What is a modem and how does it work?

Most people are aware of the fact that computers process information in binary language. If computer A wants to communicate with computer B, A will have to send a bunch of ones and zeros to B, in some meaningful (to them) order, across some medium. But how is it actually done?

Let us concentrate on the communications part. Taking things in order, we first must have something with which to connect the two machines. We'll run a wire between the two computers.

Next, we need to send a bunch of ones and zeros across that wire. We'll worry about making the stream meaningful later. First we electrify the wire. We'll pick a standard voltage of +5V DC. We'll make that equal to a "one." A "zero" will be represented by OV DC, or ground. So, by applying and removing voltage on the wire we have ran between the two machines, we can send across a bunch of ones and zeros. It looks like it

We did say something about making the stream meaningful, though. What if we want to send across fifteen ones in a row? For convenience let's refer to ones and zeros by the more generic term "bits." We can arbitrarily assume a standard transmission time for each bit of one second. So if machine A wants to send the stream "101101110100," it would take twelve seconds and look something like this:

Since the machines have agreed on sending a new bit once per second, all machine В has to do is wait until A has begun transmission, wait another 1/2 second (to ensure that it isn't trying to read the value of the line while machine A is changing it), and start checking the voltage on the wire each second thereafter, as shown by the hash-marks in the following diagram:

So now В knows what A is sending. How does В know when A is ready to send bits? How does it know when A is finished sending?

We need another wire.

This new wire will hang out at OV DC until A is ready to send data, then A will "take it high," or put +5V DC on it. When В notices that the new wire is now carrying 5 volts, it waits 1/2 second and starts reading bits. It continues to read a new bit every second until A takes the new wire low again (drops the voltage on the wire to OV DC), then stops.

We really are only looking at half the problem. A can talk to B, but В is going to have to talk to A, if only to let A know В has received the data. We can add two more wires providing the same functions but in the opposite direction.

This is very inefficient, but it would work.

Requiring four wires to implement bi-directional (sometimes called "full duplex") communication would not win an award from anyone.

We coulddo it with two.

Say we chose, instead of+5V DC and OV DC, +5V DC and -5V DC. We made the machines agree that once A took its line high, В would wait a predetermined amount of time and start reading. Knowing the amount of time В would wait, A would wait 1/2 second less and start transmitting. Communication in the other direction would work the same way.

We could do it with one.

The above paragraph describes a technique that works just as well with one wire as with two, with a small exception. Whoever takes the line high becomes the transmitter, the other machine becomes the receiver. The only drawback is we can only operate in half-duplex mode, since both machines can't be manipulating the voltage on the same wire at the same time.

In reality there are many ways for machines to decide what is a one and what is a zero, and many more ways of sending said ones and zeros all over the place. In reality even an inexpensive modem can handle data transmission rates 19,200 times faster than our example.

Vocabulary notes:

bunch – связка

stream meaningful – значащий поток

generic – общий

arbitrarily – произвольно

full duplex – полная двойная связь

half-duplex mode – полудуплексный способ

III. Match the sentences according to the text:

1) We are aware of the fact … 2) It’s necessary to run a wire between the two computers… 3) By applying and removing voltage on the wire… 4) Computer B continues to read a new bit every second until… 5) Requiring four wires to implement bi-directional communication…

a) …it is very inefficient b) …we can send across a bunch of ones and zeros c) …that computers process information in binary language d) …in order to connect them e) …computer A takes the new wire low again

IV. Complete the following sentences according to the text:

1) Computers process information …;

2) We can arbitrarily assume a standard …;

3) It is very inefficient to add …;

4) We can operate in half-duplex mode …;

5) Actually even an expensive modem can …

V. Read the text “Introduction to telephony” and find English equivalents for the following Russian words and word combinations:

Передача; провод; изучить возможные варианты; переключатели, локализующиеся в определённых областях; высокопроизводительный; протоколы передачи данных; границы пространства и времени.

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 notbased 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”