On 09/20/2011 08:57 PM, Don Kelly wrote:
This is a scary answer—you’re saying that what should be simple “TDM” FXS to PRI does not work?
There is no TDM connection on a PCI or PCI-Express bus. Transferring data between two cards in the system either requires a direct connection between them (there are older, more expensive systems out there that use H.100 for this purpose) or the data has to go through the computer and its software.
This means the data has to be packetized (even DAHDI's 1ms chunks are still packets), delivered to the host memory, an interrupt sent and acknowledged, and some code on the CPU then has to copy the data to another place in host memory for it to be sent out to the other card. Modern systems are of course fast enough to do this, but there are a lot of variables here, and it's possible for data to be occasionally delayed on its way between cards. With a voice call, this will likely not be noticed at all, but with a modem call or FAX call, it can be catastrophic.
Are you suggesting this is an Asterisk problem or a Digium hardware problem?
It's not a 'problem' with the hardware or the software; it's the nature of the beast. If you want 100% guaranteed TDM reliability, you have to use TDM connections, not packetized connections. Packetized connections can strive for 100% reliability, but most of them will never achieve it.
For many people, with modern CPUs, current versions of DAHDI and Asterisk, and appropriate configuration (using the faxbuffers option in chan_dahdi.conf, for example), such a system can be setup to work very, very close to 100% of the time.
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