If there were only three or four telephones in a locale, it would make sense to connect each phone to all other phones and find a simple method of selecting the desired one. However, if there are three or four thousand phones in a locale, such a method is out of the question. Then it is appropriate to connect each phone to some centrally located office and perform switching there. This switching could be a simple manual operation using plugs and sockets or could be done with electromechanical devices or with electronics. In any case, this CO solution is the one that has been chosen by the telecommunications industry.
As we connect each of these thousands of telephones to the central office, we have what is a star configuration; all lines are particular to one and only one station, and all terminate on the nucleus of this starthe central office (CO).
These connections are called the local exchange plant, and the telephone company handling this function is called the local exchange carrier (LEC). The connections themselves are often called the local loop; at other times we refer to them as the last mile. In more technical terms, the section closest to the customer's premises is called the distribution plant and that section closest to the CO, the feeder plant (see Figure 1).
Figure 1. Particular Names Are Applied to the Various Parts of the PSTN; End Offices Are Class-5 Offices; Toll Tandem Offices Are Generally Class-4 Offices
(Note: This is certainly a generalization, as will be much that follows. Although the feeder plant usually consists of one or more cables leading to some point of demarcation [a terminal box or an enclosure] after which the lines are spread out going in many smaller cables to the customer premises [the distribution plant], there are cases where there is no need for a point of demarcation. Then what do we call the plant? We will not struggle with such semantic difficulties here.)
But what if a particular telephone call is not originated and terminated within the particular CO's geographic coverage? How do we get to another city or another state or even another country?
The answer, of course, is to connect these COs to a higher-echelon CO (see Figure 2). We apply numbers to these levels of offices; the local office, also called the end office, is called a Class-5 office. The office to which it connects is called the Class-4 office. The top level, the Class-1 office, appears in only a few places in the country. Please note that the only office that has people as its subscribers is the Class-5 office. The other offices in this hierarchy have lower-level COs as their subscribers. Those lines connecting switching offices to switching offices, rather than to subscribers, are called trunks.
Figure 2. The Hierarchy of Switching Systems in its Most Basic Form Consists of Five Classes of Offices
This section of the telephone infrastructurethe section leading upward from the Class-5 officesis handled not by the LECs but by the interexchange carriers (IXCs), the long-distance carriers. This entire structure has been titled the "hierarchy of switching systems." The total network is called the public switched telephone network (PSTN).
In days of old there was only one long-distance carrierAT&T. Hence, any time a telephone number was dialed with an area code up-front, the LEC knew that it must be handed off to AT&T. But then came MCI, Sprint, and hundreds of other long-distance carriers. What was an LEC to do with a particular long-distance call? To whom should it be handed off? This was and is a technical challenge. In political terms, it was called "equal access," which means that a requesting long-distance carrier could require that the LEC examine the number and hand off the call to the proper long-distance carrier. This handoff was from the CO of the LEC to the point of presence (PoP) of the IXC. This PoP could be in a building adjacent to the telco's CO, or it could be in some convenient site in the suburbs where it could serve several of the telco's COs. The pure hierarchy of switching systems was becoming somewhat corrupted; new hierarchies in the long-distance part of the network were being applied on top of the old one.
Although it is not pertinent to the topology of this network, it should be recognized that the interconnections between these various COs can be twisted copper-pair carrier systems utilizing copper pairs (e.g., T1), microwave, satellites, and certainly fiber.
However, this hierarchical network is not the only network in the telephone system of today. There are many others including the following:
The PSTN we have been describing utilizes a star configuration. However, this is not the only configuration being applied in today's telecommunications world. The cable-television (CATV) companies, for instance, use a tree-and-branch technology. In this case, the head end (equivalent to the CO) receives programming from satellites and sends all signals downstream, out on the trunk. At various points along the way, branches extend outward, toward various neighborhoods. These branches are split several more times before the coaxial cable (the media of choice in past CATV systems) reaches the customer's premises. Frequently the signals must be amplified along the way, and therefore power must be sent along with the TV signal. In any case, because the intent of the CATV system is broadcastthat is, send the signal to everyonethere is no need to send an individual and distinct wire to each and every subscriber, as was the case with the telephone system.
However, this methodology has proven to be disadvantageous to the CATV companies, because it is extremely difficult to send signals upstream. Of course, in a telephone system, signals (voices) must be sent in both directions. CATV companies are spending billions of dollars to upgrade their systems not only by utilizing fiber instead of the coax but by adding electronics to the many nodes that permit both upstream and downstream transmission.
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Last modified Monday, 16-Aug-1999 03:25:52 EDT