[asterisk-users] IAX2 and Faxing

[Lee Howard]

Davis Sylvester III wrote:

> Thanks for the quick response.  But how do other VoIP Service 
> Providers like vonage provide fax capabilities?


Inevitably what I say here is going to be countered from some Vonage 
customer saying, "You're wrong!  It always has worked for me."  Vonage 
even advertises fax service.  I think that you have to pay extra for it, 
but they claim to provide it.  However, I've also known many unhappy 
Vonage customers who couldn't get a fax through to save their life.  
Read the doc I suggested, and hopefully you'll understand why their 
experience can simply be a luck of the draw.

Now, I'm not a Vonage expert... so I don't know exactly what they're 
doing.  So I don't know what is happening differently for those happy 
Vonage customers.  But let me tell you some things that could be 
happening to mitigate their situation.

1.  The distance or number of hops between the user and their 
Vonage-controlled gateway could be very, very short... which would allow 
Vonage a lot better chance of controlling the problematic fax-over-VoIP 
factors.

2. Vonage could actually be using T.38 (or perhaps T.37) in their ATAs 
and gateways.  I think that this is unlikely, but possible.

3. Undoubtedly the Vonage ATA uses QoS, and if everyone in the path 
between the user and the Vonage gateway honors that QoS to some degree 
or another, then maybe that's helping and making the difference.

4. Vonage could be employing some fax-specific kind of jitterbuffer on 
both ends of the VoIP connection.  This would help control the 
jitter-related problems... but again, it wouldn't solve the problem of 
audio corruption happening.

5. Vonage could be acting as a store-and-forward fax relay, employing an 
extremely tolerant set of fax protocols to mitigate the jitter issues.  
I think that this is unlikely, but possible.

6. They could be using some combination of the above or something else 
that I've not thought of right now.

As for my guess... I'd suspect that they're using #3 and #4 together, 
and that #1 comes into play more frequently than we would normally think.

Lee.