[Asterisk-Users] Digium FXO Interfaces don't support groundstart???

[Frank Cofer]

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: <ghost at silcon.com>
To: <asterisk-users at lists.digium.com>
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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