How Modems Work

The purpose of a modulator is to modulate (vary) the carrier properties, while a demodulator detects the modulation on the carrier wave and recovers the original waveform at the destination.

Today we don't have all-digital or all-analog networks; we have a mix of the two. Therefore, at various points in a network, it is necessary to convert between the two signal types. To infuse digital data onto transmission facilities and vice versa a two-way computer conversation, the MOdulator and DEModulator are combined into a single device called a 'modem'.

To understand better how modems work, think of how you use the telephone. You don't just pick up the receiver and start talking. You have to go through a series of quite orderly steps: you have to lift the receiver, wait for the dialing tone, dial each number so it's recognized, wait for the sound of the ringing at the other end, listen for the other person's voice, say hello, and then alternate your speech with theirs. If there's no answer, you have to know when to replace the receiver and hang up the call.

Modems have to behave exactly the same way, exchanging information in a very 'orderly' conversation. If you've used a dialup modem, you'll have noticed that your modem opens the line, dials the number, waits for the other modem to reply, and ''handshakes'', before any real data can be sent or received. If there's no reply, it'll hang up the line and tell you there's a problem. ''Handshaking'' is the initial part of the conversation where two modems agree the speed at which they will talk to one another. If you have a very fast modem but your ISP has only a slow one, the two devices will be forced to communicate at the slower speed.

Every dialup modem works according to a particular international standard (a number prefaced by a capital letter V)—and this tells you how quickly it sends and receives data in bits (binary digits) per second (usually abbreviated bps). The older standards, such as V.22 (1200 bps), assumed the connection between two computers was mostly analog; newer standards like V.90 (56,000 bps) achieve higher speeds by assuming the connection is at least partly digital.

When sending information, the modulation process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states) to analog audio-frequency tones. Digital highs are converted to a tone having a certain constant pitch; digital lows are converted to a tone having a different constant pitch. These states alternate so rapidly that, if you listen to the output of a computer modem, it sounds like a hiss, a screech, or roar. At the other end of your modem connection, the computer attached to its modem reverses this process. The receiving modem demodulates the various tones into digital signals and sends them to the receiving computer. Actually, the process is a bit more complicated than sending and receiving signals in one direction and then another. Modems simultaneously send and receive signals in small chunks. The modems can tell incoming from outgoing data signals by the type of standard tones they use.

Depending upon how your computer is configured and your preferences, you can choose between several modem types.

External modems have their own power supply and connect with a cable to a computer's serial port. The advantages of external modems are that they do not drain any power from the computer, you can turn off the modem to break an online connection quickly without powering down the computer and you can monitor your modem's connection activity by watching the status lights.

Internal modems, usually installed in the computer you buy, are activated when you run a communication programme and are turned off when you exit the programme, which is especially useful for novice users. The major disadvantage with internal modems is their location: when you want to replace an internal modem, you have to go inside the computer case to make the switch.

PC Card modems, designed for portable computers and fit into the PC card slot on notebook and handheld computers. Except for their size, PC Card modems are like a combination of external and internal modems. The cards are powered by the computer, which is fine unless the computer is battery-operated: running a PC Card modem drastically decreases the life of your batteries.

Wireless modems transmit the data signals through the air instead of by using a cable. They sometimes are called a radiofrequency modem. This type of modem is designed to work with cellular technology, and wireless local area networks. Wireless modems are not yet perfected, but the technology is rapidly improving.

Дата добавления: 2014-12-27