[asterisk-users] FAX Issues

[Lee Howard]

Steve Totaro wrote:
> On Tue, Aug 9, 2011 at 7:22 PM, Lee Howard <faxguy at howardsilvan.com> wrote:
>   
>> Ryan McGuire wrote:
>>     
>>> Unless your network is under load and you are seeing dropped packets
>>> and high jitter, I would absolutely not do T.38. The cheapest and
>>> easiest approach that I have found is to buy yourself an FXS gateway
>>> and just make sure you are using ulaw.
>>>       
>> As SIP is usually running over UDP/IP it doesn't take much to produce
>> dropped packets.  Dropped packets mean lost audio which means lost data and
>> possible demodulation difficulties for the modems.  If you're in an
>> environment where dropped UDP packets don't occur you're in a very rare
>> scenario.
>>     
>
> I suppose you are talking about from the provider and not on the LAN?
>   

You certainly can (and usually will) have UDP packet loss on an 
uncontrolled LAN.

> At Equinix in Ashburn VA, I have never had a dropped packet via the
> crossconnect from our cage to Level3's cage.  Sub ms pings.  Putting
> the primary PBX in Equinix and a 100meg speed for all VoIP calls in
> our out.  100meg DIA and 100meg layer 2 fiber to corporate.

I have no reason to doubt your claims, but if this is true, then your 
arrangement there clearly mitigates the likelihood of UDP packet loss.  
Nevertheless, this arrangement is not something that the typical user 
who asks "how do I fax over SIP/VoIP" is going to have.  Without being 
very clear about the environment and explaining the pitfalls of not 
following your example exactly, you're not doing them any favors by 
encouraging them to attempt it in their environment.

For every one user who I've ever heard from saying that they have 
reliable G.711 faxing over their SIP channels I've heard from a dozen 
who don't.

Thanks,

Lee.