Using A Modem for Dial-In Service

What a modem does:

A modem is a peripheral device that allows your computer to exchange information over a phone line with other computers. It doesn't matter if it's between two desktop computers or a desktop computer and a large mainframe computer. The two computers don't even have to use the same operating system. A modem converts a digital signal from your computer into an analog signal that can be handled on a phone line and the modem on the other end converts the signal back to a digital signal. The modem MOdulates/DEM odulates... hence Modem.

Modems can be either internal or external.

External modems are attached to the serial port of your computer. Internal modems for desktop computers (if available) are usually cheaper than external modems; however, on laptops the opposite is true -- internal modems are somewhat more expensive. E xternal modems require a power source (voltage converter), a cable which runs from the modem to the computer, and a phone line from the wall to the modem). Internal modems only need a phone line to run from the wall to the modem jack in the computer. Inte rnal modems are more convenient for those who need to take a computer on the road for business, field work, etc. However, if you plan to do e-mail, data communications, etc. from one place, then an external modem will probably suit your needs. The size of a modem these days is not of great importance as they are relatively small and take up little space on your desktop.

Modem Speeds

Modems transmit data to other computers and their speed is measured in bits per second (bps). [a bit is the smallest unit a computer communicates with -- 8 bits comprise a single character]. Modems currently sold are rated at 1200, 2400, 9600, 14,400, 19, 200, etc. The University of Michigan currently supports dial-in modems at the 1200-28,800 speeds. While a 2400 baud modem is adequate for e-mail, it is slow if you're downloading (receiving) or uploading (sending) a large file from your computer.

Rules of Thumb for Modem Speeds

To provide some perspective to these speeds, it's helpful to look at some elementary computer math. A single character is comprised of 8 bits of information.

     8 bits = 1 byte

     1024 bytes = 1 K = approx. 170 words

     1024 K = 1 megabyte = approx. 175,000 words = 3-4 average sized books

A modem that can transmit 2400 bps (bits per second) is sending 240 characters per second. (10 bits are required to send a single character). Since each word roughly 6 characters long, we're talking of a rate of 40 words/second or 2400 words per minute. A 14.4K modem which is six times that speed can send 235 words/second or 14,000 word/minute. Roughly speaking, a modem's bps speed is the number of words per minute it can transfer. So, a 2400 bps modem transfer about 2400 words per minute and a 14.4K modem can transfer 14,000 words per minute.

However, not all files will be text (e.g. graphics) so it's also helpful to know tranmittal rates by file size. A 2400 bps modem will transfer 1 MB in about 60 minutes and a 14,400 bps modem will do the same job in about 10 minutes.

Hayes Compatibility

The Hayes modem company pioneered the AT command set and set the state for the de facto standards. Virtually every modem on the market today supports this standard. Software that runs your modem, called communications software (examples are SmartCo m, VersaTerm, ProComm) uses these commands. It is important to be sure that your modem is "Hayes compatible."

You will often see a modem associated with a "V number". These numbers reflect a set of international standards called the CCITT Standards (Comite Consultatif Internationale Telegraphique et Telephonique) for data communication. The standards and capabili ties are: 

     V.22       1200 bps data
     V.22bis    2400 bps data
     V.27ter    4800 bps fax
     V.29       9600 bps fax
     V.32       9600 bps data
     V.32bis    14,400 bps data
     V.34 &
     V.34 FC    28,800 bps data (not yet standardized)
     V.17       14,400 bps fax
     V.42       The standard for point-to-point error control 
     V42bis     The standard for two similarly equipped
                modems to compress data, enabling faster
                transmissions.

Using SLIP or PPP

The Connectivity Software that the University of Michigan distributes (LAN Workplace for DOS and PPP/MacTCP for the Mac) requires a minimum of 9600 bps second to work and really, one should be using a 14.4K modem. PPP (point to point protocol) will allow for use of NCSA Telnet (which allows multiple windows within one session), NCSA Mosaic, TurboGopher, Fetch, etc.and thereby eliminate the messier details of knowing how to get around the net via Unix commands.

University of Michigan faculty, staff, and students can get the connectivity software from the swdist server.

Fax Modems

These allow you to send and receive faxes. The fax part of the modem sends/receives data that is interpreted as a picture. One can create documents on a computer (resumes, papers, thesis proposals) and send them direclty from the application to a fax machine or a computer with a fax modem. Instead of sending the print job to the printer you're sending it to the fax modem. The information is sent over telephone lines to a remote fax machine or fax modem. That is, any file in your computer can be sent to a fax machine or another computer with a fax modem. One can fax résumés, thesis proposals, etc. to a fax machine in a matter of minutes.

Selected Modem Manufacturers