[Asterisk-Users] Digium cards, so disappointing !

[Rich Adamson]

Rusty Dekema wrote:
> On 4/14/06, Rich Adamson <radamson at routers.com> wrote:
>> I believe the TDM2400 has the capability of doing on-card fxo->fxs data
>> flows (without hitting the pci bus), but that function has not yet been
>> implemented. Its basically "required" to support faxes in an analog
>> environment. When it is implemented, that card should work. The TDM400
>> card will not work in 99% of the deployments.
> 
> I apologize if I'm being dense, but I don't understand why the fact
> that a call traverses the PCI bus would kill a fax transmission. I
> made the following setup, and it consistently gets 31200 and 33600
> connects, no disconnects, good throughput:
> 
> Modem --- SIP ATA(G.711u) --- (LAN) --- Asterisk --- (TDM400FXS) --- Modem
> 
> I get the same thing (although alwyas 31200 connects, never 33600) with:
> 
> Modem --- (TDM400FXS) --- Asterisk --- (TDM400FXS_same_card) --- Modem
> 
> If this works, I don't see why a fax transmission wouldn't work. Is it
> because the fax protocol doesn't have error correction? Is that even
> true?
> 
> I realize that clearing up my confusion about this isn't probably
> going to result in the problem being fixed, but I sure would like to
> know...

The fax issue revolves around the fact that fax signals are analog audio 
(eg, modem) that have to be accurately reproduced end-to-end (whatever 
that happens to mean in your environment).

If a fax machine is attached to a sip ata device, the network 
infrastructure has to be 100% rock solid (no dropped packets, no 
congestion, relatively low utilization, no contention for resources 
anywhere between asterisk and the adapter). If those items are unknown 
or poor, the analog fax signal will not be accurately reproduced at any 
sip ata device.

Likewise, the transfer of data across the pci bus has to be 100% 
accurate with no dropped/slipped packets, no jitter, etc. "That" has 
been an issue with an estimated 95% of the TDM implementations to date.

If you go visit some of the sites where developers work with real time 
audio, you'll find lots of comments relative to the inadequacies of the 
pci bus as implemented on many many mobo's. Most of what I've read 
relates to the North/South pci bridge chipsets, and design errors in 
those chipsets including some of the Intel products. Those same issues 
seem to be impacting the TDM card in one form or another, and no one has 
openly put a finger on exactly why.

The majority of implementations that need fax capability is related to 
an internal fax machine (however its connected) to an "fxo" (pstn) port. 
If you have "that" working via a TDM400 card, everyone on this list 
would love to know "exactly" which mobo you are using.

A modem's error detection / error correction capability (which does not 
exist in a lot of fax machines and point of sale devices) can handle 
small amounts of those issues noted above. However, it cannot handle 
anything more then a glitch here and there without significantly 
impacting data throughput.