Re: [Asterisk-Users] T.38 fax summary
On Sun, Feb 27, 2005 at 05:32:49PM -0800, Lee Howard wrote: Quite right. I'm sorry to have misled. What happens is this (as an example scenario): The receiver will, for an example, receive the post-page message. The sender expects a response to this. The receiver, however, is required to wait between 55 and 95 ms before transmitting the response. The sender will likely be looking for the post-page response immediately after transmitting the post-page message. Per spec the sender will only wait about 3 seconds (per-spec between 2550 and 3450 ms) before giving up wating and retransmitting the post-page message (and then re-expecting the response). Thank you. So the 1 second lag I suggested is too much, but the principle is sound. Say we change it to half a second you're well under the limit. The question then becomes, is a fixed half-a-second jitterbuffer good enough to remove all the problematic jitter from the signal. This is a testable assertion (though unfortunatly I don't have the necessary equipment), simulating jitter is possible and hopefully the jitterbuffer itself is tunable. A tunable jitterbuffer sounds like a good idea, anyone actually thinking of implementing it though? Have a nice day, -- Martijn van Oosterhout Ecomtel Pty Ltd ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Lee Howard wrote: On 2005.02.27 11:28 Jon Gabrielson wrote: You wouldn't happen to know how to do this would you? I currently have a box with both hylafax and asterisk installed. asterisk handles the dedicated voice lines over a t100p and hylafax handles the dedicated fax lines over a 4port serial card with external modems. It would be really nice if I could have them share the lines instead of having all the lines dedicated to either one or the other. The problem is that the only way to detect whether it is a fax is for asterisk to answer it first and then there is no way that I know of to send it on to hylafax. Sure there is a way. A couple of ways (at least). 1) Get a 4-port TDM card and install it into your Asterisk box. Connect the TDM ports to your modem ports. Then forward incoming calls on fax DIDs to those TDM ports. Currently this doesn't work. A FAX machine connected to a TDM card fails almost every call. It used to work OK. The TDM driver seems to be buggy right now. 2) Get another T1 port in the Asterisk box and get a T1 fax modem and do the same thing. You probably don't have the fax-demand to justify the hardware expense, though. Regards, Steve ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Lee Howard wrote: On 2005.02.27 09:30 Martijn van Oosterhout wrote: On Sun, Feb 27, 2005 at 09:10:48AM -0800, Lee Howard wrote: Fax cannot handle a one-second delay. As Steve mentions in the article, per-spec fax has some timings (particularly silence in direction switching) set at 75 ms +/- 20 ms. So if the delay gets much larger than 75 ms, then there's likely to be trouble. Now, some fax machines may tolerate larger delays, but that tolerance is beyond the spec, and thus should not be used as a gauge. Something's not right here. Quite right. I'm sorry to have misled. What happens is this (as an example scenario): The receiver will, for an example, receive the post-page message. The sender expects a response to this. The receiver, however, is required to wait between 55 and 95 ms before transmitting the response. The sender will likely be looking for the post-page response immediately after transmitting the post-page message. Per spec the sender will only wait about 3 seconds (per-spec between 2550 and 3450 ms) before giving up wating and retransmitting the post-page message (and then re-expecting the response). This is also slightly wrong. The gaps in the audio stream are specified as 75+-20ms. The response is specified as occuring a *minimum* of 75ms after the received carrier has ceased. So if there is a steady 1000 ms lag between the sender and the receiver (both ways, meaning we assume that both ends could have the 1-second jitter buffer), what will happen is this: The sender will finish transmitting the post-page message. One second later the receiver finishes getting it. The receiver will introduce its own required pause, and add to that the overhead of any processing required, and then it will return the signal. The sender will not get that signal for another 1000 ms. That means that for the total processing of that to occur the 2550 ms danger-zone time is nearly reached. Add to that buffer-time the latency time, and I'd say that you'd be looking at a signal failure quite certainly. In real-world action, however, the 2550-3450 ms danger-zone time is practically never reached. In normal use that time is often very close to 400 ms. So yes, 75 ms latency is not accurate for a command-response interaction between two fax machines. And, per-spec the response could, in theory, sustain a 1000 ms lag. However, that would far-exceed normal behavior, and I'd be surprised if it would not prove fatal to most fax communications. I think this still allows significant buffering - say 500ms - without causing trouble. Extreme buffer would, however, be troublesome. 500ms, less the rollover time needed for the FEC, should give pretty good jitter buffering. Regards, Steve ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Hi, my name is Pedro Caria I'm new to this list. I live in Portugal and find myself in the position to talk often to various parts of the world, very often the Telco line has a delay superior to 1s, I also fax in the same conditions, so to my experience faxes do work with delays far superior to 75ms. Am I missing something ? Pedro Caria On 27/fev/2005, at 17:10, Lee Howard wrote: On 2005.02.27 08:34 Martijn van Oosterhout wrote: Hi, I read it and found it very enlightening. I do have one question regarding Modems don't like relativity. It says modems need a constant delay; is there a limit to what it can handle. For example, would it be possible to configure a jitterbuffer right at the endpoint before the fax to put a constant delay of 1 second relative to the sender. This should be enough time to weed out any jitter. Basically, fix the jitterbuffer so the delay is constant. If a fax can handle a constant delay of up to a second you're home. Fax cannot handle a one-second delay. As Steve mentions in the article, per-spec fax has some timings (particularly silence in direction switching) set at 75 ms +/- 20 ms. So if the delay gets much larger than 75 ms, then there's likely to be trouble. Now, some fax machines may tolerate larger delays, but that tolerance is beyond the spec, and thus should not be used as a gauge. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Hi, Questions keep comming up about this, so I started writing something at http://www.soft-switch.org/foip.html . I think I covered the FAX over VoIP issues fairly completely. T.37 is pretty simple to explain. There is rather more to say about T.38, but at least this is a start. If anyone wants to suggest corrections or additions, just blurt them out. Regards, Steve Rich Adamson wrote: Steve Underwood, Would you mind summarizing where/how T.38 functions, and maybe how it compares to the analog fax environment for the asterisk-users arhives? Seems to be some misunderstanding, and a lot of interest in handling faxes in various forms via asterisk. If some these approaches were summarized in one posting, a lot of us could reference it to remind us of limitations, current state, etc. A few short paragraphs would be helpful. Rich ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Steve, Excellent summary thus far!!! Will the summary stay on that url for a lengthy period of time, or would it be possible to copy it to the wiki when complete? One item you might add to the T.38 discussion is a relatively short paragraph that describes/relates the current analog fax environment to T.38; by that I mean I don't have a clear picture in my mind as to whether T.38 has any interconnecting relationship to analog machines. Maybe some additional words that says something like... T.38 capable fax machines encapsulate the digital fax bits into tcp/udp packets and sends those across a data network. To receive a T.38 data stream on an analog fax machine, the equivalent of a modem or protocol converter would be required to change the digital fax bits into an analog format compatible with the receiving analog fax machine's modem. Since there is a very high interest in the asterisk community to accomplish the T.37 approach (or stated slightly different, an interest in receiving faxes in a pdf or tiff form instead of paper), could you include a reference or two for products (open source or otherwise) that accomplishes that task? (I'm assuming spandsp is one such approach, and there are likely others. Something like... an analog fax modem when combined with the linux app called xxx will provide the economical conversion, but cannot be reliably used across voip.) The thought process (at least for me) is that many commercial voip/asterisk implementations really need the emergency Red Phone support (eg, US 911 fall back), and in many cases that analog pstn line could be the same as an incoming fax line. (Rather small price to pay to cover both legal liablilities and fax images in the current voip environment.) Back in the olden days, I recall several modem vendors bundling PC fax software with their products. All of those old Win v3.1 apps created a fax file (eg, pdf or otherwise) that could be distributed via email. What are the equivalent low-overhead approaches (sort of T.37) today? I'd be very happy if I could simply connect an old analog 28.8 modem to the asterisk serial port, and drop a low-overhead app onto the system to address the fax stuff for today. Something very basic. Rich Hi, Questions keep comming up about this, so I started writing something at http://www.soft-switch.org/foip.html . I think I covered the FAX over VoIP issues fairly completely. T.37 is pretty simple to explain. There is rather more to say about T.38, but at least this is a start. If anyone wants to suggest corrections or additions, just blurt them out. Regards, Steve Rich Adamson wrote: Steve Underwood, Would you mind summarizing where/how T.38 functions, and maybe how it compares to the analog fax environment for the asterisk-users arhives? Seems to be some misunderstanding, and a lot of interest in handling faxes in various forms via asterisk. If some these approaches were summarized in one posting, a lot of us could reference it to remind us of limitations, current state, etc. A few short paragraphs would be helpful. Rich ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users ---End of Original Message- ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On Sun, Feb 27, 2005 at 04:14:46PM +0800, Steve Underwood wrote: Questions keep comming up about this, so I started writing something at http://www.soft-switch.org/foip.html . I think I covered the FAX over VoIP issues fairly completely. T.37 is pretty simple to explain. There is rather more to say about T.38, but at least this is a start. If anyone wants to suggest corrections or additions, just blurt them out. Hi, I read it and found it very enlightening. I do have one question regarding Modems don't like relativity. It says modems need a constant delay; is there a limit to what it can handle. For example, would it be possible to configure a jitterbuffer right at the endpoint before the fax to put a constant delay of 1 second relative to the sender. This should be enough time to weed out any jitter. Basically, fix the jitterbuffer so the delay is constant. If a fax can handle a constant delay of up to a second you're home. Maybe also allow people to setup a jitterbuffer to do a special interpolation for fax, which might just amount to sending a special tone representing zeros. Is there a possiblity of creating a app allowing people to configure the jitterbuffer for a particular call? In any case, if it can't be done this way, could you explain why? -- Martijn van Oosterhout Ecomtel Pty Ltd ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Hello Steve. It's an excellent read. In two places you mention that V.34-Fax is 28,800 bps. Actually, V.34-Fax has speeds ranging from 2400 baud to 33600 baud all using V.34. And, while most V.34 connections are going to not probably be more than 28,800 bps, I have seen sustained analog V.34 fax connections at 33600 baud. I think that it may be fair to mention ECM protocol a bit, in the fax-over-VoIP sections. In the example you give of 20 ms of audio missing from the image data portion of the call, *if* the receiving modem doesn't declare end-of-page and actually is able to ride-over the audio loss, then yes, if the fax session used 2D-MR or 1D-MH compression and did not negotiate ECM protocol, then there will likely just appear a corrupt horizontal stripe or a few more. However, if 2D-MMR image compression is used, no image data corruption can be tolerated at all, or the image will be truncated at the point of corruption. This is why usage of 2D-MMR image data compression requires usage of the ECM protocol, which will, in effect, cause the receiver to request that the sender retransmit the corrupted image data portions. ECM protocol tends to make faxing a bit more resiliant, anyway, in practice, because the ECM protocol has methods of flow control that the non-ECM fax protocol lacked, which allow both the sender and the receiver to essentially pause the session at key moments when a slow-printing or slow-decoding or slow-framing machine may need extra time. Consequently, most newer fax machines are going to use ECM with all image compression methods, including those that do not require it (MH and MR). Plus, I think that it is attractive for fax manufacturers to limit copy-quality complaints by standardizing on ECM usage. Sometimes I tend to believe that ECM is what's making fax-over-VoIP work at all reliable for a lot of people using it (granted, some can do fax-over-VoIP reliably because they have an IP network path with low jitter/loss/latency/etc). I think that ECM will mask the real trouble of the line conditions from the users unless they actually are able to go in and look at a log of the fax session or if they analyze the time that is required to send/receive faxes over VoIP. Of course, this is good I guess, to make faxing work sometimes over VoIP - however, I think that the downsides are that it hides the truth of the line condition problem from the end users, and it significantly increases the time required to communicate fax images. So there may be no savings whatsoever in faxing-over-VoIP if you are paying $0.02 per minute but it takes you 3 times as long to complete the call. I'v had some experience working with newer Panasonic Panafax fax machines over the last four years. These machines do have an RJ-45 jack in them, and do connect to a TCP/IP network. They do have a feature that they call internet fax, although it is actually neither T.37 or T.38. It is closer to T.37, as it pretty much works like a scan-to-email system. I think that there are some HP systems that do something similar. So just because a fax machine has an RJ-45 jack and says something about internet fax in the features does not necessarily mean that T.38/T.37 is being used... or even just an adulterated version of them, either. I concur with your statement, T.37 is the sane way to handle FAX at this time. My personal opinion for those businesses that are using VoIP exclusively and yet still need to support fax is to do whatever can be done (T.37 comes to mind) to get all of the faxes from all of the business to a common fax gateway system and then provide that system with the requisite PSTN capacity to push out the peak fax load. I also think that those businesses should be using DID for receiving faxes on that common gateway and routing them internally. If DID isn't an option, then a trusted office receptionist can be used instead. Now, doing this is going to require additional hardware, most likely, in the form of TDM ports and perhaps some additional wiring, but I think that it's the necessary evil of the time for businesses transitioning into VoIP before FoIP is better supported. For my clients I use HylaFAX, of course, to serve as that gateway fax device, but I suspect that spandsp+Asterisk can be used in that capacity with perhaps about thesame amount of glue work. Anyway, it was a good read. Thanks, Lee. On 2005.02.27 00:14 Steve Underwood wrote: Hi, Questions keep comming up about this, so I started writing something at http://www.soft-switch.org/foip.html . I think I covered the FAX over VoIP issues fairly completely. T.37 is pretty simple to explain. There is rather more to say about T.38, but at least this is a start. If anyone wants to suggest corrections or additions, just blurt them out. Regards, Steve ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 04:26 Rich Adamson wrote: Back in the olden days, I recall several modem vendors bundling PC fax software with their products. All of those old Win v3.1 apps created a fax file (eg, pdf or otherwise) that could be distributed via email. Well, I don't remember any of them doing PDFs. PDF wasn't really around when Windows 3.1 was in its prime. And even those fax win-apps that were distributed with fax modems as recently as 2001 tended to write the fax images in TIFF or some proprietary format. What are the equivalent low-overhead approaches (sort of T.37) today? Any computer fax application that I know of will write the image to file, modern ones usually use TIFF. HylaFAX, mgetty+sendfax, efax, spandsp, etc. Then you just write the glue to make it deliver that fax image by e-mail. HylaFAX already has that built-in, and just requires some minor configuration to make it turn on. I don't know much about the delivery systems of the others. I'd be very happy if I could simply connect an old analog 28.8 modem to the asterisk serial port, and drop a low-overhead app onto the system to address the fax stuff for today. Something very basic. If you're happy to use the hardware and have a PSTN line availble to it, you're talking about HylaFAX. But I suppose that any fax package would work, with the exception of spandsp, which would require that you use an Asterisk-compatible FXO instead of that modem. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 08:34 Martijn van Oosterhout wrote: Hi, I read it and found it very enlightening. I do have one question regarding Modems don't like relativity. It says modems need a constant delay; is there a limit to what it can handle. For example, would it be possible to configure a jitterbuffer right at the endpoint before the fax to put a constant delay of 1 second relative to the sender. This should be enough time to weed out any jitter. Basically, fix the jitterbuffer so the delay is constant. If a fax can handle a constant delay of up to a second you're home. Fax cannot handle a one-second delay. As Steve mentions in the article, per-spec fax has some timings (particularly silence in direction switching) set at 75 ms +/- 20 ms. So if the delay gets much larger than 75 ms, then there's likely to be trouble. Now, some fax machines may tolerate larger delays, but that tolerance is beyond the spec, and thus should not be used as a gauge. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On Sun, Feb 27, 2005 at 09:10:48AM -0800, Lee Howard wrote: Fax cannot handle a one-second delay. As Steve mentions in the article, per-spec fax has some timings (particularly silence in direction switching) set at 75 ms +/- 20 ms. So if the delay gets much larger than 75 ms, then there's likely to be trouble. Now, some fax machines may tolerate larger delays, but that tolerance is beyond the spec, and thus should not be used as a gauge. Something's not right here. In 75ms light has just made it from here to the other side of the world. Even a PSTN network will provide a longer delay than that calling across the world. And the time to get a response back will be around twice that, which is well beyond that tolerated range. If you're saying there is a limit to the round-trip-time then within the fax specification is a predefined maximum physical distance you can send faxes. Faxing across the world does work so there is something else going here... -- Martijn van Oosterhout Ecomtel Pty Ltd ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Any computer fax application that I know of will write the image to file, modern ones usually use TIFF. HylaFAX, mgetty+sendfax, efax, spandsp, etc. Then you just write the glue to make it deliver that fax image by e-mail. HylaFAX already has that built-in, and just requires some minor configuration to make it turn on. I don't know much about the delivery systems of the others. I'd be very happy if I could simply connect an old analog 28.8 modem to the asterisk serial port, and drop a low-overhead app onto the system to address the fax stuff for today. Something very basic. If you're happy to use the hardware and have a PSTN line availble to it, you're talking about HylaFAX. But I suppose that any fax package would work, with the exception of spandsp, which would require that you use an Asterisk-compatible FXO instead of that modem. Since there is a fairly large percentage of asterisk implementations using the x100p/tdm soho interfaces that have problems with missed interrupts (or whatever), approaches such as spandsp certainly isn't the answer. At the higher end (larger corporations that may have a much larger fax volume), there seems to be multiple commercial and open source products. So, for the smaller soho environments, using one/two analog pstn lines as both fax lines and emergency backup seems very realistic. Particularily if the fax software could run on the same box as asterisk. Is it at all realistic to assume that HylaFax could coexist with Asterisk on one box? If not, is there another known low-impact package that could at least address the incoming fax to email kind of thing? Rich ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
You wouldn't happen to know how to do this would you? I currently have a box with both hylafax and asterisk installed. asterisk handles the dedicated voice lines over a t100p and hylafax handles the dedicated fax lines over a 4port serial card with external modems. It would be really nice if I could have them share the lines instead of having all the lines dedicated to either one or the other. The problem is that the only way to detect whether it is a fax is for asterisk to answer it first and then there is no way that I know of to send it on to hylafax. Jon. On Sunday 27 February 2005 11:06 am, Lee Howard wrote: I'd be very happy if I could simply connect an old analog 28.8 modem to the asterisk serial port, and drop a low-overhead app onto the system to address the fax stuff for today. Something very basic. If you're happy to use the hardware and have a PSTN line availble to it, you're talking about HylaFAX. But I suppose that any fax package would work, with the exception of spandsp, which would require that you use an Asterisk-compatible FXO instead of that modem. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 09:30 Martijn van Oosterhout wrote: On Sun, Feb 27, 2005 at 09:10:48AM -0800, Lee Howard wrote: Fax cannot handle a one-second delay. As Steve mentions in the article, per-spec fax has some timings (particularly silence in direction switching) set at 75 ms +/- 20 ms. So if the delay gets much larger than 75 ms, then there's likely to be trouble. Now, some fax machines may tolerate larger delays, but that tolerance is beyond the spec, and thus should not be used as a gauge. Something's not right here. Quite right. I'm sorry to have misled. What happens is this (as an example scenario): The receiver will, for an example, receive the post-page message. The sender expects a response to this. The receiver, however, is required to wait between 55 and 95 ms before transmitting the response. The sender will likely be looking for the post-page response immediately after transmitting the post-page message. Per spec the sender will only wait about 3 seconds (per-spec between 2550 and 3450 ms) before giving up wating and retransmitting the post-page message (and then re-expecting the response). So if there is a steady 1000 ms lag between the sender and the receiver (both ways, meaning we assume that both ends could have the 1-second jitter buffer), what will happen is this: The sender will finish transmitting the post-page message. One second later the receiver finishes getting it. The receiver will introduce its own required pause, and add to that the overhead of any processing required, and then it will return the signal. The sender will not get that signal for another 1000 ms. That means that for the total processing of that to occur the 2550 ms danger-zone time is nearly reached. Add to that buffer-time the latency time, and I'd say that you'd be looking at a signal failure quite certainly. In real-world action, however, the 2550-3450 ms danger-zone time is practically never reached. In normal use that time is often very close to 400 ms. So yes, 75 ms latency is not accurate for a command-response interaction between two fax machines. And, per-spec the response could, in theory, sustain a 1000 ms lag. However, that would far-exceed normal behavior, and I'd be surprised if it would not prove fatal to most fax communications. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 10:36 Rich Adamson wrote: Is it at all realistic to assume that HylaFax could coexist with Asterisk on one box? Yes, you could have both Asterisk and HylaFAX running on the same box. HylaFAX does not require a lot of resources (CPU, RAM, HDD) for small deployments. I've had HylaFAX running on P-75s before successfully as a one-line receive-only system. I wouldn't recommend it, but that should give you an idea of the minimum-requirements. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 11:28 Jon Gabrielson wrote: You wouldn't happen to know how to do this would you? I currently have a box with both hylafax and asterisk installed. asterisk handles the dedicated voice lines over a t100p and hylafax handles the dedicated fax lines over a 4port serial card with external modems. It would be really nice if I could have them share the lines instead of having all the lines dedicated to either one or the other. The problem is that the only way to detect whether it is a fax is for asterisk to answer it first and then there is no way that I know of to send it on to hylafax. Sure there is a way. A couple of ways (at least). 1) Get a 4-port TDM card and install it into your Asterisk box. Connect the TDM ports to your modem ports. Then forward incoming calls on fax DIDs to those TDM ports. 2) Get another T1 port in the Asterisk box and get a T1 fax modem and do the same thing. You probably don't have the fax-demand to justify the hardware expense, though. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On Sun, 27 Feb 2005, Lee Howard wrote: On 2005.02.27 11:28 Jon Gabrielson wrote: You wouldn't happen to know how to do this would you? I currently have a box with both hylafax and asterisk installed. asterisk handles the dedicated voice lines over a t100p and hylafax handles the dedicated fax lines over a 4port serial card with external modems. It would be really nice if I could have them share the lines instead of having all the lines dedicated to either one or the other. The problem is that the only way to detect whether it is a fax is for asterisk to answer it first and then there is no way that I know of to send it on to hylafax. Sure there is a way. A couple of ways (at least). 1) Get a 4-port TDM card and install it into your Asterisk box. Connect the TDM ports to your modem ports. Then forward incoming calls on fax DIDs to those TDM ports. 2) Get another T1 port in the Asterisk box and get a T1 fax modem and do the same thing. You probably don't have the fax-demand to justify the hardware expense, though. Interesting idea, how about using one or two HFC-S BRI cards instead (very low cost) or even a quad BRI for that? The pri would be used for the PSTN connection and the BRI cards to connect to the faxes. Anyone ever tried it? ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
I have some extra FXS ports on my channel bank that I could plug the modems into, but is asterisk's fax support good enough for a production system? Everything I seem to read seems to state that asterisk fax detection and fax send/receive support is still very unreliable or is this only over long distances? Jon. Sure there is a way. A couple of ways (at least). 1) Get a 4-port TDM card and install it into your Asterisk box. Connect the TDM ports to your modem ports. Then forward incoming calls on fax DIDs to those TDM ports. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.27 22:20 Jon Gabrielson wrote: I have some extra FXS ports on my channel bank that I could plug the modems into, but is asterisk's fax support good enough for a production system? Everything I seem to read seems to state that asterisk fax detection and fax send/receive support is still very unreliable or is this only over long distances? You don't need to use faxdetect if you detect faxes by way of their DID. Dedicate some of your DIDs to fax. Route those DIDs to the fax extensions. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On Feb 25, 2005, at 11:41 AM, Steve Underwood wrote: Mark, In the time it took to write all that you could probably have read up enough about T.38 to realise you were talking complete rubbish :-) Gee thanks Steve. And your insight has been absolutely beneficial as well. -mark -- Mark Eissler, [EMAIL PROTECTED] Mixtur Interactive, Inc. [EMAIL PROTECTED] http://www.mixtur.com ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
[Asterisk-Users] T.38 fax summary
Steve Underwood, Would you mind summarizing where/how T.38 functions, and maybe how it compares to the analog fax environment for the asterisk-users arhives? Seems to be some misunderstanding, and a lot of interest in handling faxes in various forms via asterisk. If some these approaches were summarized in one posting, a lot of us could reference it to remind us of limitations, current state, etc. A few short paragraphs would be helpful. Rich ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On 2005.02.25 05:53 Rich Adamson wrote: Steve Underwood, Would you mind summarizing where/how T.38 functions, and maybe how it compares to the analog fax environment for the asterisk-users arhives? I don't mean to speak for Steve, so I hope that Steve will still reply if he chooses to, but I like the question, and since I know enough about T.38 and fax to answer at least in a general sense, I will. In a traditional analog fax you have modulated audio data, that is, the data stream is converted into an audio representation by the transmitter, and the receiver demodulates the audio stream to produce the data stream. A lot of data gets packed into very small portions of audio, which is why fax over VoIP (T.38 is not VoIP, it is FoIP) is unreliable - any jitter will likely cause data loss. There are no modulators in T.38. So take the fax procedure, but instead remove the data modulation/demodulation part. T.38 devices communicate raw data through the IP network, and the IP network is as good at communicating data as the PSTN is as good at communicating audio. So if you could have a full T.38 delivery route from fax sender to fax receiver, the data never once gets converted into an audio signal - it doesn't need to be. That's the gist of things. Lee. ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
On Feb 25, 2005, at 10:20 AM, Lee Howard wrote: In a traditional analog fax you have modulated audio data, that is, the data stream is converted into an audio representation by the transmitter, and the receiver demodulates the audio stream to produce the data stream. A lot of data gets packed into very small portions of audio, which is why fax over VoIP (T.38 is not VoIP, it is FoIP) is unreliable - any jitter will likely cause data loss. There are no modulators in T.38. So take the fax procedure, but instead remove the data modulation/demodulation part. T.38 devices communicate raw data through the IP network, and the IP network is as good at communicating data as the PSTN is as good at communicating audio. So if you could have a full T.38 delivery route from fax sender to fax receiver, the data never once gets converted into an audio signal - it doesn't need to be. Sort of...but no. Fax requires a codec that supports the frequency spectrum of a POTS audio channel. Currently, that means that anything other than g.711 won't work since the other popular codecs achieve their efficiency by dumping frequencies humans can't hear (just like mp3). The problem isn't typically g.711 because that's the codec that is generally used by the digital telco world. A common problem when discussing g.711 often is packet size vs bandwidth limitations. T.38 can alleviate this problem because it doesn't rely on a codec. The bigger problem with faxing over VOIP is related to lost packets and timing issues (jitter). Lost packets are the death knell for fax because it isn't very tolerant of missing data. How do you complete an image with missing data??? AFAIK T.38 can't do anything to recover from packet loss...the fax machine needs to be tolerant of it. Ironically, ECM was introduced to recover from information loss when transmitting faxes over analog lines but ECM can actually cause problems when used with T.38. If you can turn ECM off that's the best thing to do when using T.38. Besides lost packets though if you have to consider packets arriving at weird timing intervals (jitter). The fax machine needs to get its data in a steady stream. This is supposed to be a realtime transmission after all. While T.38 can absorb some of the problems triggered by latency and jitter, when the problem becomes too excessive it tanks just as quickly as faxing without T.38. So with those barriers out of the way what is it that T.38 tries to accomplish? Instead of sending a fax over VOIP as a stream of sampled audio, the protocol intercepts the audio at the endpoints and packetizes it as blocks of data instead. The receiving gateway must know how to handle the data stream so it can convert the fax back into a T.30 fax data stream for POTS. During the session, progress is faked so that the two fax machines don't think the transmission has stopped...that's a crucial step because it takes time to convert and send/receive the fax reliably. I think the best arsenal for faxing over VOIP today is to have a good broadband connection, g.711, and a fax machine where YOU can set the max transmission speed. Sadly, the last part seems to be missing quite often. I've noticed that HP actually mentions faxing over VOIP in the documentation for their 7410 all in one machine and, more importantly, they include support for changing transmission speeds. Way to go HP! -mark -- Mark Eissler, [EMAIL PROTECTED] Mixtur Interactive, Inc. [EMAIL PROTECTED] http://www.mixtur.com ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [Asterisk-Users] T.38 fax summary
Mark, In the time it took to write all that you could probably have read up enough about T.38 to realise you were talking complete rubbish :-) Regards, Steve Mark Eissler wrote: On Feb 25, 2005, at 10:20 AM, Lee Howard wrote: In a traditional analog fax you have modulated audio data, that is, the data stream is converted into an audio representation by the transmitter, and the receiver demodulates the audio stream to produce the data stream. A lot of data gets packed into very small portions of audio, which is why fax over VoIP (T.38 is not VoIP, it is FoIP) is unreliable - any jitter will likely cause data loss. There are no modulators in T.38. So take the fax procedure, but instead remove the data modulation/demodulation part. T.38 devices communicate raw data through the IP network, and the IP network is as good at communicating data as the PSTN is as good at communicating audio. So if you could have a full T.38 delivery route from fax sender to fax receiver, the data never once gets converted into an audio signal - it doesn't need to be. Sort of...but no. Fax requires a codec that supports the frequency spectrum of a POTS audio channel. Currently, that means that anything other than g.711 won't work since the other popular codecs achieve their efficiency by dumping frequencies humans can't hear (just like mp3). The problem isn't typically g.711 because that's the codec that is generally used by the digital telco world. A common problem when discussing g.711 often is packet size vs bandwidth limitations. T.38 can alleviate this problem because it doesn't rely on a codec. The bigger problem with faxing over VOIP is related to lost packets and timing issues (jitter). Lost packets are the death knell for fax because it isn't very tolerant of missing data. How do you complete an image with missing data??? AFAIK T.38 can't do anything to recover from packet loss...the fax machine needs to be tolerant of it. Ironically, ECM was introduced to recover from information loss when transmitting faxes over analog lines but ECM can actually cause problems when used with T.38. If you can turn ECM off that's the best thing to do when using T.38. Besides lost packets though if you have to consider packets arriving at weird timing intervals (jitter). The fax machine needs to get its data in a steady stream. This is supposed to be a realtime transmission after all. While T.38 can absorb some of the problems triggered by latency and jitter, when the problem becomes too excessive it tanks just as quickly as faxing without T.38. So with those barriers out of the way what is it that T.38 tries to accomplish? Instead of sending a fax over VOIP as a stream of sampled audio, the protocol intercepts the audio at the endpoints and packetizes it as blocks of data instead. The receiving gateway must know how to handle the data stream so it can convert the fax back into a T.30 fax data stream for POTS. During the session, progress is faked so that the two fax machines don't think the transmission has stopped...that's a crucial step because it takes time to convert and send/receive the fax reliably. I think the best arsenal for faxing over VOIP today is to have a good broadband connection, g.711, and a fax machine where YOU can set the max transmission speed. Sadly, the last part seems to be missing quite often. I've noticed that HP actually mentions faxing over VOIP in the documentation for their 7410 all in one machine and, more importantly, they include support for changing transmission speeds. Way to go HP! ___ Asterisk-Users mailing list Asterisk-Users@lists.digium.com http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
