what is modem?

i don't know about modem?
so please tell me how it works?
and what is the use of this?
where it is used mostly?
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kosiAuthor Commented:
By definition MODEM is an abbreviation.  It stands for Modulatar/Demodulator.
1) How it works?
   The modem uses a standard Analog signal such as a phone line, most common, to generate a connection with another Modem.  The computer prompts the modem to attempt to contact to another modem connected to a computer.  The modem at the other end must answer for a connection to be established.  Once the modem on receiving end answers the two modems will negotiate a compatiable connection speed and protocol.  The connection speed varies depending on the type of modems used and can range from 200Baud to 56K Baud.  Baud rate is defined as a change in state from the positive to negative side of sinusodial wave(analog signal of connection).  At each change of state a bit(1 or 0) can be communicated between modems.  A group of 8 bits(normally) makes up a byte which can be mapped to a character in ASCII.  There are some compression techniques and algorithms in use now days that allow for less than 8 bits of information per character.
2) What is the use?
  The use of the modem is for the exhange of information between two computers.  At this point in time only one computer can use a modem at a time.
3) Where is it used mostly?
  The majority of modems are used by home users on computers to connect to the Internet.
Celtics, Nice definition but you got one thing wrong: Baud is generally the wrong term in referencing modem speeds. As you state, Baud rate is defined as a change in state. There can be multiple bits defined by a single state change. In 28.8 modem, it uses a technique known as Quadrature Amplitude Modulation (QAM). This allows for four different bits per state change. Thus, it runs at 7200 baud (7200 state changes/sec) but has a transfer rate of 28.8kbps (bits per second), which is what modem speed is defined by. Thus, you should be using bits/sec rather than baud to define modem speeds. In earlier versions of modems, each baud referenced one bit so baud rates and bits/sec were the same for but now it's not. On the low end, 200 baud modems are 200bps but on the upper end they don't match up. Not exactly sure about the 56k since it uses quite different techniques than other modems.
They have a good, simple explaination of modems here too:

A definition of baud:

Quote from this page proving my statement:
The terms kilobaud (kbaud) and kilosymbols/sec (ks/s) are occasionally used to express modem speed, but you've got to know whether dibits, tribits, etc. are being transmitted before you can convert these to kilobits/sec (kb/s). However usually the person is just mistaken, and is using 'baud' incorrectly.

Article on QAM if intersted
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A modem is a part inside or outside of your computer.

It is used to connect two computers together or to connect your computer to the Internet over a telephone line.

A modem takes information from your computer and changes it into sounds that are sent over a phone line just like your voice is. A modem at the other end of the phone line changes the sounds back into computer information.

Modems are used by millions of people around the world who share computer information or connect to the Internet or World Wide Web (WWW).


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Your answer is a brief synopsis of the comments provided.  A modem does not change the information to sound.  It changes it into a signal that humans can perceive as sound.  The computer looks for voltage changes in the line that it is able to detect.
Modems are not used just on telephone lines, they use many different media.  Fiber optics and serial cables are just some.
The majority of modem users are people around the World are used to connect to the WWW.  The internet is is a butchered term that has been incorporated into the WWW lingo.  Internet refers to a connection between two or more remote computers over a communication medium.  This term was used to describe early computer connections and had nothing to to with modems or WWW.
Beyond that modems are also used, in large numbers, to connect remote computers directly to each other for exhanging data and troubleshooting problems remotely.  This is how a lot of automated systems are set up such as ATM's.
Please reject the answer from ijf and award the points to ggilman.
I have to agree with celtics. ijf seems to be posting another's comments as his "answer." Kosi, you should certainly reject it.
As for the points though, they should go to celtics. He had the real answer. I just cleared up one little mistake he had. Most people I have talked to make the same mistake & that's why I thought it was important to cear it up.
As for anything else I may clear up (or possibly muck up??):
A MODEM takes a digital signal from the computer, converts it to analog, and transmits it over some medium (normally telephone lines but could be something else). This is a voltage, not a sound. Yes, if you put this voltage across speaker terminals you will have a sound but it is a voltage that is being sent. It's really the same with with telephone as well. You don't send sounds over a telephone. You speak into a mic, the mic creates a voltage, the voltage is transferred over some telephone lines, and at the recieving side, a speaker converts that voltage back to a sound.
As for other devices that are modems: I don't think you can count serial line/fiber optic as modem. For that matter, you can't consdier a cable modem a modem or even an ISDN modem a modem. Sounds bad since the last two have "modem" in the name. The true definition of MODEM comes from "Modulate/Deodulate" like celtics mentioned. This basically means digital-analog/analog-digital conversion. This is required over a telephone line since telephone lines are inherently analog (or were a few years back). However, the others mentioned are inherently digital, they are not truely "modems". I'm not as positive about the cable modem and would have to do research on it but I think it's a digital transmission. Pretty sure about the serial cable though, why convert to digital going directly to another computer? ISDN and Cable modems are called "modems" because they act like traditional modems that basically allow a computer to connect to a remote network, be it WWW, RAS, or otherwise. So are they really modems? Depends on how strict your definition is but generally no. Most fiber optic systems that I have seen are digital too. Never really heard of a "fiber optic" modem. Generally modems are used to connect to "remote" networks, like I mentioned. Normally fiber optic would not be remote but I guess you could have a fiber optic modem similar to a cable modem?? Actually looking at celtics, that's his definition too: "remote network". Good choice.
BTW, the phone system in most areas is now digital. What happens is your telephone or modem creates an analog signal, sends it on the telephone line. At some point the telephone company converts it to a digital signal and sends it to the destination (probably over fiber optic), where it converts it back to analog before entering your modem. Without going into too much detail, 56k modems take advantage of this fact and send analog signals that can basically be easily converted into a digital stream to send a "nearly digital" transmission from the modem. With 56k though the reciever must have a direct digital connection to the phone company, not analog. 56k only works right if only one segment (from your computer to the telephone company) is analog. If this isn't the case, 56k will convert back to standard 33.6k.
Actually it looks like I made a mistake in the last post. The sender, in this case your ISP (internet service provider, here being used generally as it can be any dial-up service) , needs to have a direct digital line, not the reciever. The 56k link is from the ISP to your computer. 56k is one directional. You have 56k from ISP to you and 33.6 from you to the ISP.
Okay, so you caught me. I was simply trying to help kosi understand how a modem works buy using the sound analogy to help kosi understand that the information is transferred in the same was as sound over a telephone wire. You are all very right that modems detect voltage from the telephone line.

It seems to me that kosi was not looking for such a technical answer that some of you provided and so I was simply trying to write things in a more simple form. Terms like BAUD, ISP, BPS, QAM and Modulatar/Demodulator seemed, to me (by reading kosi's question), to be irrelevent (to the question) and possibly confusing.

Sorry for any inconveniences.
ijf, I realize that mine was a bit wordy and probably in more detail than anyone really cares about but I wasn't really trying to answer, just trying to keep kosi from getting any wrong information. And if kosi does care about it, all the better. I didn't think Celtics was too awful bad though. Maybe a little detailed but then again kosi never mentioned level of detail & it wasn't a whole lot higher than yours. I would think most novices could read Celtics and get the general idea of what was going on and I just didn't think yours had any extra information that his didn't. Yours was just a brief summary of his.
Actually reading your answer you don't explain "so please tell me how it works?" but just explain what a modem is. "A modem takes information from your computer and changes it into sounds that are sent over a phone line just like your voice is" explains its functionality but not really how it works. Was the user more interested in what it was than how it works? Probably I would guess but that's only part of what was asked. To know how it works I think you have to get a bit detailed. Maybe not to the level I did but more like Celtics which lies somewhere between yours and mine.
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