5 Jun
2019
5 Jun
'19
8 p.m.
On 6/5/19 3:08 PM, John Labovitz via cctalk wrote:
being a BBS aficionado I?d heard of a technique called
?callback?
that some BBSes implemented, which allowed for using a POTS line for
both dial-up modem and for voice. The idea was that if you wanted
to dial into a BBS with callback, you?d dial up, let it ring once,
hang up, then call back; the modem on the BBS system would only answer
on the *second* call.
That's a decidedly different meaning for callback than I'm used to hearing.
I'm used to it being applied in a security context. As in you would
call into a system, identify yourself, hang up, and then it would call
back the number stored on file for the account. So calls going in
opposing directions.
I guess this could have also been used to alter which end paid for
calls. The first call would be short and relatively inexpensive. The
second call (back) would be longer and could be expensive and take
advantage of better long distance rates as applicable.
The software (in my case, the BYE modem/IO handler
that run on
my CP/M system) would watch the ring-detect line on the serial port,
and once it went high, it would set a timer for a little more than
six seconds (2 for the ring, 4 for the silence). If the timer expired,
it would set a flag to answer on the *next* ring ? which would have
been the second call. If a ring came in before the timer fired, it
would ignore the call ? assuming it was *not* a computer calling in,
but rather a voice call. And if no call came in within a few seconds
more, the state would reset for a new call. It worked pretty well, as
long as the BBS callers knew how to use callback. After a few months
of those shenanigans my parents allowed me to order a second dedicated
line, so they wouldn?t be woken up in the middle of the night.
Intriguing technique.
--
Grant. . . .
unix || die