Modem stands for Modulator / Demodulator. A standard modem translates (modulates) a computers digital information into sounds (an analog signal) that can go over a phone line. It also translates (demodulates) incoming sounds from the phone line back into digital information that the computer can understand.

Most modems have a top speed associated with them. Most new modems are 56K modems, meaning they have the capability of receiving data at a rate of 56,000 bits (56 kilobits) per second.  Other modem speed ratings are 33.6k, 28.8k, 14.4k and 9.6k

Types of Modems

There are many different varieties of Modems. Some are internal, meaning they plug into a slot inside the computer (PCI or ISA). Others are external Modems that are built into boxes that sit outside the computer and connect to it through a serial or USB cable.

An internal modem will take up a COM Port just like a serial port will. As far as the computer is concerned, the modem looks just like a serial port with an external modem connected. Internal modems also need to have a unique IRQ. With Plug & Play, IRQ's are automatically assigned. On older computers the modem needs to be set with a jumper to an IRQ that is not being used by the computer.

Internal Win Modems

These are also sometimes known as SOFT MODEMS, HSC or HCF Modems. These Modems go inside the computer and plug directly into the motherboard. These are generally the poorest types of modem. Instead of having all the chips built into the modem, they use software to run part of the modem which slows down the computer. If the computer does not have enough processing speed while doing other tasks, this will cause the modem to have problems. Because they have less hardware on them, they are the cheapest types of modems and, unfortunately, nearly every new computer comes with one of these fitted. Most PCI modems are WIN Modems as they require the higher speed transmission to the chip. It is rare but not impossible to find an ISA WIN Modem. RPI modems were a precursor to the WIN modem, which did error correction and compression in software. The only advantage to WIN Modems is that they are easy to upgrade, as nearly everything is done in software.

Internal Non-Win Modems

These modems also go inside the computer, but unlike WIN Modems, these have all the chips they need on the modem. This generally makes them more reliable, but also more expensive. They can normally still be upgraded through flash ROMs on the modem, although they are less flexible than the WIN Modem. Usually these are ISA modems as they do not require the higher speed transmission to the chip since the speed of the modem is much less than the bus speed in the computer. Some  Non-Win Modems, however, may be PCI.

External Modems

These modems are a separate box that sits outside the computer and usually plug in through a serial port. On PC's, the serial port is either a 9 pin or a 25 pin port. The 25 pin port is also known as an RS-232 port. On older Macintoshes this is a circular port. External modems can now also be connected using a USB port. This means they can be plugged into a USB port on either a PC or a MAC. External modems are all non-Win Modems (although with USB, Win Modems are theoretically possible and may exist someday). External modems cost the most but have the benefits of being easily installed, having lights that tell you the status of your connection, and of generally not being a Win Modem. On PC's, there is a chip in the computer called a UART (Universal Asynchronous Receiver Transmitter) that handles the communication to the modem. On 486 computers and below this chip was usually an 8250 chip, which can only reliably support speeds up to 19200. On Pentiums and above the chip is a 16550 which can support speeds up to 115200. For this reason External 56k modems will not generally work on 486 computers.

Macintoshes do not have the issue with supporting 56k modems, but some cables that come with modems for the MAC do not support Hardware Flow Control.