larry@uunet.uu.net (Larry Lippman) (04/09/91)
In article <telecom11.256.6@eecs.nwu.edu> john@zygot.ati.com (John Higdon) writes: > > While I have not dug too deeply into this aspect, one advantage of at > > least the two party Bell method was ANI. While the trick with the > > tapped ringer coil added some noise, it DID allow the CO to figure if > > Mr. Tip or Ms. Ring was calling Fargo without a "numberpleeze." > So how in the hell did that actually work? I remember that before > party lines were abolished in metro areas in the state, the sure-fire > way to tell that a friend had a party line when using his phone was by > the "tick-kunk" that came immediately after dialing and just before > the originating register dropped. Those of us with private lines had > no such noise. We all knew that sound had something to do with > identifying the tip or ring party, but to this day no one has ever > explained what was going on. The "tip party" on a two-party line has a balanced connection to earth ground using a portion of the ringer and network circuitry. Since the effective connection has approximately equal impedance from both the tip and ring of the line to ground, there is (or *should* be) little longitudinal imbalance and therefore little 60 Hz hum and noise. In addition, the impedance at voice frequencies should be high enough to cause little shunt attenuation at voice frequencies. The rotary dial is placed in the circuit "ahead" of the ground, so there is little effect that could cause dial pulse distortion. The "ring party" on a two-party line has no such balanced ground. The "party test" is therefore concerned with ascertaining the presence or absence of this balanced ground. Electrically, the test is performed while a station is off-hook through comparing the current flow on the tip side of the line with that on the ring. The conditions for the test are those of resistance battery on the ring side of the line and resistance ground on the tip, with the resistance usually being supplied through a dual-winding supervisory relay. If the current is equal (less some small allowance for cable leakage resistance) then there is no balanced ground and the station is the "ring party". If the current is significantly less on the tip side than the ring, the station is the "tip party". In an electromechanical CO, such as No. 5 XBAR, the differential current measurement described above is simply provided using a dual winding 280-type or equivalent polar relay. If the relay operates, then there is sufficient current unbalance to indicate the "tip party". If the relay remains non-operated, then it is the "ring party". In some ESS offices the party test is made though current measurement across tip and ring series resistors using differential ampliers and a comparator circuit. The party test is usually made in the originating register in a XBAR or ESS CO, and in an outgoing ANI trunk in a SxS CO so equipped. With respect to the "sound" made by an originating register during the party test, I suppose this is possible if the party test relay were switched in and out of the circuit. However, in originating registers that I have seen, I seem to recall that the party test relay was a polar differential type that was always in the circuit. The relay was sensitive and had little series resistance, so there was no need to switch it in or out of the circuit. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry