Communications
Volume Number: 1
Issue Number: 8
Column Tag: Mac Meets Ma Bell
“A Communications Primer”
By Bruce Lieberman, Software Engineer, MacTutor Contributing Editor
Welcome to the wild and wacky world of Telecommunications, this article is not
meant to be the definitive introduction to this subject (as it covers far too vast an
assortment of related areas), just a bit of dipping your toes in the water and getting
familiar with the terminology commonly in use.
In a nutshell, you are going to try and have your computer communicate a stream
of binary digits to another computer, have everyone understand the process involved
and make sure that the information got to its destination in an unadulterated form.
There is a catch, you are going to do this across a very restricted (slow, limited
frequency bandwidth), noisy, error prone medium ..... the telephone companies phone
lines and switching centers.
There's the rub. For speech, phone lines are good enough to maintain
intelligibility (most of the time), for music ..... forget it, and for a stream of digital
data ...... no way!
Enter the MODEM (MOdulate / DEModulate), this little goodie can take your
digital data bits and convert them to an audio tone that represents 1 or 0 and convert
them back again. The frequency of these tones is predetermined by an accepted standard
for the person originating or answering the call (Bell 103, Bell 212, etc.) and the
tone frequencies themselves easily fit within the limitation bandwidth of the phone
system.
In the old days there were acoustically coupled modems that placed the old style
phone handset into a box that had two holes surrounded with rubber (to make a tight
seal around the mouthpiece and earpiece), within each hole was a complementary
microphone and speaker to send and receive to and from the phone hand set. This
approach had many drawbacks, not the least of which was induced physical noise.
Nowadays we have direct connect modems that connect electronically to the phone lines
and bypass the entire step of noise producing acoustical to electrical conversion. Direct
Connect modems have been taken a step further, to what is now termed "Intelligent
modems (most of which are based on the Hayes Smartmodem which uses a standard
licensed from Bizcomp, Inc.) that can perform a number of functions without the need
of software; although all of these modems utilize software to make their functionality
that much more flexible and approachable.
Now that we have a way of handling the limited frequency response of the phone
lines, what do we do about all the noise and garbage on the line ? The loss of one
character of information can corrupt an entire file (program or text), so we must
provide a way of dealing with the errors that are part and parcel of the phone system.
Basically, all we can do is contrive a way to determine if the data survived the
transmission intact, and if it has not, send it again until it is correct (if there is a
severe problem, the system will "time-out" after a predetermined number of
retries). Each of the error correction protocols you will encounter will have there
own way of accomplishing this task, the only thing you must keep in mind is that you
have to run the same transfer protocol on each side of the transmission. We will take a
quick look at some of the communications software packages now becoming available
for the Macintosh; following that, I have included a very good glossary of
telecommunications terms.
As you get started contacting the many public and private communications
services available (BBS's, Dow Jones, Compuserve, The Source, etc.) you may
encounter certain problems in making a viable connection, some of the most common
problems are: wrong Baud Rate, the other system will not lock carrier (probably only
supports 300 baud, switch to 300 and try again); wrong Data Format, you have locked
carrier but there is garbage coming in on the screen (typically change from NO parity
to EVEN).
For more in-depth information and for a wealth of telecommunications
resources, look into these books : The OMNI series on Telecommunications (including
the Online Database Directory), Technical Aspects of Data Communications (Digital
Press), The RS232 Solution (Sybex Books) and for a look at how some of these
services work, read The Complete Handbook of Personal Computer Communications
(St. Martin's Press).
I have had the opportunity to briefly use three new telecommunications packages
available now for the Macintosh:
inTouch by Software Masters, 3330 Hillcroft 'BB', Houston, Tx. 77057
713-266-5771, $149.95. (128 or 512K)
MacLine by TouchStone Software, 909 Electric Ave, Seal Beach, Ca. 90740
213-598-7746, $145.00. (128 or 512K)
SmartCom II by Hayes Microcomputer Products, 5923 Peachtree Industrial
Blvd, Norcross, Ga. 30092, 404-449-8791, $149.00. (128 or 512K)
Each of these packages has unique capabilities of their own and all have more
functionality than MacTerminal from Apple. All of these programs allow you to
configure the communications protocol settings that you desire.
"inTouch" is a very well written telcomm progam capable of many styles of
Terminal Emulation (TTY, Beehive, Televideo 925, IBM 3101, ADDS Viewpoint, DEC
vt100 and even VIDTEX graphics!) direct and modem, it supports text file and binary
file transfers including Xmodem & Crosstalk protocols and a special "inTouch" protocol
for Mac to Mac file transfers. "inTouch" has 8 funtion (macro) keys and also
programmable keys for "username" & "password", additionally there are keys to
represent the DELETE, ESCAPE & BREAK keys found on terminals. These keys are all
softkeys addressed with a click of the mouse.
There is an onscreen clock that also functions as an online timer; a phone number
setting (with an auto redial function if the number is busy); a screen scroll buffer
(40 lines on a 128K, 400 on a 512K); the ability to print incoming data in real time;
save text to disk; manipulate incoming and outgoing CR&LF information; utilizes a
MiniFinder for selecting files for transfer; the ability to view text files; supports
auto-answer mode; an auto-execute funtion "inTouch" calls Communications Command
Language, which is a very flexible batch command system that allows unattended
telecommunications at predetermined times ....
You can write CCL files that will dial up Compuserve and logon for you, change to
the appropriate area of the system, extract the information you want and log you off
..... or you could write a CCL file that would call a number of other Macs running
'inTouch' and transfer specific files to each machine at specific times. All in all a very
nicely done, nicely documented package.
[An added feature is the IBM 3101 terminal emulation. Think you need Apple's
$1000 box to talk to IBM networks? Not so. Since IBM started making an ASCII
terminal (the 3101), they have had to support it on their networks. The end result is
that most IBM networks have IBM software that provides full IBM 3278 full screen
editing emulation for IBM 3101 Ascii terminals on the network. You dial up and log on
as an 3101 character mode terminal, then switch to block mode transmission, and the
IBM software emulates 3278 protocol for you. Hence, all that is needed to get full
3278 emulation is a software package capable of emulating an IBM 3101 ASCII
terminal sufficiently to "fool" the IBM software into providing 3278 emulation rather
than "dumb terminal" emulation. We have tested the "inTouch" program and while it
does communicate with IBM systems as a 3101 terminal, it presently lacks the
necessary block mode transmission to invoke the 3278 support from IBM's side.
However, we have talked to the company and they are very enthusiastic about
attempting to add whatever is necessary to get the 3278 support. - Ed.]
MacLine is a rather unique telcomm product, while it performs admirably as a
stand alone package, it is how it fits into a family of software written for many
operating systems that is unique. Presently TouchStone markets PCworks for the