Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!mips!pacbell.com!lll-winken!telecom-request From: wb8foz@mthvax.cs.miami.edu (David Lesher) Newsgroups: comp.dcom.telecom Subject: Return to the Land of Selective Ringing Message-ID: Date: 31 Mar 91 20:56:24 GMT Sender: Telecom@eecs.nwu.edu Reply-To: David Lesher Organization: NRK Clinic for habitual NetNews abusers Lines: 76 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 260, Message 5 of 5 John asked about a noise while dialing those with party lines, and how the ANI worked. Larry is surely the man for the both questions, and I'll defer to him. But there was ANOTHER distinctive noise you could hear during ring cycle: bridge lifters. Here's the story, as it was explained to me: Party lines were intended for conservation of wire. The typical application was: CO-----------------------------------------Mr. Tip |-Ms. Ring where the distance between them was small. But when private lines became widely available, Bell had a problem. Most folks would opt for them, and only one party would be out there (Say Ms. Ring) by herself. She still WANTED party line service. A similar situation existed when Mr. Tip called the business office and raised Cain because Ms. Ring was ALWAYS on the phone to her commodities broker. Ma's options were: a) Force Ms. Ring to change to private line service. b) Give her a private line without charging extra. c) Leave her as the ONLY party on a given party line. d) Party her with someone NOT next door. Now Ma often tried tactic a), but the problem with it was the PSC and/or the FCC frowned on such coercion. The Beancounters LOATHED c) and b) cuz they wanted every penny. That left d). Now the reason you could NOT just do this: ___________________________Mr. Tip co/ \ \______________________Ms. Sleeve was a guy named Farad. Cable is capacitive. When Mr. Tip was dialing, he was breaking the loop current with the pulser in his dial. BUT, the stub going off to Sleeve's condo had a lot of capacitance in in, and it terminated in a good sized ringer cap, too. (Maybe several, if Ms. Sleeve had a set in each room.) That (total) capacitor distorted Mr. Tip's dial pulses, and confused his Strowger Switch. It could also, I suspect, shunt audio to ground during talking, but I have never bothered to do the needed math to prove or disprove that. [Of course, there is another solution: Touch-tone. As I recall TT was first proposed to eliminate the problem of dial pulse distortion on ultra-long rural loops.] So Ma called her elves at Murray Hill, and they invented a bridge lifter. It goes between the CO and the outgoing pair and lifts (disconnects) Mr. Tip when Ms. Sleeve is off-hook, and vice versa. I'm now speaking out of my hat, but I THINK it only affected things during dialing. Otherwise, how could the other party demand surrender of the pair for an emergency? In any case, you can HEAR the bridge lifter, in an office with ringing sidetone, (as opposed to those that give you the switch generated tone) as a "raspy" quality to the ring. I often notice this while calling a doctor's office. In closing, I was sure that you could get party line service here in Miami in 1989, but I now see no mention of it. [Moderator's Note: Do you think the ringing you heard on the call to the doctor's office was due to the doctor's line probably being bridged to a live answering service in some other exchange, etc? PAT]