Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!NADC.ARPA!prindle From: prindle@NADC.ARPA (Frank Prindle) Newsgroups: comp.dcom.telecom Subject: Re: Three wire lines Message-ID: <8806011249.AA04333@NADC.ARPA> Date: 1 Jun 88 12:49:01 GMT Article-I.D.: NADC.8806011249.AA04333 Sender: daemon@bloom-beacon.MIT.EDU Organization: The Internet Lines: 35 Approved: telecom@xx.lcs.mit.edu Many homes are wired with 3-wire cable. This supports a single phone line with 2-party service. The third wire is connected to ground (earth) and serves two functions: it is the return path for ringing current for both parties, and a fixed resistance from tip to ground (when the phone goes off- hook) is used by the CO equipment to identify one party (i.e. the "tip" party) for billing purposes. Such a line requires a phone modified for tip or ring party connection (rearrangement of the ringer connections and use of an additional hookswitch contact pair). Needless to say, this is virtually impossible with any phone except the old AT&T 500s and 2500s and their equivalents. Modular cables for today's single line phones don't even carry the third wire through to the phone! Plugging a new phone into a ring party jack might just work, but in a tip party jack, it will likely bill calls to the wrong party if they go through at all. This service is no longer offered by most LOCs, but customers who have kept the service through the years must be permitted to retain it (it is charged at a flat rate for local calling, usually several dollars a month cheaper than individual line flat rate). Due to the sparsity of the remaining 2-party line service in major cities, most such lines are, it reality, private lines. It is less of a bargain in light of the fact that only a very limited number of phones can be made to work with it. There is generally no service charge if a customer willingly wants to do away with it; the LOC will gladly switch you to private service, but you are on your own to rewire (or toss) the phones. The three-wire house wiring will remain behind for centuries to confuse all but the dedicated phone historians. Four-wire wiring became the rule much later (probably in the late 50s and early 60s) when lighted phones started to appear ("It's little, it's lovely, and it lights!"). Sincerely, Frank Prindle Prindle@NADC.arpa [thanks to Patrick Thompson of Cup.portal.com and Rich Wales of UCLA for submitting similar responses to this question. --jsol]