Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/03/2011 07:08 PM, Steve Underwood wrote: On 01/04/2011 04:22 AM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: Hi folks, I was hoping that someone might be able to help clarify some confusion I have on DAHDI Fax detection after spending some time searching. My understanding is this: I'll try. 1.) Echo cancellation is automatically disabled upon recognition of a CNG tone, regardless of the faxdetect setting. This can only be disabled at compile time. No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. That's not true. Modern echo cancellers normally disable completely. It is arguable whether they should disable completely for FAX, but they need to behave properly for all modems. For any duplex modem, disabling only the NLP is useless. They need to cancel end to end, so they don't get upset by a continuously adapting canceller, and so they can minimise the issues caused by the highly non-linear G.711 channel. This doesn't match up with what the manufacturers of the two G.168 ECs that Digium distributes have told me personally about their products. Their ECs behave differently for FAX and 'regular' modems, but they do that based on the detection of the V.21 preamble, ANSam and other signals in addition to CED, which seemed to be much more detail than was warranted in my response to the OP :-) 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call to the fax extension. If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes. 3.) faxdetect=outgoing will ?? The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used. Also, do Digium cards with HW Echo Cancellation detect the CNG tones in hardware? If so, how does the faxdetect setting in DAHDI affect that behavior? No, none of the Digium HW ECs detect and report CNG tones via the DSP; CNG tone detection is still done on the host CPU. 'faxdetect' is not set in DAHDI, it's set in chan_dahdi.conf. Steve -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users -- Kevin P. Fleming Digium, Inc. | Director of Software Technologies 445 Jan Davis Drive NW - Huntsville, AL 35806 - USA skype: kpfleming | jabber: kflem...@digium.com Check us out at www.digium.com www.asterisk.org -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/03/2011 06:47 PM, Thomas Rymes wrote: On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: [snip] 1.) Echo cancellation is automatically disabled upon recognition of a CNG tone, regardless of the faxdetect setting. This can only be disabled at compile time. No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. OK. Either way, though, the changes to echo cancellation are not affected by the faxdetect setting, right? That is correct; the faxdetect setting and the echo canceller behavior are completely unrelated. 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call to the fax extension. If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes. Great. This is the typical usage, I presume, directing fax machines to FFA, Hylafax, another fax machine, or hangup (if this isn't a fax line). Is there a time limit to when DAHDI listens for faxes (say the first 10 seconds of a call?), or might it detect one in the middle of a ten minute call? I haven't double-checked, but I believe the software DSP will be in place on the call until it sees a CNG tone, regardless of when that happens during the call. 3.) faxdetect=outgoing will ?? The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used. [snip] I figured that must be it. Presumedly you might use this to perform some activity on an outgoing fax prior to sending it, such as logging something, etc? Maybe send it to FFA, receive it, and e-mail it to another server that faxes it out on a local number to save toll calls, etc? Thanks for the clarification, there's a lot of conflicting info out there. Feel free to comment on wiki.asterisk.org if any of the information there led you astray; we'd like to get that to be the most accurate place for people to find this sort of information. -- Kevin P. Fleming Digium, Inc. | Director of Software Technologies 445 Jan Davis Drive NW - Huntsville, AL 35806 - USA skype: kpfleming | jabber: kflem...@digium.com Check us out at www.digium.com www.asterisk.org -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/04/2011 8:55 AM, Kevin P. Fleming wrote: On 01/03/2011 06:47 PM, Thomas Rymes wrote: On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: [snip] OK. Either way, though, the changes to echo cancellation are not affected by the faxdetect setting, right? That is correct; the faxdetect setting and the echo canceller behavior are completely unrelated. Excellent. [snip] Is there a time limit to when DAHDI listens for faxes (say the first 10 seconds of a call?), or might it detect one in the middle of a ten minute call? I haven't double-checked, but I believe the software DSP will be in place on the call until it sees a CNG tone, regardless of when that happens during the call. Wouldn't it make sense to be able to specify a time period after which chan_dahdi disables fax detection? Only calls that begin with a voice call and end with a fax would benefit from detection after the initial ~8 seconds of a call, unless I am overlooking something. If the DSP keeps listening and detects a spurious fax tone (I know I have seen the human voice incorrectly identified as CNG), it will send the call off to the fax extension if one exists in the same context. In fact, we ran into some issues with exactly that happening. [snip] Thanks for the clarification, there's a lot of conflicting info out there. Feel free to comment on wiki.asterisk.org if any of the information there led you astray; we'd like to get that to be the most accurate place for people to find this sort of information. I'll give it a look. I had not specifically looked at the asterisk wiki, but Google searches brought up lots of messages confusing the fax operation of the echo canceler with the faxdetect= setting for DAHDI/Zaptel. Thanks again, Tom -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/05/2011 02:39 AM, Tom Rymes wrote: On 01/04/2011 8:55 AM, Kevin P. Fleming wrote: On 01/03/2011 06:47 PM, Thomas Rymes wrote: On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: [snip] OK. Either way, though, the changes to echo cancellation are not affected by the faxdetect setting, right? That is correct; the faxdetect setting and the echo canceller behavior are completely unrelated. Excellent. [snip] Is there a time limit to when DAHDI listens for faxes (say the first 10 seconds of a call?), or might it detect one in the middle of a ten minute call? I haven't double-checked, but I believe the software DSP will be in place on the call until it sees a CNG tone, regardless of when that happens during the call. Wouldn't it make sense to be able to specify a time period after which chan_dahdi disables fax detection? Only calls that begin with a voice call and end with a fax would benefit from detection after the initial ~8 seconds of a call, unless I am overlooking something. If the DSP keeps listening and detects a spurious fax tone (I know I have seen the human voice incorrectly identified as CNG), it will send the call off to the fax extension if one exists in the same context. In fact, we ran into some issues with exactly that happening. It is very normal for many people to chat and then start their FAX machines, especially domestic FAX users with a FAX machine attached to their home land line. If you don't care about those your proposal is OK, otherwise. There is no excuse for false detection of FAX tone. It takes a very poor detector to mistake voice for FAX, unless the person is specifically trying to whistle the right tones (which some people are quite good at). [snip] Thanks for the clarification, there's a lot of conflicting info out there. Feel free to comment on wiki.asterisk.org if any of the information there led you astray; we'd like to get that to be the most accurate place for people to find this sort of information. I'll give it a look. I had not specifically looked at the asterisk wiki, but Google searches brought up lots of messages confusing the fax operation of the echo canceler with the faxdetect= setting for DAHDI/Zaptel. Steve -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/04/2011 09:53 PM, Kevin P. Fleming wrote: On 01/03/2011 07:08 PM, Steve Underwood wrote: On 01/04/2011 04:22 AM, Kevin P. Fleming wrote: No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. That's not true. Modern echo cancellers normally disable completely. It is arguable whether they should disable completely for FAX, but they need to behave properly for all modems. For any duplex modem, disabling only the NLP is useless. They need to cancel end to end, so they don't get upset by a continuously adapting canceller, and so they can minimise the issues caused by the highly non-linear G.711 channel. This doesn't match up with what the manufacturers of the two G.168 ECs that Digium distributes have told me personally about their products. Their ECs behave differently for FAX and 'regular' modems, but they do that based on the detection of the V.21 preamble, ANSam and other signals in addition to CED, which seemed to be much more detail than was warranted in my response to the OP :-) Well, that makes a bit more sense, but I am very skeptical about this. The Octasic canceller is highly problematic with various modems and tones, so they aren't exactly a reference model for how to do things. Reports I here of the other canceller are much more positive. Its obvious why they want to keep the canceller alive. Long echoes over VoIP channels, combined with slow responding FAX boxes, can lead to a FAX machine hearing its own output heavily delayed, and it may mistake this for the response from the far end. T.38 largely avoids this kind of issue. The start of a FAX call doesn't really have a good signal on which to train a canceller. They can use the first V.21 burst in each direction (The FAX signals for G3 or the V.8 exchange for Super G3), and then lock down the canceller, but those signals aren't wide band enough to be ideal. The canceller could adapt very oddly. If they continue adapting once the wideband signals from the fast modems start, they are likely to upset modem operation there. If they just accept that, and rely on the fast modem retrying, it will usually step down in speed. I believe I have seen this behaviour in setups where the signal looks very clean, but the FAXes always exchange at 12000bps. Steve -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On Jan 4, 2011, at 7:37 PM, Steve Underwood wrote: It is very normal for many people to chat and then start their FAX machines, especially domestic FAX users with a FAX machine attached to their home land line. If you don't care about those your proposal is OK, otherwise. Well, I was suggesting an OPTION to disable the detection after a specified period, so anyone who wishes to do as you describe could just leave the setting to indefinite if they wished. Having said that, I don't think that sort of behavior is particularly common anymore. More importantly, I would argue that practically all users doing that are, as you mention, using a phone connected to a fax machine, eliminating the usual needs for sending the call to the fax extension. Either way, if defined as an option, users could choose how they wanted it to work. There is no excuse for false detection of FAX tone. It takes a very poor detector to mistake voice for FAX, unless the person is specifically trying to whistle the right tones (which some people are quite good at). While that might be true, false detections are anything but unheard of, so being able to disable detection after the first few moments of a call might be useful. Tom -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/03/2011 11:26 AM, Tom Rymes wrote: Hi folks, I was hoping that someone might be able to help clarify some confusion I have on DAHDI Fax detection after spending some time searching. My understanding is this: I'll try. 1.) Echo cancellation is automatically disabled upon recognition of a CNG tone, regardless of the faxdetect setting. This can only be disabled at compile time. No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call to the fax extension. If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes. 3.) faxdetect=outgoing will ?? The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used. Also, do Digium cards with HW Echo Cancellation detect the CNG tones in hardware? If so, how does the faxdetect setting in DAHDI affect that behavior? No, none of the Digium HW ECs detect and report CNG tones via the DSP; CNG tone detection is still done on the host CPU. 'faxdetect' is not set in DAHDI, it's set in chan_dahdi.conf. -- Kevin P. Fleming Digium, Inc. | Director of Software Technologies 445 Jan Davis Drive NW - Huntsville, AL 35806 - USA skype: kpfleming | jabber: kflem...@digium.com Check us out at www.digium.com www.asterisk.org -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On Jan 3, 2011, at 3:22 PM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: [snip] 1.) Echo cancellation is automatically disabled upon recognition of a CNG tone, regardless of the faxdetect setting. This can only be disabled at compile time. No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. OK. Either way, though, the changes to echo cancellation are not affected by the faxdetect setting, right? 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call to the fax extension. If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes. Great. This is the typical usage, I presume, directing fax machines to FFA, Hylafax, another fax machine, or hangup (if this isn't a fax line). Is there a time limit to when DAHDI listens for faxes (say the first 10 seconds of a call?), or might it detect one in the middle of a ten minute call? 3.) faxdetect=outgoing will ?? The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used. [snip] I figured that must be it. Presumedly you might use this to perform some activity on an outgoing fax prior to sending it, such as logging something, etc? Maybe send it to FFA, receive it, and e-mail it to another server that faxes it out on a local number to save toll calls, etc? Thanks for the clarification, there's a lot of conflicting info out there. Tom -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
Re: [asterisk-users] Clarification on DAHDI Fax Detection
On 01/04/2011 04:22 AM, Kevin P. Fleming wrote: On 01/03/2011 11:26 AM, Tom Rymes wrote: Hi folks, I was hoping that someone might be able to help clarify some confusion I have on DAHDI Fax detection after spending some time searching. My understanding is this: I'll try. 1.) Echo cancellation is automatically disabled upon recognition of a CNG tone, regardless of the faxdetect setting. This can only be disabled at compile time. No. CNG tone is never used to affect the state of an echo canceller. All G.168 compliant echo cancellers will respond to the CED tone (generated by the answering endpoint) and will reconfigure the echo canceller appropriately. Most modern ECs will *not* be disabled, but will enter a 'linear' mode where they can do some echo suppression but not complete cancellation. DAHDI will detect CED when most software echo cancellers are in use and will disable them (since none of the available software ECs can go into linear mode). The Digium HPEC software EC will detect CED on its own and enter linear mode. That's not true. Modern echo cancellers normally disable completely. It is arguable whether they should disable completely for FAX, but they need to behave properly for all modems. For any duplex modem, disabling only the NLP is useless. They need to cancel end to end, so they don't get upset by a continuously adapting canceller, and so they can minimise the issues caused by the highly non-linear G.711 channel. 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call to the fax extension. If the CNG tone arrives from the network side of the DAHDI channel (the far endpoint), then yes. 3.) faxdetect=outgoing will ?? The same thing, but if the CNG tone is being sent towards the DAHDI channel (from the near endpoint). This is rarely used. Also, do Digium cards with HW Echo Cancellation detect the CNG tones in hardware? If so, how does the faxdetect setting in DAHDI affect that behavior? No, none of the Digium HW ECs detect and report CNG tones via the DSP; CNG tone detection is still done on the host CPU. 'faxdetect' is not set in DAHDI, it's set in chan_dahdi.conf. Steve -- _ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- New to Asterisk? Join us for a live introductory webinar every Thurs: http://www.asterisk.org/hello asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
