modems

A modem-(Modulator/Demodulator) is a device that enables data to be transmitted between computers, generally over telephone lines.

 Computer information is stored  in  digital form  while  information transmitted over telephone lines is transmitted in the analog  form. A modem converts between these two forms. There are internal  and external modems Internal modems are  found  on a printed  circuit board inserted into the computer (they come as expansion boards that you insert into a vacant expansion slot) while external modems are outside the computer system. Normally external modems are considered superior to internal ones given that they have lights which indicate how the modem is functioning and because they can easily be moved from one computer to another.

Modem interfaces are standardizee. However, there are different protocols a number of different agreed-upon formats  for transmitting databetween two devices  Some, like CCITT V.34, are official standards, while others have been developed by private companies. Most modems have built-in support for the more common protocols -- at slow data transmission speeds at least, most modems can communicate with each other. At high transmission speeds, however, the protocols are less standardized.

Aside from the transmission protocols that they support,there are some features that distinguish modems from one another. Some are:

1.  How fast the modem can transmit and receive data.  At slow rates, modems are measured in terms of baud  rates. The slowest rate is 300 baud (about 25 characters per second). At higher speeds, modems are measured in terms of bits per second (bps). The fastest modems run at 57,600 bps, although by compressing the data they can have even  faster rates of transmission. Obviously, the faster the transmission rate, the faster you can send and receive data. Note, however, that you cannot receive data any faster than it is being sent. If, for example, the device sending data to your computer is sending it at 2,400 bps, you must receive it at 2,400 bps. It does not always pay, therefore, to have a very fast modem. In addition, some telephone lines are unable to transmit data reliably at very high rates.
    
2. Many modems support a switch to change between voice and data modes. In data mode, the modem acts like a regular modem. In voice mode, the modem acts like a regular telephone. Modems that support a voice/data switch have a built-in loudspeaker and microphone for voice communication.
    
3. (auto-answer) An auto-answer modem enables your computer to receive calls in your absence. This is only necessary if you are offering some type of computer service that people can call in to use.
    
4. The ability to perform data compression, which enables them to send data at faster rates. However, the modem at the receiving end must be able to decompress the data using the same compression technique.
    
5. The existence of flash memory  as opposed to the conventional read only memory (ROM)
    
6. the ability to receive and send faxes. Those that can are called Fax modems