A PBX can connect to the PSTN via several line types, with the most common being "ground start" lines on smaller PBXs, then either E&M lines to an on-site T1 channel bank (typical in older installations) or built-in T1 interfaces (still "emulating" E&M on individal timeslots, though) in newer PBXs. The local CO will provide answer supervision on calls according to the rules for "ground start" or "E&M" lines. These rules are buried somewhere in the Bellcore specs, but I don't recall where.
PBXs should NOT use "loop start" lines (which would show answer supervision by battery reversal) because of the possibility of "glare," which is the telco term for simultaneous seizure of the line from "both sides." This is quite likely to happen on a "loop start bothway" trunk on a PBX, because the only way that an incoming call towards the PBX can "make its presence known" is to start ringing the line. The standard ring cycle is 2 seconds of ringing followed by 4 seconds of no ringing, and the incoming call can "hit" anywhere in this ring cycle. So, it is possible for an incoming call to be connected to a PBX trunk line for up to 4 seconds before ringing is applied to the line. If a user on the PBX tries to make an outgoing call in that period, the PBX will assume that that line is available, and then you have "glare." - John At 03:28 PM 7/15/98 +0800, you wrote: >Dear Vic, > >My question pertains to call accounting. How can the PABX determine the >duration and cost of the call made by the guest so that the hotel can bill >the guest correctly ? When a guest makes a call, the PABX needs to know : > >1) Whether the call is answered by the callee. >2) Time at which the call was answered. >3) Time at which the guest or callee hungup. > >With this info and the number dialled, the call billing system can then >calculate the cost of the call. > >The PABX probably needs some form of call answer supervision from the PSTN >for 1) and 2). I'd like to know what is available in the USA, please. > >Thank you for taking your time on answer my question. > >YhLoke > >---------- >From: Victor L. Boersma >Sent: Wednesday, 15 July 1998 12:54 >To: INTERNET:[email protected] >Subject: Question about PSTN Call Supervision in the USA > >> How does a hotel/motel in USA determine when to charge for long distance >calls >> made by guests ? Could they be using : >> >> 1) Reverse Battery Answer Supervision or Line polarity reversal >> 2) Pulse Metering >> >> or something else ? >> >> Can anyone tell me what types of call answer supervision options are >available >> on loop start lines from local phone companies in the USA ? >> >Hotels use PABXs (Private Automatic Branch Exchanges) and supervising what >hotel guests do is not done by the Telephone Company (unless they provide >the PABX). > >Hence, your question would appear to be How does the PABX supoervise what >the guests do, rather than what the PSTN does. The PSTN only knows that >the hotel is making long distance calls, not who in the hotel is doing >that. > >There are various supervision schemes in use in North America between PABXs >and the PSTN. It all depends on whose PABX from what vintage is in use. >Pulse metering is not one of the options used in North America. > >I need to have a clearer understanding of what exactly you intended to ask. > >Ciao, > >Vic > > > > ________________________________________________________________ John Combs, Senior Project Engineer, ITS/TestMark Laboratories Email: [email protected] URL: http://www.testmark.com
