Category : Science and Education
Archive   : PCL54.ZIP
Filename : MODEM.TUT

Output of file : MODEM.TUT contained in archive : PCL54.ZIP




Two concepts best summarize computer/modem telecommunications:
addiction and mild frustration.

Put simply, modem communication is perhaps the most addictive,
useful and powerful application of a computer - albeit a process
fraught with the need for patience and persistence. No pain, no
gain, as they say. First let's look at the pieces of the puzzle.

Computer telecommunications, simply stated, is the process of
hooking one (or more) computers together via a telephone line to
exchange files of information and programs or carry on a live
conversation. Your computer will need a device called a MODEM
which allows data to be converted into a special signal which
can be transmitted over phone lines to another computer which
awaits the information.

There are two types of modems: internal and external. External
modems are slightly more expensive than internal modems and are
housed in an external box or enclosure. External modems require
a socket on the computer or port (usually the serial port on the
back of your computer.) The serial port is also called a COM
port, com for communications.

Most PC's come with a serial port, a few must have it installed.
If you are already using the serial port for something else
(e.g., a mouse or pointing device) you may need an additional
port installed or temporarily unplug the device using the serial
port. External modems take up a bit more space and add a bit
more clutter (internal modems mount inside the computer.)
However some people like the external modem with its blinking
lights which instantly displays call status - something a hidden
internal modem cannot do unless special display software is also
installed or activated. Another consideration is that external
modems are portable, you can move from machine to machine by
simply unplugging the external modem and taking it home or to
the next job site.

Next some basic terminology to help things along. The "host
computer" refers to the computer your PC is calling. Sometimes
this is a commercial service such as The Source or Compuserve
which are large interactive services. Other times the host may
be another pc your friend is using on the other side of town or
perhaps a BBS (bulletin board system) which is a computer
operated for general public access by a private individual or
company with files and messages of general public interest and

Another important term is the baud rate associated with the
modem transmission process. Essentially baud is a reference to
the speed at which your modem and the host modem exchange
information. Some common rates are 300 baud, 1200 baud and 2400
baud. A 1200 baud modem is roughly 4 times faster than a 300
baud modem. And since you may be transmitting information via
long distance lines, speed and brevity of the modem link is of
interest. Some BBS (bulletin board systems) will not allow
slower 300 baud modems to load or access software files since
this ties up the "board" for other users. As an aside, a 2400
baud modem can be switched to transmit and receive at the slower
1200 and 300 baud rates so there is some "downward" flexibility
in purchasing a high speed modem which allows lower baud rates
as needed.

Curiously the software which controls the modems and computers
linking during this exchange will "synchronize" the two modems,
determine which modem is using the slower speed and adjust the
faster modem downward to a that speed. Many BBS systems use the
2400 baud rate and some are even capable of 9600 baud - in this
case the BBS host computer will slow to your rate. If you plan
to do much long distance modem work, the 2400 baud modem is the
wisest choice. The somewhat higher initial purchase price which
will obviously be offset by savings in reduced long distance

Next we dip into the software pool. While DOS controls the
internal workings of your computer, you'll need communications
software (sometimes called terminal software) to enable the pc,
modem and telephone line to work together. Many commercial
software packages have a communications package built in or
added as a separate module.

Shareware and public domain packages also exist (which are
themselves obtained from BBS systems via modem!). Some of the
more dependable software packages include Qmodem, Procomm,
Boyan, Telix, and GTpowercom. Good communications software is
flexible and powerful without being difficult to start and use.
The best packages have help screens and tutorials as is common
with application software in general. Telix, for example asks a
few simple setup questions then attempts to set the variety of
numerical settings needed without operator intervention or

The favorite modem software package for the author of PC-LEARN
is the shareware software product: TELIX.

Although baffling at first, the modem software setup process
allows configuring the software package to your specific PC and
modem. The modem software needs to be told if you are using
serial port (com port) one or two. You will need to tell the
communications software which directory or drive where files
will be sent or downloaded. The baud rate must be set as well as
noting if the modem is Hayes compatible. Hayes compatibility
refers to a standard set of transmission instructions which many
modems share. In some ways this is similar to the DOS standard
within operating systems for IBM compatible computers.

The modem software must also be told other basic details such as
PARITY, and DATA BITS which refers to how the data is "grouped
or packaged" during the actual modem transmission. Generally
data bits are set to 8. Parity is usually set to NO. Commercial
services such Compuserve offer exception to these settings and
will tell you these settings in their literature.

An example: If parity is required, set data bits to 7. Set stop
bit to 1. This is a total of 1 + 7 = 8 Bits. The final bit is
usually used to check parity/accuracy. The above setting is
somewhat cryptically referred to as 8N1 which means 8 bits, no
parity, one stop bit. Don't worry about this too much now -
frequently this is a one time setup which you will not have to
fiddle with once the software has been set.

Other settings can include ANSI standard terminal emulation (a
standard selection you should check if given the choice). ANSI
terminal emulation is usually the default setting and should
only rarely be changed.

PROTOCOLS must also be specified. In simplest terms a protocol
is a method of checking for errors between modems as the data
transmission progresses. In a precise fashion, these protocols
tell the first modem to transmit a small packet or "chunk" of
data then the receiving modem will play back a short verifcation
"checksum" or "password." The transmitting modem stops, listens
for the checksum and if that item matches the information which
it sent, the first modem then transmits the next packet/chunk of
the message. This continues, one small chunk of data at a time.
The concept here is to protect data integrity over noisy,
static-prone telephone lines.

UPLOADING data to the host (you transmit data to the other
computer) and DOWNLOADING data (you receive data from the other
computer) requires selection of a protocol method.

Several common protocols include Xmodem, Ymodem, Xmodem-CRC and
Zmodem. Xmodem is generally offered by most BBS systems and is a
safe bet if you are not sure. Other protocols are somewhat
faster and/or slightly more dependable, but Xmodem will usually
do the job. Trial and error or simply logging onto the BBS
system with Xmodem and then inspecting the menu of other
protocols available will give you more detailed information and
clues about other possible protocols.

Ymodem offers a faster transmission rate than Xmodem, better
data security through a refined CRC checksum method and finally
a method for "batching" or sending/receiving several files one
after another unattended. Zmodem is faster still, somewhat more
secure with respect to data loss, likewise offers batch
transmission of many files and additionally allows a file to be
partially stored if transmission connections are lost and then
completed when the connection is restablished so you do not have
to transmit the entire file again!

General considerations in selecting software for modem use

* The dialing directory is of crucial importance. The idea is
that the package will store and dial your most frequently dialed
numbers. Some packages hold only a few, some quite a lot! Does
the package store your individual password and comments next to
each number? You may wish to use different passwords for each
BBS system you call.

* Consideration of transfer protocols offered. Xmodem is a
basic, but you may want more efficient protocols later. Make
sure the package supports several or can be upgraded to more
protocols later. Zmodem one is the best at the time this
tutorial was prepared.

* How does the package dial numbers from the list? Will it call
back and continue trying? Can it dial numbers from the list in a
certain order. Can it do circular dialing (several numbers are
called in a row, then the program jumps from the bottom of the
list and starts at the beginning again - around and around in a
circle)? This type of dialing efficiency is important if you
deal with busy communications services and BBS systems where it
may take time and several attempts to log on to a system
especially during the busy evening peak time.

* Does the package offer scripts or macros? Simply stated a
script is a way of having the package automate several steps.
Without further input the package can call a BBS system, type in
your password and then continue typing further keystrokes as if
you were entering them from the keyboard yourself - in essence a
script capability is a programming-like language which gives the
communications software package a customized way to enter many
keystrokes while you only input one or two strokes to start the
chain reaction. Many programs let you even record the sequence
of keystrokes during a communications session for playback and
retrieval later. Beware though, if the system which you are
calling changes log on sequences or you may have to go back and
later revise your original script - not a major problem, just a

* Is it possible to capture screen information to your disk
drive? Can you output screen information to your printer? Long
communication sessions on line with your modem produce endless
screens of information which scroll by quickly. The ability of
your software package to output all or selected parts of your
session to permanent disk file or printer output can be a tidy

Local neighborhood BBS systems can offer the most entertainment
and information for the first time user. The cardinal rule of
behavior: "act as if you are in someone else's home." Courteous
behavior is a must. How do you find telephone numbers for these
systems? Consult local computer clubs for a few numbers for BBS
systems and then call the BBS systems with your modem for lists
of MORE local telephone numbers. BBS operators and users are
quick to answer questions for new users and can serve as helpful
"experts" on any computer question no matter how arcane.
Operators of these BBS systems are frequently call SYSOPS and
many provide BBS systems as a hobby activity much as other
hobbyists enjoy short wave radio operation.

Most BBS systems will allow you to tour within the various
activities and parts of the system for 30 minutes to an hour on
your first call. After that you are expected by some "boards" to
leave your name and a phone number where you can be reached for
verification. Some boards are free, some request a nominal use
fee (usually around $25 per year.) Some boards request that you
upload (send to them via modem) shareware files or public domain
software in order to download their software. The process is
something like two kids with bubble gum trading cards: "I'll
trade you 125,000 bytes of this shareware file for 200,000 bytes
of your files."

On your first visit into a BBS board you usually are allowed
access only to certain "low level" but interesting areas of the
BBS "board." The SYSOP (system operator) may be available to
come "online" if you experience difficulty. The software of the
BBS system itself will safely keep you from damaging the host
computer even if you hit an incorrect key or enter an erroneous
command. You may leave messages for the SYSOP, leave messages of
a general nature, ask for help on a specific question, download
public domain software, play a game, or just generally explore
the information within the system of the host computer.
Downloading files is easy. For many boards the command is a
simple "D" for download. The BBS will ask you the name of the
file you wish to download and then give you an estimate of how
long it will take and the modem protocol you will (or should) be
using. After that you key in a keystroke or combination which
tells the software in your computer to begin the process. When
the transfer is complete, the host BBS takes over and presents
the main menu of choices or selections - in essence you can
continue to browse as before.

Lists of phone numbers for online services and BBS numbers are
available from the boards themselves and many local newsletters
of computer clubs and many computer store salespersons.

Most public domain and shareware software from BBS systems is
stored and transmitted in a compacted form of either an ARC or
ZIP format. You will need a small public domain program (ARC or
NARC or PKZIP) also available from the BBS board to unpack the
software. This archiving or "arcing" process is done to compact
the software for shorter, more efficient transmission - it saves
time and disk space for both the BBS host computer and your
computer. The easiest way to visualize arc'd or zip'd software
is to think of the many files of a typical software package
compacted in size and then placed into one single file. You
might also wish to compact your other files for storage on a
hard drive aside from the modem process. File compaction is a
useful tool unto itself! ARC and ZIP software are shareware
packages available from BBS systems.

Working with modems and telecommunications software sounds
arcane and daunting at first, but it is a thoroughly enjoyable
and rewarding pastime - and also a nice way to make friends and
delve deeper into the global network of information and
friendships this new form of communication represents.