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