[asterisk-users] FXO Cards vs. Channel bank with T1

[Jay R. Ashworth]

On Wed, Nov 01, 2006 at 08:29:32PM +0200, Tzafrir Cohen wrote:
> On Wed, Nov 01, 2006 at 09:08:43AM -0600, Ejay Hire wrote:
> > This is incorrect.  The data is still packetized and passed through IP which
> > provides the same echo cancellation and distortion issues as a call that
> > passed through an FXO/FXS card.
> 
> The issue here is an "implementation bug" of Zaptel rather than a
> fundemental archtectual flaw.
> 
> For fax or modem to work well you need a good line. One of the problems
> that may cause line quality problems is different clock speeds of
> different components of the system. They may cause an occasional "click"
> every number of seconds.

The jargon is "clock slip", and it happens when you don't have your T-1
clocking master/slave hierarchy set up correctly -- or when you have
drops from two different switches from two different carriers (local
and LD spans, for example).

> The problem I referred to is that different Zaptel cards may have a
> different clock. Asterisk uses the clock of the "master" zaptel device,
> but it is not exactly clear who that master device is (basically: the
> first Zaptel device). No other device tries to get clocking from it.
> 
> If you use an external channel bank you work around the problem by
> connecting all the external connections (both PRI lines and channel bank
> FXO/FXS lines) through the same PRI card. That card will not have a
> problem being in sync with itself.
> 
> As for our device: our short-term solution is to sync the PC clock from
> Zaptel as we can already sync our device from the PC. But the long term
> solution is to sync our device (and other zaptel devices) from the
> master zaptel device.

Well, optimally, every T-1 card should be slaved to it's span, and
buffering should take care of keeping various spans in sync with each
other.

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
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