John McHarry <m21198@mwvm.mitre.org> (08/22/90)
In Vol 10: Iss 585 PAT commented that setting the modems to never time out would eliminate the call waiting problem, but would cause the system to remain off hook (and hung up by never hanging up!) in the event of a disconnect. Some, but I think not all, lines have a feature called "forward disconnect." This is invoked by the switch on a disconnect, and causes the line to go either open or short, interrupting the -48v feed. I think it was provided to knock down hold buttons when the far end hung up. The hold button set a relay which was held in by the -48v feed, keeping the off hook condition when the phone was on hook. Picking up a phone drew off enough current to drop the relay, but it was necessary for the switch to interrupt the current when the other end hung up. I believe this is also used by some answering machines to detect and hang up on no message or the end of a short message. At any rate, it could be used to knock the modems down on a disconnect, if the modems were designed to sense it, or if some ancillary circuitry were provided to handle the case, perhaps by cycling the power on the computer (?!) My opinions only, with the exception of the last suggestion, which even I cannot accept. * John McHarry (703)883-6100 McHarry@MITRE.ORG * [Moderator's Note: But didn't you know that call-waiting plays the same kinds of tricks on the voltage in the phone? For example, I know someone with a five line / six button phone. He foolishly had call waiting installed on a couple of those lines, which were all independent and not in hunt or anything like that. If a line was on hold when a call-waiting signal came through, it would always knock down the hold and dump the party holding, allowing the call-waiting to force its way onto the line instead. Call-waiting is not compatible with any electronic device which depends on changes in line voltage or what it 'hears' on the line to decide what to do. Modems, hold circuits, you name it. It even makes some PBX's think the call they are handling is complete and should be disconnected. PAT]
Steck Thomas <steck@aplcen.apl.jhu.edu> (08/28/90)
The Moderator wrote, some time ago: >force its way onto the line instead. Call-waiting is not compatible >with any electronic device which depends on changes in line voltage or >what it 'hears' on the line to decide what to do. Modems, hold >circuits, you name it. It even makes some PBX's think the call they >are handling is complete and should be disconnected. PAT] PAT - I beg to differ with you on this one. On the older switching systems, this may have been true. However, on the new ATT 5ESS and the Northern Telecom DMS-100, call waiting is not implemented as a voltage changee - simply a tone introduced to the line. There is no true interuption of the line and no 'switching' sounds (clicks, etc..) like there used to be. Tom Steck