[asterisk-users] Fax for Asterisk success rates?

[sean darcy]

On 10/09/2012 07:40 AM, Steve Underwood wrote:
> On 10/09/2012 12:28 AM, Brett Lehrer wrote:
>>> How many fax and voice calls (which codecs for tha latter ones ?) are on
>>> average using your DSL line ?
>>> 1. Previously, I experienced failures during the process of converting
>>> incoming PDF documents into ready-to-send fax image files while the
>>> reverse
>>> process (from a fax file into a PDF or whatever document) never failed.
>>> I would be curious to check if a greater failure rate for outbound
>>> faxing
>>> (greater than inbound faxing failure rate) could simply comes from image
>>> processing, before any transmission.
>>> 2. Though your DSL line may have enough bandwidth from your location
>>> to its
>>> DSLAM, chances are packets are dropped or delivered too late for T.38
>>> faxing.
>>> An interesting test would be to use an Asterisk PBX hosted somewhere at
>>> "close range" from netVortex fax gateways : that would remove most
>>> networking issues out of the equation.
>> I'll have to look more closely into what codecs we traditionally use,
>> but g.722 up and ulaw down is common.  Generally don't have more than
>> 2-3 calls active at once.  At most, 5, and that's a rarity.  Record
>> for fax is 4 simultaneous send/receive, but typically just 1, maybe
>> 2.  I imagine that's encroaching on the upper limits of the 768 kbps
>> upspeed.  I've wondered about how lag might impact the problem but I
>> just don't know how I'd go about testing it properly without spending
>> a bunch of money on hosting.
>>
>> I do my PDF -> TIFF conversion on another machine with ghostscript.
>> Here's the line:
>>
>> gs -q -dNOPAUSE -dBATCH -dSAFER -sDEVICE=tiffg4
>> -sOutputFile=<TIFF_FILENAME> -f <PDF_FILENAME>
>>
>> I changed from tiffg3 to tiffg4 because the filesize got cut in half
>> assuming that the less time spent transmitting, the less chance there
>> was to run into a problem that might stop the fax.  However, most
>> failures that I've looked at seem to occur immediately or fail to
>> connect at all, rather than get cut off due to a hiccup in the
>> connection.
>>
>> Brett Lehrer
>>
> A FAX can only be sent in ECM mode when using tiffg4 format. It will
> have to be recoded into tiffg3 format if ECM is inhibited, which it far
> too often is. On the other hand, if you are using ECM any decent FAX
> system (e.g. spandsp) will recode into tiffg4, and really good ones
> (e.g. the very latest spandsp) may recoed into T.85/JBIG, for faster
> transmission times. Digium don't seem to specify what FFA does in this
> area.
>
> Steve
>

A little puzzled. Do you mean:

1. tiffg4 encoded fax will(might?) fail if ECM is inhibited at either 
send or receive.

2. tiffg3 will work if ECM is inhibited.

3. If ECM is not inhibited, any decent fax system, will reencode tiffg3 
to tiffg4.

Therefore we should encode to tiffg3 and let spandsp determine if it 
should be rencoded to tiffg4 (or T.85/JBIG)?

sean