[Asterisk-Users] T.38 support

[Steve Underwood]

Darren Nickerson wrote:

> James H. Thompson jht at lj.net 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