Msg: 195 *Conference*

07-09-91 22:35:43

From: ED DAHLGREN

To : MARK BARTNIK

Subj: MODEMS & PBXS

Er, ah ....
 
Two points from Richard's reply (#3357) are right on the mark: the very best
way to transfer data between systems is over a direct link ... the more you put
in the way, the more problems you'll encounter AND the character you're getting
is due to unwanted junk on the line (caused by putting too much distance &
equipment between your two systems ....).
 
However, it's not the PBX's fault necessarily.  Direct telephone lines are
quite subject to noise, and you'll see the same doofy character.  In fact, if
you're actually dialing out into the outside world on one line and back into
the very same building on another line, the chances are that it's the outside
lines that are generating the noise.  You could try dialing from one extension
to another and see if that eliminates the problem.  Best bet, though, is still
to go to a cable that runs from point A to point B.
 
Richard, PBX's ARE multi-user systems and ARE run by computers, and salespeople
of modern systems DO like to call telephones "terminals," but ....  In a
time-shared computer system, you're actually sharing some of the system's
resources, while in a PBX you're (typically) not.  Once the channel is
assigned, there's no resource-swapping going on.
 
If the PBX uses electronic phones, with displays and dialing memories and all
kinds of junk like that, typically the link between the phone and the PBX
consists of a voice path PLUS extra bandwidth to accomodate the extra stuff.
The system doesn't need to poll the voice connection because any changes of
"terminal" status are indicated by hanging up or are otherwise initiated by the
phone, like when it goes on-hook for, say, 1/2 second to tell the system it's
going to tell it something.
 
There are (fortunately, obsolete) technologies around like TASI --
Time-Assigned Speech Interpolation -- that actually do swap a resource (the
long-distance circuit) among users.  You'll know if you get on one of these,
though, because you won't be able to sustain a connection at all.  Modems hate
this kind of nonsense.  This type of technology was developed in the days of
laying copper cable across the Atlantic, when long-haul data communications was
a pipe-dream and satellites didn't exist.
 
The one type of telephone switcher that really IS a killer for data is the old
electro-mechanical kind.  I doubt that there are any of these in modern
businesses, just in way-rural, mostly private, telephone companies.  These are
switches that hunt across a series of electrical contacts, and at every
make/break they generate tremendous noise on the lines.  Not healthy for data
links at all.  But again, this is extremely rare, and I think the junk is being
caused by the actual, outside-world circuits themselves rather than any of the
equipment attached to it.
 
One final comment: GO DIRECT!  (8*)  Save what little hair your poor
phone-system manager has left, and push your data at far greater speeds than
you can over ordinary phone lines, anyway.
 
                                                   -- Ed