>> However, based on the comments you give I'd suspect that you're
having
>> what people seem to be calling "frame slipping". There seem to be
>> some motherboards that react poorly with Zap cards (or their
>> respective drivers) and cause that. Your zttest results should be
>> revealing here.
Frame slips are NOT motherboard related!
For those that happen to be following this thread, be careful with the
above statement as one person is "assuming" T1/E1 interfaces while
another is "assuming" the statement applies to analog interfaces such as
the TDM400 card. The statement applies to one assumption but not the other.
A Frame slip is due to clocks at opposite ends of a circuit such as a T1
running at different speeds. Either a buffer overflows and one frame is
thrown away or there is no data when a frame is needed so the previous
frame is repeated.
The solution is to have one end of the circuit supply the clock and the
other end derive the clock from the incoming signal.
Don Pobanz
How would you check clocks speeds at opposite ends of a circuit (T1, E1,
BRI, ...) ?
As it seems frame slips occur from time to time (for instance, on 10% of
received faxes), do you imply that Asterisk settings should be changed
so that on every fax received, it should adopt opposite clock speed
(unlike today where by chance, 90% of circuit clock speeds are the same) ?
The issue on T1/E1's is not clock speed itself but rather the low level
"synchronization" of the clocks. The timing signals necessary for clock
synchronization are "always" embedded in the transmit side of every
T1/E1 data stream. Its part of the T1/E1 design specifications and
cannot be removed under any circumstance by anyone. Whether you use it
or not is defined in /etc/zaptel.conf.
The telco's and other T1/E1 service providers "never" listen to your
synchronization; rather, they have a very well understood hierarchy
where synchronization is always derived from their upstream providers
(whoever they happens to be). Therefore, if you synchronize your clocks
to the T1/E1 provided to you, your clocks will be synchronized to the
rest of the world (in total).
FWIW, the folks at sangoma have said that one of there design
verification tests includes ensuring that faxing works since it is one
of "the" most critical tests that validates overall design.
So, if you are absolutely sure that you've specified the correct T1
synchronization parameters in your /etc/zaptel.conf and you still have
fax reliability issues, look elsewhere in your implementation for the
root cause.
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