Does Your T.38 Stack Measure Up?
So, what distinguishes one T.38 stack from another? If they both conform to the ITU T.38 recommendation and are interoperable, shouldn’t they be the same…that is, indistinguishable? Yes, there’s price, compute-resource requirements, and ease of integration if you’re evaluating commercial stacks, but is there more? The answer is an emphatic yes.
You see, the T.38 spec doesn’t tell you how to make your T.38 resilient, and that’s the key to in-the-field performance. Of course, much of that resilience comes from effective integration with the analog modems if the stack is being used in a PSTN-IP gateway, rather than a fax server. For example, does the integration of the stack and the modems effectively handle line echoes? Your T.38 stack can be completely interoperable with other vendor’s T.38, but fail to work with certain fax terminals.
But the big differences show up when the T.38 stack is designed by someone who has previously designed a T.30 protocol engine and nursed it through at least a few years of field experience. That’s the only way all the tricks of the trade can be marshalled to design that “resilience” into the stack. It’s called “spoofing”. If it’s done correctly it can add several seconds to the round-trip-delay tolerance of the stack without creating fax-terminal-specific interop problems.
Here’s a few things to check on to see if your stack measures up:
- Does your stack support T.38 version 3 with V.34?
- Does it maintain the outgoing packet stream when idle to prevent half-duplex disconnects?
- Does it retransmit terminal frames, keeping poorly written relays from waiting forever if a terminal signal is lost?
- Does it use a Class 2-style interface with the modems, introducing hundreds of milliseconds of additional delay?
- Does it effectively resync in the presence of multiple lost packets?
- Does it effectively handle late T.38 reINVITEs?
We would be pleased to discuss your fax-technology requirements with you. Just give us a call at 770-449-7775 press 1 or email email@example.com.