How do a telephone system operate?

Answer 1

http://electronics.howstuffworks.com/telephone.htm For other information concerning history, components and internal workings of a small business telephone systems check this Wikipedia entry: http://en.wikipedia.org/wiki/Private_branch_exchange

Answer 2

A carrier in the form of a very low frequency RF signal is placed on the phone line at the exchange that is then audio modulated in the phone instement, it is then detected (converted back to audio) in the instrement on the other side.

Answer 3

2.5.1 Structure of the Telephone System

Soon after Alexander Graham Bell patented the telephone in 1876 (just a few hours ahead of his rival, Elisha

Gray), there was an enormous demand for his new invention. The initial market was for the sale of telephones,

which came in pairs. It was up to the customer to string a single wire between them. The electrons returned

through the earth. If a telephone owner wanted to talk to n other telephone owners, separate wires had to be

strung to all n houses. Within a year, the cities were covered with wires passing over houses and trees in a wild

jumble. It became immediately obvious that the model of connecting every telephone to every other telephone,

as shown in Fig. 2-20(a), was not going to work.

Figure 2-20. (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

To his credit, Bell saw this and formed the Bell Telephone Company, which opened its first switching office (in

New Haven, Connecticut) in 1878. The company ran a wire to each customer's house or office. To make a call,

the customer would crank the phone to make a ringing sound in the telephone company office to attract the

attention of an operator, who would then manually connect the caller to the callee by using a jumper cable. The

model of a single switching office is illustrated in Fig. 2-20(b).

Pretty soon, Bell System switching offices were springing up everywhere and people wanted to make long-

distance calls between cities, so the Bell system began to connect the switching offices. The original problem

soon returned: to connect every switching office to every other switching office by means of a wire between them

quickly became unmanageable, so second-level switching offices were invented. After a while, multiple second-

level offices were needed, as illustrated in Fig. 2-20(c). Eventually, the hierarchy grew to five levels.

93

By 1890, the three major parts of the telephone system were in place: the switching offices, the wires between

the customers and the switching offices (by now balanced, insulated, twisted pairs instead of open wires with an

earth return), and the long-distance connections between the switching offices. While there have been

improvements in all three areas since then, the basic Bell System model has remained essentially intact for over

100 years. For a short technical history of the telephone system, see (Hawley, 1991).

Prior to the 1984 breakup of AT&T, the telephone system was organized as a highly-redundant, multilevel

hierarchy. The following description is highly simplified but gives the essential flavor nevertheless. Each

telephone has two copper wires coming out of it that go directly to the telephone company's nearest end office

(also called a local central office). The distance is typically 1 to 10 km, being shorter in cities than in rural areas.

In the United States alone there are about 22,000 end offices. The two-wire connections between each

subscriber's telephone and the end office are known in the trade as the local loop. If the world's local loops were

stretched out end to end, they would extend to the moon and back 1000 times.

At one time, 80 percent of AT&T's capital value was the copper in the local loops. AT&T was then, in effect, the

world's largest copper mine. Fortunately, this fact was not widely known in the investment community. Had it

been known, some corporate raider might have bought AT&T, terminated all telephone service in the United

States, ripped out all the wire, and sold the wire to a copper refiner to get a quick payback.

If a subscriber attached to a given end office calls another subscriber attached to the same end office, the

switching mechanism within the office sets up a direct electrical connection between the two local loops. This

connection remains intact for the duration of the call.

If the called telephone is attached to another end office, a different procedure has to be used. Each end office

has a number of outgoing lines to one or more nearby switching centers, called toll offices (or if they are within

the same local area, tandem offices). These lines are called toll connecting trunks. If both the caller's and

callee's end offices happen to have a toll connecting trunk to the same toll office (a likely occurrence if they are

relatively close by), the connection may be established within the toll office. A telephone network consisting only

of telephones (the small dots), end offices (the large dots), and toll offices (the squares) is shown in Fig. 2-20(c).

If the caller and callee do not have a toll office in common, the path will have to be established somewhere

higher up in the hierarchy. Primary, sectional, and regional offices form a network by which the toll offices are

connected. The toll, primary, sectional, and regional exchanges communicate with each other via high-

bandwidth intertoll trunks (also called interoffice trunks). The number of different kinds of switching centers and

their topology (e.g., can two sectional offices have a direct connection or must they go through a regional

office?) varies from country to country depending on the country's telephone density. Figure 2-21 shows how a

medium-distance connection might be routed.

Figure 2-21. A typical circuit route for a medium-distance call.

A variety of transmission media are used for telecommunication. Local loops consist of category 3 twisted pairs

nowadays, although in the early days of telephony, uninsulated wires spaced 25 cm apart on telephone poles

were common. Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely

used.

In the past, transmission throughout the telephone system was analog, with the actual voice signal being

transmitted as an electrical voltage from source to destination. With the advent of fiber optics, digital electronics,

and computers, all the trunks and switches are now digital, leaving the local loop as the last piece of analog

94

Answer 4

The telephone network works using three technologies. They include a wireless network, a private network and a fixed line network.