[asterisk-dev] Proposal for T.38 transparent gateway design in Asterisk

[Klaus Darilion]

Am 28.04.2010 22:52, schrieb Kevin P. Fleming:
> Klaus Darilion wrote:
...

> channels. If you can describe a scenario where Asterisk would need to
> listen for the FAX preamble on an incoming channel, I'll certainly
> listen :-)

Just for more flexibility. I know service providers whose gateways 
support T.38 but they always require the SIP client to initiate T.38, 
regardless if it is an outgoing or incoming call. Also some clients are 
really really slow in detecting fax (10 seconds or more), so allowing 
fax-detecing and T.38 initiation in all directions would help in some 
scenarios.

>> IMO it is very important that the gateway object is not only for TDM
>> channels, but for ALL channels, TDP+IP, also T.38 capable channels.
>
> As I said I above, I disagree. We should not do anything specifically to
> support audio FAX over non-TDM channels. If the code that gets written
> can be used that way and people attempt to do so, that's their choice,
> but no specific efforts should be made to support those configurations.
>
>> For example
>>     SIP-ATA<---SIP+T.38--->Asterisk<----IAX+G711---->  IAX peer
>> is a valid scenario, for example when trunking with other Asterisk
>> servers or using IAXmodem+Hylafax over LAN or high quality networks
>> (e.g. direct peerings between service providers).
>
> There are least two other ways to do this: HylaFAX can support T.38
> directly using t38modem (so then the T.38 session would just pass

Have you every tried t38modem? I tried several times and always failed. 
There are also many more people reporting that they failed to use t38modem.

> through Asterisk), and IAX2 could be extended (fairly easily) to be able
> to transport T.38 directly. In the latter case, the 'IAX2 peer' would
> need to have a FAX origination/termination application, or be able to
> pass the T.38 session onwards over a T.38-supporting channel technology.

Of course having T38 support for IAX and IAXModem would be nice. But 
anyway - if you have the proper setup than it works also with IAX 
rock-solid.

I have several installations with IAXmodem and Hylafax and it is really 
very stable. It is just a matter of network quality. I also have a setup 
with 
Iaxmoden<-iax->Asterisk<-sip/g711->Asterisk<-sip/g711->openser/rtpproxy<-sip/g711->gateway 
from one service provider to another one and it is really stable! If you 
have a good network with low delay/jitter it just works!

I use the t38-gateway patch from the bugtracker with SIP-T.38<->IAX and 
it just works.

And there are really lots of users which use Asterisk with IAXmodem and 
it works. Thus, it would be a pitty if TDM<->IAX works but T.38<->IAX not.

Of course it will not work if you try to use FoVoIP over networks with 
higher delay/jitter. But if you increase delay even more then also T.38 
will fail. That would be something like "we do not implement T.38 
because it does not work over satellite links".

>> Also
>>     SIP-ATA 1<---SIP+T.38--->Asterisk<---SIP+G711---->  SIP-ATA 2
>> is a valid scenario and should be supported. In this case Asterisk
>> should reINVITE to G711 if the T.38 reINVITE was rejected by SIP-ATA 2.
>
> I would say this should *not* be supported; if someone has a FAX machine
> attached to a SIP ATA, and that ATA does not support T.38, then they
> should get one that does. Even if we did support this scenario (assuming

There are lots of installed gateways (e.g. Asterisk < 1.8 servers) which 
do not support T.38 gatewaying but wont be replaced because of this. 
Gateways are often connected to other equipment via good networks (LAN). 
Thus in such cases it would work reliable.

So please, just implement the T.38 gatewaying so that it may work will 
all channels - do not limit the gatewaying object itself to TDM channels 
only.

regards
klaus