What is a modem, anyway?
This is a copy of:
http://www.globalmark.com/bluecomm/whatismo.html
A modem is a device that can occupy an internal slot in a computer (called a "half card" or "internal" modem). Or it can be an external device (usually referred to as a stand-alone modem) and hook-up to a computer via cables.
Modems convert a computer's binary language into audio tones. Those tones can then be sent anywhere in the world using ordinary telephone lines. But, in order to receive these tones, there has to be a modem at the receiving end, too. At the receiving end, the modem "hears" the tones and converts them back into the binary language used and understood by computers.
This process is called Modulation Demodulation (hence MO-DEM for short).
In order for the modem to work, there has to be something between the modem and the computer. An intermediary that makes sure the data transmitted is useable . . . that's what the communications software program does. This is the program that instructs and directs both the computer and the modem so the computer will know what to do with the data it has received.
A modem's speed is measured by bits-per-second ... usually expressed as "bps". It wasn't too long ago that 2400 bps was considered as being high speed. Today, however, even the home user can buy an affordable 14,400 bps modem! Which is to say that the 14,400 bps modem will transmit data six times faster than a 2400 bps modem.
V-DOT WHAT?
V-dot-this. V-dot-that.
V.32 - The Consulting Committee for International Telephone and Telegraph (or "CCITT") standard for 9600 bps and 4800 bps modem operation. V.32 modems fall back to 4800 bps when line quality is impaired.
V.32bis - The CCITT standard for a fairly broad range of modems capable of higher operational speeds; 14,400 bps, 12,000 bps, 9600 bps, 7200 bps, 4800 bps. V.32bis will automatically seek a lower operating speed when line quality is poor ... and when line quality improves, the modem will automatically move forward to the highest speed.
V.42 - The CCITT standard for detection and negotiation for LAPM error control. V.42 also supports MNP Levels 2 through 4.
V.42bis - An extension of V.42 defining the data compression 4 to 1 ratio protocol for use with V.42.
DATA/FAX MODEMS
Data/FAX modems are engineered for transmission of data, but they also have an added capability that enables you to send and receive FAXes from your PC. There comes the obvious advantage of not having to run to another machine to send a FAX. But there are several other significant advantages:
Consider that the FAX you send from your PC can more easily be treated as "confidential".
Sending FAXes to several people is quite easy ... easier than doing the job repeatedly at a stand-alone machine.
You can keep FAXing costs in control by programming your PC to do the job when rates are lower than peak use time rates.
You'll even have the benefit of being able to print out a FAX on plain paper.
FAX MODEMS HAVE A LOT OF CLASS.
Check out the specifications for a FAX modem, and you'll find reference to the modem's compliance with a certain "class" ... and an additional reference to the "group" the FAX modem belongs to. That's OK as long as you understand the differences between them:
CLASS 1 MODEMS utilize a series of Hayes-type AT software commands to control FAX boards. Class 1 modems do not have the ability to automatically detect whether an incoming call is data or FAX. The modem will have to be set for one or the other.
Comment from John Navas (JNavas@NavasGrp.com): USRobotics modems
support Adaptive Answer in Class 1 mode.
CLASS 2 MODEMS have hardware that provide the ability to automatically detect whether the incoming call is a data call or a FAX call.
GROUPS, TOO?
As far as FAX modem "groups" are concerned, the differences between them are easy to understand.
Depending on its operating speed, and level of technological sophistication, a FAX machine/FAX modem will fall into one of four groups:
GROUP 1 is for FAX modems/machines that transmit a standard 8-1/2" x 11" page in approximately 6 minutes.
GROUP 2 indicates that the FAX modem or machine will transmit a standard 8-1/2" x 11" page in approximately 3 minutes.
GROUP 3 will be a digital machine/9600 bps machine or modem. FAX modems in this group will send an 8-1/2" x 11" page in as few as 20 seconds!
GROUP 4 at this point in time, includes those machines and modems that represent the state-of-the-art; operate at 64,000 bps! That's fast!
A rule of thumb: faster modems cost more to buy, but less to use than slower
modems.