Bases Of Network Configurations
In this section we discuss basic network configurations that may apply anywhere in the telecommunication community. Networks more applicable to the local area are covered in Chapter 2, and those for the long-distance plant are discussed in Chapter 6.
9.1 Introductory Concepts
A network in telecommunications may be defined as a method of connecting exchanges so that any one subscriber in the network can communicate with any other subscriber. For this introductory discussion, let us assume that subscribers access the network by a nearby local exchange. Thus the problem is essentially how to connect exchanges efficiently. There are three basic methods of connection in conventional telephony: (1) mesh, (2) star, and (3) double and higher-order star (see Section 2 of this chapter). The mesh connection is one in which each and every exchange is connected by trunks (or junctions) to each and every other exchange as shown in Figure 1.11A. A star connection utilizes an intervening exchange, called a tandem exchange, such that each and every exchange is interconnected via a single tandem exchange. An example of a star connection is shown in Figure 1.11B. A double-star configuration is one where sets of pure star subnetworks are connected via higher-order tandem exchanges, as shown in Figure 1.11C. This trend can be carried still further, as we see later on, when hierarchical networks are discussed.
(fll Star
(CI Double star
Figure 1.11. Examples of star, double-star, and mesh configurations.
(fll Star
(CI Double star
Figure 1.11. Examples of star, double-star, and mesh configurations.
As a general rule we can say that mesh connections are used when there are comparatively high traffic levels between exchanges, such as in metropolitan networks. On the other hand, a star network may be applied when traffic levels are comparatively low.
Another factor that leads to star and multiple-star network configurations is network complexity in the trunking outlets (and inlets) of a switch in a full mesh. For instance, an area with 20 exchanges would require 380 traffic groups (or links), and an area with 100 exchanges would require 9900 traffic groups. This assumes what are called one-way groups. A one-way group is best defined considering the connection between two exchanges, A and B. Traffic originating at A bound for B is carried in one group and the traffic originating at B bound for A is carried in another group, as shown in the following diagram:
One-way and both-way groups are further discussed in Section 11.
Thus, in practice, most networks are compromises between mesh and star configurations. For instance, outlying suburban exchanges may be connected to a nearby major exchange in the central metropolitan area. This exchange may serve nearby subscribers and be connected in mesh to other large exchanges in the city proper. Another example is the city's long-distance exchange, which is a tandem exchange looking into the national long-distance network, whereas the major exchanges in the city are connected to it in mesh. An example of a reallife compromise among mesh, star, and multiple-star configurations is shown in Figure 1.12.
9.2 Higher-Order Star Network
Figure 1.13 illustrates a higher-order star network. It is simply several star networks of Figure 1.11B stacked on top of each other. Another high-order star
Figure 1.12. A typical telephone network serving a small city as an example of a compromise between mesh and star configuration. A is the highest level in this simple hierarchy. A might house the "point of presence'' (POP) in the U.S. network. B is a local exchange. C may be a satellite exchange or a concentrator. Consult Ref. 24.
- Figure 1.13. Higher-order star network.
network is shown in Figure 1.11C. In these types of networks the higher layers are given more importance than the lower layers. The incisive reader will say that we describe a hierarchical network. The reader is correct, but we wish to reserve our detailed discussion of hierarchical networks for Chapter 6, Sections 6 and 7.
We illustrate the order of importance of the several levels in a high-order star network in Figure 1.13. There are three levels or ranks of exchanges in the figure. The smallest blocks in the diagram are the lowest-ranked exchanges, which have been marked with a "3" to indicate the third level or rank. Note that there are restrictions or rules of traffic flow. As the figure is drawn, traffic from 3A1 to 3A2 would have to flow through exchange 2A1. Likewise, traffic from exchange 2A2 to 2A3 would have to flow through exchange 1A. Carrying the concept one step further, traffic from any A exchange to any B exchange would necessarily have to be routed through exchange 1A.
The next consideration is the high-usage (HU) route. For instance, if we found that there were high traffic intensities (e.g., >20 erlangs) between 2B1 and 2B2, trunks and switch gear might well be saved by establishing a HU route between the two (shown by a dashed line in Figure 1.13). Thus we might call the high-usage route a highly traveled shortcut. Of course, HU routes could be established between any pair of exchanges in the network if traffic intensities and distances involved proved this strategy economical. When HU routes are established, traffic between the exchanges involved will first be offered to the HU route, and overflow would take place through a last choice route or, as shown in Figure 1.13, up to the next level and down. If routing is through the highest level of higher-order star network, we call this route the final route, a hierarchical network term. (See Chapter 6, Sections 6 and 7.)
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