[asterisk-users] Fax detection ...

[Steve Underwood]

Jay R. Ashworth wrote:

>On Mon, Oct 02, 2006 at 10:43:44AM +0800, Steve Underwood wrote:
>  
>
>>Jay R. Ashworth wrote:
>>    
>>
>>>On Sun, Oct 01, 2006 at 02:58:37PM -0700, Lee Howard wrote:
>>>      
>>>
>>>>Well, fax detection isn't entirely reliable anyway.  Even if you assume 
>>>>that your fax detection feature and operation is flawless in properly 
>>>>detecting fax tones (and that most likely would be a specious 
>>>>assumption), not all calling fax machines send fax tones.
>>>>        
>>>>
>>>So, y'know, that assertion gets made a lot.
>>>
>>>What's the turn rate of fax machines in the market? 3 years?  5?  CNG
>>>tones are *well* over 10 years old, no?
>>>      
>>>
>>What relevance does that have to CNG? It was a feature of the original 
>>spec 30 years ago.
>>    
>>
>
>Well, perhaps I wasn't paying attention, but I thought that CNG tones
>*had as their purpose* making receive FAX detection trivial.  That
>would tend to make the question on-point, would it not?
>  
>
It is the age of the machines which has no relevance.

>>>What percentage of fax calls are sent without CNG tones these days?
>>>      
>>>
>>Quite a lot. A large number of FAX machines have CNG turned off. On many 
>>machines, if select features like sharing a line between FAX and 
>>answering machine CNG, CED and various other useful behaviour might be 
>>disabled.
>>    
>>
>
>My personal experience is that I've never seen a consumer-grade fax
>machine with send-CNG turned off, and I don't *think* I've ever seen
>one on which there was a knob *to* turn it off; I would be less sure
>about fax modems -- those may have a knob, but I would expect it to
>default on.
>
>Could you expand on what behaviour you think CNG breaks?  Cause I'm not
>modeling it, mentally...
>  
>
I guess you touched many consumer grade fax machines, since *most* above 
the very basic ones can do this in some way.

Some have really fun behaviour. I used to suffer some Olivetti ones that 
had several modes of calling and answering - Delay a while, to give 
someone a chance to pick up first; Pick up, but remain silent and see 
what the machine can hear; etc. Those Olivettis would only properly send 
a fax to another Olivetti when they were in straight forward standard 
mode. Now there's compatibility for you. :-) A lot of other machines 
offer similarly dumb modes of behaviour, but nothing quite so extreme as 
those. When you investigate one of these issues, and ask the user why 
the machine is not in simple answering mode, they are usually unaware it 
is not. These modes get set largely at random on installed fax machines.

Now, it seems like these special modes should only affect answering. It 
would seem they are mostly about doing what Asterisk is doing - waiting 
silently for the 1100Hz tone. However, that's just too clean and simple 
for the fax industry. They do a bunch of other dumb stuff to make things 
more awkward, like call and only send something when they here 2100Hz.

Steve


Steve