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 <[EMAIL PROTECTED]> > 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 > > _______________________________________________ > -- Bandwidth and Colocation Provided by http://www.api-digital.com -- > > asterisk-users mailing list > To UNSUBSCRIBE or update options visit: > http://lists.digium.com/mailman/listinfo/asterisk-users _______________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
