Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!bionet!hayes.fai.alaska.edu!accuvax.nwu.edu!nucsrl!telecom-request From: cellar!martin@bellcore.bellcore.com (Martin Harriss (ACP)) Newsgroups: comp.dcom.telecom Subject: Re: Leaving Brief Messages With Free Collect Calls Message-ID: <12269@accuvax.nwu.edu> Date: 17 Sep 90 20:55:04 GMT Sender: news@accuvax.nwu.edu Reply-To: "Martin Harriss (ACP" Organization: Bellcore Lines: 33 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 10, Issue 654, Message 10 of 12 In article <12126@accuvax.nwu.edu> Dave Lockwood writes: >In various articles, various people write: >About answering the phone before it rang... >In the UK, before the advent of the digital switch, all the >electromechanical (Strowger and Crossbar) switches generated ring >current and tone by means of a rotating motor with a commutator pair. >Examining this device showed that the ring tone (to caller) and the >ring current (to callee) would be exactly "out of phase", ie the ring >current was sent in the gaps between the ring tones. Not so. The standard BT ringing machine generates three phases of both ring tone and ring current. These three phases are distributed around the exchange. I don't think any attempt is made to synchronize - you just get what happens to be the phase on any given selector rack. What kind of ringing machine were you looking at? There are some small ones used by PBX's that may do funky things with the phases of the generated voltages and tones. Note that TXK1's and TXK3's (X-bar) generate a short burst of ringing (both ring tone and ring current) on siezing the called line, to help prevent glare. This used to be a sure-file test for a crossbar exchange - but some electronic exchanges may do this as well these days. (On a TXK1 the initial burst of ringing is much longer, allowing you to differentiate between TXK1's and TXK3's) Martin Harriss martin@cellar.bae.bellcore.com