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- From: tad@ssc.com (Tad Cook)
- Newsgroups: comp.dcom.modems
- Subject: Re: Modems with E&M signalling support
- Message-ID: <1992Nov12.213917.26447@ssc.com>
- Date: 12 Nov 92 21:39:17 GMT
- Article-I.D.: ssc.1992Nov12.213917.26447
- References: <1992Nov9.023639.22411@news.arc.nasa.gov>
- Organization: very little
- Lines: 60
-
- In article <1992Nov9.023639.22411@news.arc.nasa.gov> medin@dscs.arc.nasa.gov (Milo S. Medin) writes:
- >Hi. I was wondering if anyone out there knew of any modems
- >that support E&M signalling. This capability, along with
- >support from the CO switch or PBX, allows the modem to
- >determine what the called number is. It requires 4 wires
- >to the switch, instead of the usual 2.
-
- E&M signalling has nothing to do with the calling number. E&M leads
- are trunk leads within a C.O. (the telco switch) that change state
- (from ground to -48 vdc) in response to hookswitch status at the
- far end. So it is like having a regular phone line, except that the
- DC signalling (on hook/off hook) is broken out on separate wires.
-
- E&M leads do not extend from the C.O. Because they are not balanced,
- they are susceptible to noise, and only run a few feet from the trunk
- to whatever terminating equipment they are hooked to. If you have
- E&M leads at a customer premise, then they are probably using
- carrier to get the circuits to the premise, and you can connect
- with a PBX that has E&M compatible trunks.
-
- But the E&M leads only run from the carrier circuit to the PBX....not
- over telco cable.
-
-
- >
- >Note this is not caller id, where the calling number is
- >transmitted, but rather getting the called number. This is
- >very useful if the PBX was programmed to route several destination
- >numbers to the modem, and you wanted the computer's software to
- >act differently depending on which phone number the originating
- >actually called.
-
- What you are talking about is Direct Inward Dialing, which is
- independent from the type of trunk signalling used. This is
- used where each PBX extension has its own 7 digit phone number,
- but the service comes into the PBX on a limited number of
- trunks, so the PBX has to be able to tell where to route the
- call. It winks back to the telco when a call comes in, and
- the telco sends the destination phone number with DTMF signaling.
-
- >
- >I'd prefer a modem that supported v.32bis and FAX, but right I'm
- >just interested in any modems that support this at all.
- >
- What you do is take a regular modem and hook it to the line
- (extension) that is on the user side of the PBX. If you hook
- the modem to one of the outside trunks, then the modem would
- have to emulate a DID trunk, and what would it do when it
- got a voice call? Remember that all of the traffic for the PBX
- comes in on these trunks. That is the reason for the wink-back
- and ID from the CO.
-
- I am sure there are no modems that do this.
-
-
-
- --
- Tad Cook | Phone: 206-527-4089 (home) | MCI Mail: 3288544
- Seattle, WA | Packet: KT7H @ N7DUO.WA.USA.NA | 3288544@mcimail.com
- | Internet: tad@ssc.com or...sumax!ole!ssc!tad
-