Darren Nickerson wrote:
James H. Thompson [EMAIL PROTECTED] wrote:
T.38 is often put forward as the solution for reliable FAX over VOIP.
With good reason ;-)
Just wondering for anyone using T.38 (with any equipment), how well does it work as compared to a FAX PSTN call?
Our experience is that it's only as good as the T.30 implementation in your T.38 gateway, softswitch etc. Since it's really them having a conversation with the remote device, you're relying on their implementation of the fax protocol. We recently had a problem with a customer's T.38 termination service ... they had disabled ECM (Error Correction Mode) as a general policy across their devices, and as a result a lot of our customer' faxes were of degraded quality. The provider claimed that when their network started to drop packets, ECM caused the faxes to take too long to send because of all image data retransmits. Our response - fix your network!
Its a pain that a T.38 gateway actually needs to support quite a bit of T.30, just so it knows when to switch between the various modems. People ask why I implemented a full fax machine in spandsp, rather than just building a modem, and plugging it into hylafax. Well, there's your reason. I am implementing T.38 for spandsp, and I need most of a T.30 implementation to make that work.
I guess that's all just another way of saying 'it depends' ;-)
One thing to be aware of when deciding between T.38 and the PSTN is that V.34 fax (speeds above 14,400) aren't possible, so you'll want to make sure that the money you save from (presumably cheaper) T.38 termination is greater than the money you save from spending less time on the wire. The speed boost from V.34 is larger for multi-page faxes than single-pagers, and penetration of V.34-capable machines varies from country to country and vertical market to vertical market.
T.38 now supports V.34, but I don't know how many gateways have been updated to provide that support. Most of the devices I see which say they provide T.38 are rather vague about exactly what they do. I still don't seen enough V.34 fax machines to consider its use that critical, anyway - unless most of your faxing is between two parties known to have V.34 capable machines.
Regards, Steve
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