The basic factor that determines a modem s performance is the maximum speed at which it is capable of sending and receiving data. All modems available today are capable of speeds up to 56 kbps. External modems are housed in a box that sits on top of your PC and connects to it via a Serial Port. Internal modems, on the other hand, are housed inside the computer s cabinet itself as a card that plugs into an empty expansion slot on the motherboard. Here is where the main issue of choosing a modem lies.
Internal modems are cheaper than external ones and perform just as well.
What s special about them is that they move all the main circuitry of the modem onto the motherboard, leaving just the phone line interface to a tiny plug-in card that is available for a lot less than standard modems. It is interesting to note that a large majority of modems, from different companies, have the same basic chipset inside them. Thus they are in a similar price range and offer more or less the same set of features. So buying one is more a question of where you get the best deal. The performance of a modem depends to a very large extent on the quality of your phone line.