Introduction to telephony

Assuming we have a workable means of transmitting bits across a wire between two machines, the next question could be, "What do we do if the machines are 6,452 miles apart?"

We'll need four wires, that's 25,808 miles of copper. Perhaps we should investigate alternatives. There are fortunately people out there who have already run wire (or run fiberoptic cable, or built microwave towers) all over the world, and are more than willing to loan us the use of their facilities for a nominal fee. They are called long-distance carriers. We'll call them LDCs.

They have a network of interconnections that stretches from one end of the globe to the other. We have the aforementioned LDCs and we have Local Exchange Carriers, or LECs. They have networks that go out to everyone's house and connect them all to big (in some cases, very big) switches in centralized locations. The LDCs have high-capacity connections to these switches.

Their facilities are not based on ones and zeros. They are, after all, voice-transmission facilities. The LDCs have networks specifically designed for data transmission, but it is not really cost-effective for you to pay to have them run a line out to your house. The alternative is to find some way to translate between voice transmission and data transmission protocols.

A protocol is just an agreement between two machines about things like how long bits should be, and what voltages are good for ones, etc. It just gets complicated trying to cover all the bases while the machines are compressing the data to shove more across in the same time, and sending simultaneously in 350 different frequencies to take advantage of available bandwidth, and stretching the bounds of space and time.

Vocabulary notes:

fiberoptic cable – оптоволоконной кабель

long-distance carriers – дальние курьеры

aforementioned – вышеупомянутый

local exchange carriers – местные обменные курьеры

cost-effective – рентабельный

to shove - пихать

available bandwidth – доступная полоса пропускания

VI. Answer the following questions:

1) What do you know about long-distance carriers?

2) What is the difference between LCDs and LECs?

3) What is a protocol?

VII. Read the text “Modulation and Demodulation”