The Telephone Network

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 central-office (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 star—the 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).

(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.

This section of the telephone infrastructure—the section leading upward from the Class-5 offices—is 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 carrier—AT&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:

• A local-area network (LAN) is a limited-distance network connecting a defined set of terminals. It could connect workstations in an office, offices in a building, or buildings on a campus.
• A wide-area network (WAN) links metropolitan or local networks, usually over common carrier facilities.
• The intelligent network is a concept that centralizes a significant amount of intelligence rather than installing this intelligence in individual COs. For instance, how does a particular CO know which long-distance carrier is to receive a particular call?
• The synchronous optical network (SONET) is a particular set of standards that allows the interworking of products from different vendors. It usually embodies a fiber-optic ring that will permit transmission in both directions.
• The Internet is really quite different from the network we have been describing. It is a packet network (rather than a circuit-switched network), but, as has been discussed, it is an overlay network.
• The common channel signaling network is especially important; it works closely with the PSTN. We also apply the term out-of-band signaling. In the original PSTN, signaling (e.g., call setup) and talking utilized the same common trunk from the originating switching system to the terminating switching system. This process seized the trunks in all of the switching systems involved. Hence, if the terminating end was busy, all of the trunks were set up unnecessarily. In the mid-1970s, the common channel signaling network was established; it utilizes the protocol called signaling system 7 (SS7). With this system, a talking path was not assigned until all signaling had been satisfactorily completed. This network, incidentally, was and is a packet network rather than a circuit-switched network.

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 broadcast—that is, send the signal to everyone—there 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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