[asterisk-users] fax / t38 gateway

[Steve Underwood]

Hi,

A lot of people talk about grooming to make VoIP work smoothly, not just 
for FAX. However, most people can only achieve grooming in one 
direction. Their ISP will not cooperate, and groom what is sent to the 
subscriber. Unless you just keep your DSL link very lightly loaded, by 
doing no browsing, file transfer, or anything else which can send sudden 
burst of data to you, grooming doesn't do a whole lot of good.

Jonn R Taylor wrote:
> This is caused by your DSL/cable modem buffering the data internally. To resolve this you need to feed the modem at a data rate that prevents it from buffering. I solved this by using a shaping bridge server that does nothing but control the data in and out of the internet connection. I have steady 21ms ping times to the server that my RTP traffic comes from.
>
> I do faxing over VOIP every day and it works! But it only works if you have a stable internet connection with an SLA, which I have. Also having an ISP that only uses a pure IP network makes a big difference. Some DSL providers use ATM to transport IP and this causes high packet loss when their network is busy.
>
> T.38 is a great idea but we need large scale implementation of it, with standards for it to work.
>
> Here is my hylafax log from last week. I removed the numbers for privacy reasons.
>
> Facsimile received since last week:
>
>        Pages     Time Pg/min Errs TypRate TypData
>           1     3:17    0.3    0   14400 2-D MMR
>          26    16:55    1.5    0    9600  1-D MH
>           2     0:46    2.6    0   14400 2-D MMR
>          13     5:13    2.5    0   14400 2-D MMR
>          14     5:48    2.4    0   14400 2-D MMR
>           5     2:33    2.0    0   14400 2-D MMR
>           1     0:28    2.1    0   14400 2-D MMR
>           1     0:31    1.9    0   14400 2-D MMR
> ------------------------------------------------------------------------------------------
> Total                                              63    35:31    1.8    
A lot of people who say faxing works actually achieve good results only 
through a lot of ECM retries. Your pages per minute figures, apart from 
the first one, look like they were probably for pretty clean transfers. 
It depends a lot on the ATA. ATAs which try to be more clever about 
voice quality tend to stop FAX working completely. Of course, they have 
some FAX related options in their menus, just to look more impressive. :-)

If you have a set up that works, stick with it. Don't expect to be able 
to repeat the result elsewhere.

Steve