i.e. the lines are short). As the telephone network users are increasing at rapid rate, the concept of mesh network is uneconomical and the technological growth of transmission of signal by multiplexing made this technique to become completely obsolete.
Star topology. It is an alternative to the mesh arrangement. The network configuration shown in Fig. 9.3 (a) is called star network. In star network, the number of lines is equal to the number of stations. As shown, a star connection utilises an intermediate exchange called a tandom exchange. Through the tandom exchange (TE) all other exchanges communicate.
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