<div>Lee and everyone else that replied,</div> <div>Thanks for the valuable and very detailed explanation. </div> <div> </div> <div>May I ask, from your own personal experience. is it not necessaritly worth (the headaches) of investing mytime into setting up SPANDSP into my asterisk system, but rather invest it into going to a company, like packet8 that offers t38 conversion? </div> <div> </div> <div>what does the future of faxing lean towards? before entering an era when all fax machines run the t38 protocol. will we see more t38 termination services or faxing through g711?</div> <div> </div> <div>from what i've read, using a service that does t38 termination, seems to be where i should go.</div> <div> </div> <div>Thanks.</div> <div><BR><BR><B><I>Lee Howard <faxguy@howardsilvan.com></I></B> wrote:</div> <BLOCKQUOTE class=replbq style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #1010ff 2px solid">On Sun, Sep 24, 2006 at 01:58:21PM
-0700, Christopher Corn wrote:<BR><BR>> A couple of faxing methods im confused about.<BR>><BR>> The pass through method, sending fax data over G711 codec<BR>> versus<BR>> Relay method, t30 to t38 conversion<BR>><BR>><BR>> Can someone explain to me why the pass through method doesn't<BR>> require t30 to t38 conversion ( or does it do it?)? i believe<BR>> the conversion to t38 is so that it can be routed through a<BR>> packet network and then back to t30 so that the fax machine can<BR>> understand. why is it that if you use a pass through method, and<BR>> your still passing through a packet network, you dont need to<BR>> convert to t38 and t30?<BR>><BR><BR>Be careful about your wording. People here generally refer to "pass <BR>through" as T.38 pass-through and not G.711 pass-through.<BR><BR>I think that if you understood how faxing works you would see that your <BR>questions here don't really make sense.<BR><BR>In traditional PSTN
faxing you have a total of two endpoints performing <BR>T.30 protocol. In a simplified form, the sender takes scanner image <BR>data and modulates it (into an audio waveform) and then passes that <BR>audio over the PSTN to the receiver which demodulates it (takes the <BR>audio and turns it into data again). As long as the demodulated data is <BR>identical to the original data, then everything should be okay... for <BR>the most part. However, if you start to consider audio corruption on <BR>the PSTN, then that's where difficulties start to ensue. If you have <BR>some audio, modulated data, and then you compress it or fracture it or <BR>otherwise corrupt it, then there's no possible way that the demodulator <BR>is going to be able to come up with the original data.<BR><BR>Now introduce VoIP telephony... where a small amount of audio corruption <BR>(jitter) is anticipated on the UDP channel... and mix it with faxing and <BR>hopefully you can see how it just doesn't work well.
VoIP is packetized <BR>audio passed over an IP network. Packetized audio is nothing new. ISDN <BR>circuits have had it for a long time now. Those circuits are digital - <BR>meaning the audio waveform is digitized at 8000 Hz... so the audio is <BR>represented with bytes and are packetized into frames. Those <BR>traditional digital circuits are designed to prevent any loss of that <BR>data. VoIP works similarly, except that the medium is lossy UDP/IP <BR>networking.<BR><BR>Since VoIP works on *IP* networks, and since IP networks already handle <BR>data communication very well, there really is no reason to perform the <BR>modulation or the demodulation - just send the raw data through. So <BR>that's basically the punchline of T.38... it's fax protocol without the <BR>traditional modems involved. Then you have FoIP.<BR><BR>However, these days the world is a hybrid of VoIP and PSTN environments <BR>(mostly PSTN still), and thus anyone using T.38 will need to have a
<BR>"gateway" involved somewhere along the call path that can do that <BR>traditional modulation/demodulation. That is what the T.38 gateway is. <BR>If a T.38 relay does not act as a gateway (i.e. no modulators) then it <BR>performs only T.38 pass-through - meaning it only is useful for <BR>situations where calls are end-to-end T.38 or where an external FoIP <BR>service provider is used.<BR><BR>Because of the way things work T.38 gateways will not only need to have <BR>traditional modems (hard or soft) but will also need to perform T.30. <BR>So when faxing with T.38 and the call is not end-to-end T.38 then you <BR>have at least three points along the call path performing T.30 (versus <BR>the traditional scenario of just two).<BR><BR>So, to answer your questions...<BR><BR>Why does using G.711 not require T.38? Because from the viewpoint that <BR>the question was given, G.711 and T.38 are competing approaches. T.38 <BR>was designed to replace G.711. You can packetize G.711
audio just fine <BR>without converting it to anything else. So when faxing with G.711 T.38 <BR>is not involved because its basically mimicking the old-style <BR>traditional PSTN faxing, except that the audio is passing over a <BR>different (less-reliable) medium.<BR><BR>So the reason that T.38 exists is because UDP/IP is lossy and is not <BR>therefore reliable for the purposes of faxing with G.711 unless the <BR>communication can be guaranteed to be nearly lossless. For those that <BR>work on lossy channels, G.711 will just not work reliably.<BR><BR>Lee.<BR><BR>_______________________________________________<BR>--Bandwidth and Colocation provided by Easynews.com --<BR><BR>asterisk-users mailing list<BR>To UNSUBSCRIBE or update options visit:<BR>http://lists.digium.com/mailman/listinfo/asterisk-users<BR></BLOCKQUOTE><BR>