[asterisk-users] FAX Issues

[Steve Totaro]

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?
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.

Works like a charm.

>
> For the most part people who claim success when faxing over SIP G.711 are
> being rescued by ECM (error correction) within the fax protocol.  There are
> very, very few who really have mitigated UDP packet loss.
>
> That said, all T.38 systems are not equal.  Certainly, the reliability of
> your T.38 provider may not be any better than that of G.711 fax over the SIP
> UDP.
>
> I only recommend faxing over TDM everything else is at your own risk.

All faxing in Asterisk is at your own risk.  It is not reliable.

>
> Thanks,
>
> Lee.
>