mdf@TUT.CIS.OHIO-STATE.EDU (Mark D. Freeman) (05/28/88)
My house was built in the 1920's and was wired for telephone with 3 wires. I can understand two or four, but three? Why? -- Mark D. Freeman (614) 262-1418 Applications Programmer, CompuServe mdf@tut.cis.ohio-state.edu 2440 Medary Avenue ...!cbosgd!osu-cis!tut.cis.ohio-state.edu!mdf Columbus, OH 43202-3014 Guest account at The Ohio State University
prindle@NADC.ARPA (Frank Prindle) (06/01/88)
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]dave@westmark.UUCP (Dave Levenson) (06/04/88)
In article <14528@tut.cis.ohio-state.edu>, mdf@TUT.CIS.OHIO-STATE.EDU (Mark D. Freeman) writes: > > My house was built in the 1920's and was wired for telephone with 3 > wires. I can understand two or four, but three? Why? In those days, two-party telephone lines were common. Your phone and someone else's phone shared a pair of tip/ring wires. Only one of you could actually use the line at a time. For incoming calls, one of your numbers caused ringing to be applied between tip and ground. The other number caused ringing to be applied between ring and ground. The bells in your phones were wired between the appropriate side of the line and ground. Thus you need three wires to each set: tip, ring, and ground. The third wire (usually colored yellow) was probably connected to the ground terminal of your protector. For outgoing calls, the party who rings tip to ground was wired so that when the set is off hook, one coil of the set ringer was connected between tip and ground, without the usual blocking capacitor in series. This high-impedence, low-resistance ground on the tip side of the line could be detected by the central office for originating party identification. -- Dave Levenson Westmark, Inc. The Man in the Mooney Warren, NJ USA {rutgers | clyde | mtune | ihnp4}!westmark!dave
dave@westmark.UUCP (Dave Levenson) (06/04/88)
In article <8806011249.AA04333@NADC.ARPA>, prindle@NADC.ARPA (Frank Prindle) writes: > 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. > Actually, when the CO applies ringing between tip and ground, it also applies ground to the ring side of the loop, thus applying a dead short accross the ringer that is not supposed to be ringing. Similarly, the tip side is grounded when the `ring party' is being alerted. A useful by-product of this is that a standard phone wired for `bridged ringing' will ring correctly when either the tip party or the ring party is rung on a two-party line. If your phone is arranged for ring party ringing, it will work correctly with private line bridged ringing, as the tip side of the line is normally at ground potential anyway. My local telephone book (from NJ Bell) indicates that customers with two-party lines are not provided modular jacks, and may not purchase their own telephone instruments! This exception to the usual equipment rules results from the opportunity for toll-fraud that results when the tip party's phone is not wired for tip party identification. NJ Bell also states that two-party is a `closed service' which means that existing customers may keep it, but no new two-party service is offered. -- Dave Levenson Westmark, Inc. The Man in the Mooney Warren, NJ USA {rutgers | clyde | mtune | ihnp4}!westmark!dave