[asterisk-users] fax / t38 gateway

[Brendan Martens]

Quite right... And so we can all stop repeating ourselves; Steven has  
already done a great writeup on all this: http://www.soft-switch.org/foip.html

Brendan Martens


On Oct 27, 2008, at 3:20 PM, Kristian Kielhofner wrote:

> On Mon, Oct 27, 2008 at 2:49 PM, Wilton Helm <whelm at compuserve.com>  
> wrote:
>> Thanks Brendan for the explanation.  There is one other idea that  
>> struck me,
>> but again, I don't know if it has any merit.  My thinking is to  
>> keep FAX as
>> FAX and electronic as electronic, rather than introducing a new  
>> hybrid
>> approach.  Obviously Entering FAX from an electronic source is as  
>> old as the
>> FAX modem, and Exiting it electronically is as old as E-FAX, not to  
>> mention
>> other alternatives.
>>
>> Is it feasible to simply specify the codec as ulaw or alaw  
>> (depending on
>> jurisdiction, I forgot the g numbers) for calls originating from  
>> the FXS or
>> whatever the FAX is coming from?  Obviously, the bandwidth would be  
>> higher
>> in that case, but you can't get around the laws of physics.  Yes it  
>> is lossy
>> compression, still, but it is the simple, predictable form of lossy
>> compression that the modem in every FAX machine already is  
>> programmed to
>> cope with.  The only problems I can see would be if the provider  
>> who handles
>> the call refuses to accept that codec, or transcodes it to  
>> something else.
>> I don't know the likelihood of either of these.
>>
>> Wilton
>>
>
> Wilton,
>
>  Many providers will "allow" you to do faxing via g711u/g711a (G711u
> mu-law is used in "T" countries, G711a a-law is used in "E"
> countries).  Of course they will "allow" it - fax modems talk to each
> other just like we do.  They're just doing it with much less tolerance
> to error and variations in the audio.  The provider's gateways,
> however, should detect the fax tone and disable echo cancellation,
> etc.
>
>  What this discussion is forgetting are the issues inherent with
> packet networks:
>
> - latency
> - jitter
> - packet loss
>
>  Standard fax machines communicating via some ATA with a G711u RTP
> stream cannot correct for these situations.  In some severe cases. the
> modems might not even be able to train.  V.x modem standards were not
> designed for packet networks.  For this reason many faxes (especially
> at higher speeds) will fail (depending on the state of the network)
> when using a G711*, "pass-through", or "clear channel" codec.
>
>  You will have a much higher rate of success faxing with G711u over
> your LAN than a congested cable modem, for instance.
>
>  That's what T.38 is for.  It doesn't even use RTP, it uses UDPTL
> (UDP Transport Layer) or TCP (rare) to manage the transport of data
> and correct for transmission errors in various parts of the OSI stack.
> As we've said before the "support" for this standard varies and often
> times just doesn't work.
>
> - G711u will fail depending on the condition of the network.
> - T.38 will fail depending on the type(s) of equipment used.
>
>  Faxing via VoIP is largely a crap shoot.  However, it is important
> to focus on T.38 because I feel these interop issues can *eventually*
> be resolved.  No one is ever going to "fix" the issues with packet
> networks*.  That's why they are packet networks.  We will have much
> better luck working towards T.38 interop.
>
>
> * Obviously they are some "fixes" like MPLS, etc, but that doesn't
> really help those of us trying to make do with the internet, for
> example.
> -- 
> Kristian Kielhofner
> http://blog.krisk.org
> http://www.submityoursip.com
> http://www.astlinux.org
> http://www.star2star.com
>
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