Tài liệu The Telephone System

The telephone system remains as the backbone of low-cost telecommunications services in North America. An understanding of the telephone system is necessary when considering its use for data communications.

Telephone: invented in 1874 by Alexander Graham Bell and patented in 1876

Single wire: In the beginning, the market was for telephones only. It is the customer who used to wire them between telephones using single wires and the earth as the ground or the other conductor to complete the circuit.

Bell foreseen the problem of jumbling wires all over the city and formed the Bell Telephone Company, which opens its first switching office in Connecticut in 1878.

The original problem soon returned: to connect every switching office became unmanageable, so second level switching offices were invented. Eventually, the hierarchy grew to five levels.

Copper: At one time, 80% 0f AT&T's capital was the Copper in the local loops, the largest Copper mine.

Now a days a variety of transmission media are used for telecommunication. Local loops consist of twisted pairs, although in the early days of telephony, uninsulated wires spaced 25cm apart on telephone poles were common. Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely used.

Local loops still carry analog signals but all interexchange lines are carrying digital signal.

In summary, the telephone system consists of three major components: