First of all I use this in humor:
Frank you ignorant slut!
I have to disagree on your analysis. I worked in telephone COs (DMS250, Stromberg/Carlson) and with PBXes for over a decade. Glare can and is controlled by ground start signaling. It does so because the ground is tested for (or supposed to be) prior to dialing. It's called the pre-seize condition. On a T1 using robbed bit signaling, tip and ring conditions are converted into A/B signaling states in the channel modules of a channel bank.
Ground start was the prefered signaling system for what was called Feature Group D trunks between Other Common Carriers and the RBOCs. Before FGD was available, we used loop start. We had incoming and outgoing trunk groups, hence no glare... Needless to say expensive. Because FGD had ground start, to cut interconnect costs, we went there as soon as it was made available.
The 150ms pulse you described is called wink start, which was funky. I most commonly say it on systems using E&M signaling. Gawd I hated those!
YA know, the asterisk list has been for a lot of walks down memory lane :)
Frank Cofer wrote:
Glare cannot be prevented on two way trunks (it is physically impossible because the two ends are separated in distance and therefore separated in time and any independent decision to use it at one end is never seen instantly at the other end).
Ground start does not decrease glare at all (it actually increases it) and use of "ground start to eliminate glare" is a common myth. This is because use of ground start (which uses only one side of the pair to earth ground to "start" a request for service) increases the time to mark a central office line busy when it is seized from the Customer Premise Equipment (CPE), owing to its clunky signaling (150ms earth ground on the ring of the line) and the fact that it uses only one half of the current to start the line as loop start. Since it increases the time to signal the distant end, it increases glare.
Its only benefit is to the central office because it stops a second seizure to the central office when a call disconnects from the central office end first, which would otherwise find a request (loop) as soon as the disconnect was effected. This is why "ground start" was introduced by the Bell System (when they owned both the PBX and the CO) since it would reduce the attempt load on the central office from large business users by 25% or more saving a lot of central office gear for a relatively small expenditure on the PBX end. Ground start has some ugly drawbacks, since it reduces signaling range, requires the normally isolated floating pair to be referenced to earth ground (which exposes the circuit to longitudinal spikes, noise and lightning) and requires the circuit to be muted during the imbalanced condition that occurs when the ring conductor is momentarily grounded to draw dial tone. Digium is right to leave it out. Most other informed, modern manufacturers do likewise.
"Ground start" signaling referred to in T1 (which is an absurd label since there is no ground placed on a T1) is really after the Grey Code (only one signaling bit transitions at a time) and has nothing to do with glare or ground start signaling and is just a carry over label.
Glare can be reduced by changing the hunt order from either end and to employ faster signaling. The former method decreases the likelihood that both ends will compete for the circuit at the same time and the latter reduces the window that a commitment has been made at one end and is still not known by the other end. Typically, the CO is set to hunt ascending and the CPE descending and this is still employed even in ISDN circuits. This is a "terminal hunt" and NOT a "round robin" hunting sequence. If you want to absolutely eliminate glare, use one way (incoming/outgoing only) circuits. I believe asterisk has a feature to set the hunt order preference.
The disconnect problems you experienced with your Agilent PBX may be more likely related to the "guard interval" that a circuit is left alone at your end after it is used. Though "ground start" will appear to fix it, there are some issues of CO message rate three way calling that have caused grief (the CO interprets the next call as a flash for a three way call and holds the circuit rather than disconnecting it). This phenomena may have been misdiagnosed as glare, since the message unit 3-way calling was imposed as a default feature in certain jurisdictions. Increasing the guard interval to 2 or 3 seconds will suffice, or specify to the carrier that the 3-way calling is to be denied for your lines.
Hope this helps.
----- Original Message ----- From: <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Thursday, July 08, 2004 4:55 AM
Subject: [Asterisk-Users] Digium FXO Interfaces don't support groundstart???
Hi All,
I was surprised to be told by a Digium support person today that Digium's FXO interfaces (X100P, TDM400P FXO modules) don't support groundstart signalling. This surprises me because as far as I know in a typical PBX configuration with analog trunk lines, groundstart signalling is the only way to prevent Glare.
I just purchased two TDM400P's for a system I'm building to replace our office PBX (Altigen). Since there are no statements anywhere on Digium's website about lack of groundstart support (Actually, to the contrary they boast about all the signalling support in their sales slick), I now need to decide if I want to return the products and switch to a T1 / channel bank configuration.
I remember when we setup our current Altigen PBX, we had problems with glare and disconnect detection and so I went through the process of figuring out what was going on and learning about groundstart. After we switched to groundstart everything worked great.
In a high use system, it's highly likely that a trunk will experience glare, which is annoying for incoming callers and system users. I'm just a bit baffled as to why Digium wouldn't support groundstart on cards designed to be PBX trunk lines.
Someone please tell me I'm missing something.
Mark
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