Central to all of this is a peripheral device attached to or installed in our computer called the MODEM. Modem is an acronym for Modulator-DEModulator. This device converts computer digital signals into analog signals which can be transmitted through the telephone system. In the scheme above, it converts the digital signals from your computer to analog signals at your computer, at the other end it converts these signals back for use in the other computer.
Our via equation has several parts in it. We will attempt to cover each part in the next few months. This month we will talk about MODEMs as this devise is drawing much attention within the Alamo PC community on the BBS and through the mail. Modems are required for transmission of data from one computer to another. It takes two modems for this transmission, one to send and one to receive. Modem technology has grown tremendously over the past ten years. It is within the area of modems that most of the "black art" has had to be overcome as we communicate with one another. What is this "black art"?
The reminder of this column is devoted to a discussion of the
characteristics which make up a modem. We will define several terms in
this summary as we go along our path to modem bliss. The characteristics
of modems include speed, compatibility, asychronous vs synchronous, full
duplex / half duplex, and various standards of communication design.
The speed characteristic is very important in computer data transmission. The bigger the bps number, the faster data moves, which has a cost impact on you the user. The faster data moves, the less time you have to pay for over the phone system time and other timed on-line services. A 9600 bps modem transmits four times faster than a 2400 bps modem. Many on-line services such as BBSs or Internet operate at 9600 bps. Some services such as our BBS and Compuserve offer 14,400 bps service. This allows transfer of very large files while keeping cost and on-line time to a minimum. Internet services currently operate at 9600 bps for the most part. Bits per second in modems is further translated into characters per second (cps) in most communication software protocols. For example, in my 14,400 bps fax/modem, the Telix (communication software) Z-modem protocol indicates transmitted data at 1400 - 1600 cps which directly relates to 14,000 - 16,000 bps rating of the modem.
Speed is further confused when bps is compared to THROUGHPUT.
Keep in mind that bps is just what it sounds like: the number of bits transmitted
by the modem each second. Throughput is a measure of USEFUL data bits transmitted
each second. The two numbers are not necessarily identical. Throughput
becomes important is describing data transmission when FILE COMPRESSION
is incorporated into the modem operation.
Duplex Another characteristic found in modems has to do with how the
modem send and receives data. A modem which operates at FULL-DUPLEX can
send and receive data simultaneously. A modem operating at HALF-DUPLEX
can send and receive data, but not at the same time. Half-duplex capability
is a holdover available in communication software from very early modems.
Half-duplex modes are very seldom utilized in today's high speed modems.
Modems are governed by the V series communications standards by this body. Hence, "V.32" and "V.42bis" indicate which standards have been built into a modem. These V series standards refer to CCITT standards regarding speed, error-checking and data compression for the most part. The V series standards have been with modems since the beginning. For example, V.21 refers to speed for 300 bps modems. V.22 and Bell 212A standards refer to speed for 1200 bps modems. V.22bis covers 2400 bps modems and is the first world standard. V.32 and V.32bis cover speed. V.42 and V.42bis cover error correction and data compression. V.Fast covers speed in 28,800 bps modems. This standard is to become V.34 when it is finally approved. V.17 refers to fax sending and receiving speeds at 14,400 bps. The "bis", i.e., V.32bis, refers to the second iteration of that standard. The "bis" is from the Latin and means second iteration.
Nearly all modems have a set of error-correcting techniques built
into them. These standards are collectively covered in the Microcom Networking
Protocol (MNP) standards. They are programmed into the modem's chip set.
These error-correcting standards range from MNP 1 through MNP 5. Modems
which support V.42 contain MNP 2 - 4. Look for modems which also have at
least MNP 5 also programmed into the chip set. New MNP standards are being
developed at this time.
The serial port used by the modem must be one of four available.
These serial ports are designated as COM 1 through COM 4. Most computers
come with two serial ports built in. This makes connecting external modems
fairly easy, unless, those two serial ports are being used by other peripheral
devises. When this is the case, one must add a serial I/O card to an open
motherboard slot to make one of the remaining serial ports available for
the modem. Serial ports and COM ports are the same thing.
Fax capability with high speed modems is available in nearly all
cases. Most high speed (14,400 bps) modems use Rockwell communication chip
sets. These chip sets have built in fax capability.