larry@uunet.uu.net (Larry Lippman) (06/30/91)
In article <telecom11.483.4@eecs.nwu.edu> bruce@pixar.com writes: > In a PBX with DID trunks, what happens if something breaks and the PBX > never does return supervision? Does a two-way audio path exist? Will > the connection stay up as long as desired, or time out? It depends upon the type of central office apparatus serving the DID customer, and it also depends upon any interface apparatus installed by the telephone company at the DID site. One extreme is best illustrated from the days of the Bell System "voice connecting arrangement". In the case of DID trunks, the voice connecting arrangement was known as the C22, and utilized the WECo 112A interconnecting unit. This arrangement contained a one-way amplifier which was switched in the circuit to permit transmission of call progress tones and intercept announcements from the PABX to the calling party; i.e., no voice transmission could exist from the calling party to the PABX. Only when the PABX made a contact closure on the supervision leads was the one-way amplifier switched out of the circuit. If the PABX were never to return answer supervision, a two-way voice path would never exist. DID installations where bi-directional transmission is "enforced" by supervision from the PABX still exist today. At the other extreme are wink-start outgoing DID trunks running directly from the CO to the customer PABX without one iota of restrictive interface apparatus. From a practical standpoint most ESS CO's monitor the duration of an outgoing DID trunk that has not seen supervision from the PABX; after a given time interval (usually three to five minutes) the connection is automatically dropped. Trunk usage reporting software in an ESS CO will monitor DID trunk usage with and without supervision, and any attempt to "cheat" on the part of the PABX will show up as abnormally long trunk holding times without supervision. I've not heard of a PABX being intentionally configured to defeat answer supervision, but I feel certain it has been tried. :-) Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry