Author Topic: Why Ground Start trunks on PABX's? Please Explain  (Read 6370 times)

Offline dsk

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Why Ground Start trunks on PABX's? Please Explain
« on: November 13, 2009, 04:50:43 AM »
Here in Norway, I have never heard about Ground start in the regular network. ???
Quit many office telephones had this button when connected to a PABX, but this was for transferring to another extension, not to get the dial tone.  Some German telephones had an extra contact set on the dial, grounding at the same interval the receiver was shorted out.

Could you tell me why it has been used, and whats the positive, and negative arguments?   :)

dsk
« Last Edit: November 26, 2012, 11:49:00 PM by AE_collector »

I have even got a regular New York number :-) 646 570 1796

Offline Stephen Furley

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Re: Why ground start? Please explain.
« Reply #1 on: November 13, 2009, 12:22:10 PM »
I don't know what the reason for ground start lines was; I did read it somewhere in the past but I've forgotten.  I think ground start lines were used mainly for analogue trunk lines to pabx systems; these days most trunks are digital, so I would guess that ground start lines are not as common as they once were.

Offline rp2813

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Re: Why ground start? Please explain.
« Reply #2 on: November 13, 2009, 01:30:01 PM »
I think Donna may be able to advise on the differences between loop start and ground start.  I have long since forgotten this from back in my implementation days.   I agree with the posts here so far that ground start usually applied to trunk lines associated with PBX systems, but that's as far as my knowledge level goes anymore.

Ralph
Ralph

Offline Phonesrfun

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Re: Why ground start? Please explain.
« Reply #3 on: November 13, 2009, 02:46:11 PM »
Its my understanding that ground start was much faster at activating the CO equipment than loop start, and on a large business system, that avoided collisions between an incoming call being intercepted by someone dialing a 9 to get an outside line.  I guess it still happened, but it was much rarer to happen than with loop start.

Prepay pay phones, where you had to deposit the coin to get a dial tone were always ground start.  This was set up so that the ground could not only be the reference for starting the call but for coin collect and return voltages.

Ground start still exists on PBX lines, but pay phones work entirely different these days.
-Bill G

Offline bwanna

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Re: Why ground start? Please explain.
« Reply #4 on: November 13, 2009, 08:52:28 PM »
this probably explains a more succinctly than i would ;) actually, is pretty much what bill said.

http://tinyurl.com/yko9lmb

coincidentally, i had occasion to deal with ground start loop today. customer ordered 14 lines as ground start. actually needed 2 to be loop start for the fax machines.

at&t does not provide coin service at all any more. the coin vendor gets a POTS line & the coin service is thru their equipment.

i do take issue with the term trunk line. it is just a line or pair. trunk line is between central offices.
donna

Offline Phonesrfun

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Re: Why ground start? Please explain.
« Reply #5 on: November 13, 2009, 10:32:44 PM »
A few years ago I did not even know ground start even existed.  I was working at a start-up community bank in Vancouver, WA and I hired a company to install our PBX before we opened the bank.  Everything seemed to be going well, and on opening day we all moved into the new building, opened the doors for business, and had the phone lines activated.  We suddenly found out that we could receive calls, but we could not access outside lines!

The company we hired had miscommunicated with the telephone company, and did not specify ground start lines, even though he had put in a system that required ground start.  The phone company had just assumed we wanted loop start.

The only phones we could use to make outside calls from were the ones such as the fax line and a couple of modem lines because they did not go through the PBX.  The phone company fixed it in a matter of hours, but I suddenly got a quick education in the fact that ground start existed, and went through the discussion of why they existed.  We had the option of reprogramming the PBX, but that would have taken about as long as it would for the phone company to change the programming at their end, and if we were to reprogram the PBX, we would not have even been able to recieve calls during the time the switch was down for reprogramming.

It worked fine after that.
-Bill G

Offline rp2813

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Re: Why ground start? Please explain.
« Reply #6 on: November 15, 2009, 12:26:33 AM »
Yeah Donna, the trunk reference was my own slip due to a product known in-house as "Supertrunk" which was often part of larger business installation requests.

Ralph
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Offline bwanna

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Re: Why ground start? Please explain.
« Reply #7 on: November 15, 2009, 08:06:54 AM »
o ralph ......sorry did not mean to take issue with you :'(

the wiki link refers to a single gst loop or single loop start loop as "trunks".  in my experience, a trunk usually refers to a "major" feed. like a pbx trunk within the building or a toll trunk between central offices, etc. :)
donna

Offline Stephen Furley

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Re: Why ground start? Please explain.
« Reply #8 on: November 15, 2009, 03:57:10 PM »
Bwanna,

maybe it depends on whose wiewpoint you are looking at this from.  To the telephone company these are subscriber lines, like any others; what is hung on the end of them, telephone, fax machine, PABX doesn't really matter.  However, from the viewpoint of the PABX they are lines which connect itself (an exchange, albeit usually a small one) to another exchange, usually a public one, and this certainly seems to fit the definition of a trunk, at least in the way we use the word here; is it used differently there?   Even on the smallest PABX system with maybe three extesions and one exchange line we would call that line a trunk; would you not do so there?

At work we have about 800 extensions actually in use, though there are another 200 or so configured on the system which are not actually in use now.  Our trunks are digital, as they are on most PABX systems these days.  The most common digital trunks here are ISDN, BT now supply only two types, there used to be others.  There's ISDN 2e which is a BRI service with 2 64 kb/s B channels and one 16 kb/s D channel, and ISDN 30e which is the PRI service with up to 30 B channels, 1 D channel and 1 syncronising channel, each of 64 kb/s, carried on a 2 Mb/s E1 connection which can take various forms.  In our case, since we're only a few hundred metres from the Croydon exchange it comes in on two copper pairs, at greater distences fibre is sometimes used,  The minimum number of B channels which BT will supply on this service is 8; if you want less than that then you have to use multiple ISDN2e services.  We have 2 ISDN30e services, one with the full 30 channels, and the other with 13, for a total of 43 trunk lines.  These two services therefore use four pairs.  Other pairs carry other services, ISDN2e, analogue and other odd thins; altogether we have 100 pairs, two 50 pair cables coming in from the street, though only about 25 pairs in total are in use at present.

At least that was true until a few days ago.  We've just added 20 SIP trunks, at present mainly as a backup in the case of failure of the ISDN trunks, but if they prove to be reliable we may well drop the second ISDN30e, at a considerable saving, so we will then have a total of 50 trunk lines, 30 ISDN and 20 SIP.  The SIP trunks use an IP connection over our Internet connection, which is 100Mb over fibre direct to a location in central London where we connect to the London Metropolitan Network which connects us to JANET, the Joint Academic network, which connects academic institutions throughout the UK to each other, and to the Internet.   The fibre comes in at the opposite end of the building to the copper, so there's less risk of somebody digging through both at the same time.  So far we're still testing the SIP trunks, but they seem to work very well, at least for outgoing calls, there are ttill a few issues with incoming ones, which should be sorted out this week.  The SIP trunks are uncompressed 64 kb/s, so with some overhead if they were all in use they would take about 2% of our Internet bandwidth.

Our PABX can connect to various types of trunks, including analogue, though we don't have any of these.  I checked yesterday, and it supports both loop start and ground start on analogue trunks.
« Last Edit: November 15, 2009, 04:11:37 PM by Stephen Furley »

Offline Phonesrfun

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Re: Why ground start? Please explain.
« Reply #9 on: November 15, 2009, 04:07:36 PM »
I greee with Stephen.  I believe that historically, a trunk was a line that connected a local exchange (Whether public or private) with another exchange and/or the outside world.  A trunk line used to refer to just one line.  So an office could have had 100 trunk lines between it and another, for instance.  For a PBX, the trunks were the incoming and outgoing lines to the PBX from the CO.  For a CO, they were the lines between COs.

With T1s and fiber, I don't doubt that the term trunk has perhaps morphed into a more large-scale channelized meaning than just a simple line.
-Bill G

Offline bwanna

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Re: Why ground start? Please explain.
« Reply #10 on: November 16, 2009, 09:25:23 AM »
bill, stephen.... i have noticed, in talking to private phone vendors, that they use a somewhat different vernacular than us outside telco.  like you say it depends on your perspective. :)
donna

Offline GG

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Re: Why ground start? Please explain.
« Reply #11 on: February 16, 2011, 05:34:23 AM »

Bill is correct: the purpose of ground start trunks on PBXs is to prevent the condition known as "glare" where an outgoing call attempt collides with an incoming call. 

From the interconnect industry, we use the term "trunk" to refer to a connection between CO and PBX where the PBX performs a function that is integral to the switching process, rather than just making a connection to a circuit.  For example with DID trunks (Direct Inward Dialing), the PBX routes the call to the station; and ground start trunks, and PRI (ISDN) trunks, both require more complex interactions with the PBX than just going off-hook.  Analog CO lines are still "lines," and can be used with conventional key systems (one key per CO line) of whatever kind. 

Lucky you, Stephen, that BT (and some European carriers) are providing ISDN circuits down to the level of 2B+D.  In the USA all we can get is PRI, which is 23 B channels + one D channel, and is expensive so is only used for PBX installations.  Twenty years ago we were all looking forward to ISDN becoming more or less universal as an alternative to analog lines.  It would have provided for DID service to homes and small businesses.  Now instead it's all about VOIP, which when implemented correctly is OK but is often implemented poorly including G.729 compression (cellphone audio, which I detest).  But in any case, VOIP does not provide DID, and does have major security problems such as susceptibility to interception by hackers.

Offline AE_Collector

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Re: Why ground start? Please explain.
« Reply #12 on: February 16, 2011, 10:48:09 AM »
I always thought that term "Glare" was perfect at describing the situation!

An incoming call has landed on the trunk from the CO to the PABX but just before the CO applies ringing voltage which would tell the PABX that a call is there on the trunk, someone on the PABX dials the outside access code and lands on ther same trunk. "Glare"!

Terry

Offline kb3pxr

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Re: Why Ground Start trunks on PABX's? Please Explain
« Reply #13 on: December 26, 2017, 11:18:13 AM »
Ground Start was used on CO lines to prevent glare along with line selection order. Incoming calls came in sequential order of 1-2-3-etc, outbound calls were 3-2-1 and that practice was actually used with the manual PBX systems. Now ground start on the extensions can serve as a failover. In normal operation loop start (lift handset) is used to get the PBX, but, a ground start button can be installed, that if the PBX looses power, an emergency call can be made by lifting the handset and pressing the ground start button. I saw this back in school as my district has a central PBX at the admin building and the schools were all off premises extensions. Some schools used a dial intercom system for internal calls (classroom to classroom, classroom to office) or a manual system with a switchboard on the PA console. In fact back when I was in High School (early to mid 2000s) most classrooms had non-dial 554 sets.