atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Telecom Service & Providers Ground Start=One Way or Two?

I was reading about ground start lines from the central office. Can a ground start pair be used for both incoming and outgoing calls, or is it always dedicated to just one or the other?

Ground start lines can defininately be two-way, in fact that's one of the reasons that they came about. They were developed to minimize "glare" or "call collisions".

With loop start lines, it's possible to grab a line for an outgoing call at precisely the same time an incoming call is arriving. I am sure you have encountered this at home: You pick up the phone to call someone and there's someone on the line calling you instead of dial tone! More than likely, it's that dreaded telemarketer that you would probably have wanted to avoid.

Ground start lines are normally incoming-only UNTIL an outgoing call attempt is made. At that time, the tip side of the line is connected to ground momentarily to tell the CO to reverse the function of the line for that call and thus, return dial tone.

I may be wrong, but I think that by grounding the tip, it tells the CO to bump the potential incoming call to the next trunk in the hunt group to avoid a call collision. That's something for the CO guys here to confirm....

These are particularly useful for hotels where only six lines (trunks) serve a facility with 100 rooms. Imagine if someone in a hotel room went to call out by dialing "9" and collided with an incoming call? Both parties would hang up and the hotel may have lost a potential wedding reception. I am not sure, but I would venture to guess that businesses such as hotels brought about today's popularity of GS trunks.

Now that PRIs are so popular where more intelligent status is conveyed both-ways, ground start trunks will likely fade into the sunset before too long.

I can elaborate in further detail if you are interested.

OK they're two-way.

Is it true that a ground-start pair that is not in use has no voltage on it? In other words, if I put my voltmeter between the tip and ring, when there is no call, will there be any measurable voltage?

tip to ring no, ring to ground yes

This is where I start getting confused. I thought the tip is always at ground potential and is actually bonded to a ground rod at the NI and back at the CO. So if there is no potential diference between ring and tip, how can there be a potential difference between ring and ground? That doesn't add up.

Battery seeks a path to ground. On a ground start circuit that path is open. Until as Ed said a ground is put on the tip.

MP did see the other part of your question, the tip side of the line on a loop start circuit is a pure ground in the C.O. It had nothing to do with the ground at the NID.

MP, caution! With the typically high impedance meters everybody has these days, you may see battery or what appears to battery across T & R on an idle G/S trunk. It will probably drain down pretty quickly if you try to measure the voltage across T & R. And SOMETIMES, you'll see voltages up in the 100's, that will drain down to close to zero.
John C.

So if I am testing a pair at the network interface, and I hear nothing with my butt set connected to the pair, and I read no voltage or amperage across the pair, then I can then touch tip momentarialy to a good ground and will then get a dial tone? I am talking here about a ground start pair.

Yes, if it's good.