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
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.
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 > >
> 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.Digium TDM 4 fxs is not really a good choice for a faxing system. I've tested it for a while. You should read old messages here about it. I'm using a linksys pap2-na now, it is working well and it does cost less than a digium tdm. Sipura SPA-2000 is also working for a fax system
>> 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. > > Digium TDM 4 fxs is not really a good choice for a faxing system. I've > tested it for a while. > You should read old messages here about it.Faxes coming in from PSTN DID's going directly to a TDM card is a very good and reliable solution. In this setup the fax call can be directly bridged from the PSTN to your fax machine with no codecs or lossy compression, etc. This is exactly the solution I've been using for two production fax setups, and they have worked under heavy usage without any issues since I put them in (about 3 months).