Link up! Communications save time, travel and money.

Link up! Communications save time, travel and money.

By James O. Scharf

JAKARTA (JP): A few days ago, while I was scrounging around in
Glodok, I noticed that not many people were selling modems. I was
surprised to learn that many dealers, who were previously doing a
good business in communications, had stopped selling them. The
general opinion was, "No market!"

In most office environments, the fax machine has taken over as
the prime communications device. I admit, I use my fax much more
than I use my modem. Let's face it, the fax is very user friendly
and gets the job done. Yet, there are many instances when the fax
is a poor substitute for high-speed communications and may often
create problems in offices where telephone traffic is intense and
phone lines are limited. Here's a couple of actual (and typical)
cases.

One CEO needed to fax large documents to his home office in
London and their other Asian-based offices. His international
phone bills were killing him. A friend of his, a CEO at another
company with multiple offices in Jakarta and remote sites
throughout Indonesia, complained that his telephone lines were so
tied up with fax-traffic that he usually had to resort to sending
documents by courier. After installing communications, the first
CEO is now saving over US$40,000 a year and the second has
reduced his number of dedicated fax lines and no longer needs
courier services.

Communications can be a real boon if properly set up and can
just as easily turn into a nightmare if they aren't. If you're
new to computers, you may be wise to stay away from them until
you're confident that you know both your machine and its
operating system.

A simple communications set-up would be a computer and modem
at each end of a telephone line. One modem won't do the job, at
least not yet. Although "modem" has become a word that everyone
uses, it is really a contraction of the electronics term
"modulator/demodulator." Without going into a lot of electronics
claptrap and 50-cent techno-jargon, you may think of it simply as
an encoder/decoder.

Actually, it works pretty much like your telephone when you
dial a number. It takes binary data, changes it into a series of
audio tones and sends these over the telephone wire to another
modem which receives the tones and turns them back into binary
data.

Buying a modem is a bit like buying a computer. They run the
gamut from simple and slow to complex and fast, with the price
running accordingly.

The first thing to consider is speed. Although you will hear
the terms "baud" and "bps" (bits-per-second) used
interchangeably, this is not always true. Baud is defined as: a
change of signal state per unit of time (seconds) and is a
function of the signal being sent. This becomes more confusing as
the various transmission standards redefine bit usage and various
data compression and error suppression techniques are employed.

To avoid confusion, it may be better for you to refer to bps
values. Since, usually, a start bit and a stop bit are appended
to the 8-bit character, you may divide bps by 10 to get
characters per second. I have included both values in the figure.

A realistic range of modem speeds will use 1200, 2400, 4800,
7200, 9600, 12000, 14400 and 19200 bps. Anything under these
speeds is too slow to consider (300 or 600bps) and above these
speeds (38400 or 57600bps and up) may be unrealistic.

Telephone lines, especially in Jakarta, are just not that
clean. On international lines, higher bps rates are possible and
initial data links can be established at 57600bps. (Although
downshifting usually occurs.)

Other than speed, communications also have direction. On a
two-wire telephone system this is called a "half-duplex" channel
and is analogous to a radio operator saying "over" to let the
other person know he is ready to receive.

Four-wire telephone systems have "full-duplex" capability
since they can use one two-wire pair to send and the other two-
wire pair to receive.

Modern modems can also have full-duplex capabilities on a two-
wire system by using half of the signaling rate capability to
send, and half to receive. Multi-duplex or "multiplex" channels
use this technique to carry more than two data signals on the
same channel. This forms the basis for networked systems.

Unlike voice communications, data communications need some
method of telling the receiver that a new character is being
started or stopped. This could be compared to two people sending
Morse code with flashlights. They may agree upon a two second
"off" between characters and a five second "off" between words.

Asynchronous transmission requires the sender and receiver to
agree on a mutual transmission format to accomplish data
synchronization. We may both agree to use one start bit, eight
data bits, and one stop bit as our transmission format.

This bit sequence is called a "frame." We may also agree to
use parity error checking within the frame. If so, another bit
will be appended to the seven data bits just before the stop bit.
(Parity error checking was covered in this column on Oct. 18,
'93.) Most users just set "None" for parity since it's not very
reliable for communications.

Synchronous transmission is based on frames being defined by
external timing signals that are added to the transmission
signal. Start and stop bits are no longer required. This requires
added modem circuitry and more advanced error correction.

As you can see, both sender and receiver must agree on just
how to set up their modems before successful communications can
occur. At minimum, these communication "parameters" include: baud
rate, number of data bits, number of stop bits and parity
setting. For example, CompuServe requires eight data bits with
one stop bit, no parity and full-duplex.

Another confusing issue for users new to communications are
"protocols." File transmission protocols provide error detection
and define how much data will be sent as a block. Some of the
more popular protocols are: Kermit (96 characters), Xmodem (128
bytes), Ymodem (1K) and Zmodem.

Which protocol is being used must also be agreed upon before
transmission starts. The problem with Kermit, Xmodem and Ymodem
is that they will retransmit the entire block of data should an
error occur somewhere in the block. On noisy phone lines, this
could slow data through-put considerably.

Zmodem sends large blocks of data but uses "checkpoint
restart." Every so often during transmission, Zmodem will issue a
checkpoint. Should an error occur, Zmodem will retransmit only
the data between the checkpoint and the error. You and a friend
may try transmitting a 20K or so file using each protocol and
checking transmission times.

Now we get to the really heavy-duty protocols that do not
retransmit data but provide error correction "on the fly." TCP/IP
stands for Transmission Control Protocol/Internet Protocol and
was originally developed by the Defense Advanced Research
Projects Agency. ISO/OSI stands for International Standards
Organization/Open System Interconnection. This protocol divides
communications into a seven layer "protocol stack" and is only
used in networked systems. Both of these protocols rely on the
Hamming, Reed-Solomon or Bose-Chaudhuri-Hocquenghem error
correction codes.

If you're just communicating locally, my advice is to keep it
cheap and simple. A 9600 baud modem running Zmodem will work
nicely and won't put you in the poor house. By the way, make sure
the modem you choose is Hayes compatible. Hayes is the de facto
standard for PC to modem communications and uses the AT command
set.