[asterisk-users] Zap channel faxing in or out fails but phone calls work.

[Jerry Jones]

I would agree with you on just about everything.

Except the op had his fax connected via channel bank directly to *  
and a pri on the other port - ie no packets involved here.

However - all faxing does involve the transfer of frames from one fax  
to the other and that is was ecm handles. But yes even with  
traditional tdm circuits in between and no voip you still get frame  
losses. Which should be minimal and ecm should be able to compensate  
for.

However, in the real world (tm), most of my customers are very  
sensitive to how long does it take to send a page vs are there any  
slight imperfections on a page. And yes we have found that MOST  
issues involve poor fax machines. Why customers whose 'business  
depends on these 100s of faxes daily' choose to buy $100 faxes and  
place on a single line is beyond my comprehension.


> Everything that you read on a wiki must be considered potentially  
> bogus or otherwise misinformed.
>
> Let me rewrite that paragraph for you...
>
> ------------------------------------------------
> ECM - error correction mode
>
> Good fax machines with proper memory and programming are able to  
> use Error Correction Mode (ECM) for error-free image transmission.  
> When ECM is used, a fax page is transmitted as a series of blocks,  
> each block consisting of a series of HDLC data frames. After  
> receiving the data for a complete block, a receiving fax machine  
> notifies the transmitting fax machine of any frames with errors.  
> The transmitting fax machine then may retransmit the specified  
> frames. This process may be repeated until all frames are received  
> without errors, and then the procedure may continue on to the next  
> block. If, for some reason, the receiving fax machine is unable to  
> receive an error-free block, the fax sender may abort and  
> disconnect, thus leaving the receiver with a partial or truncated  
> image.
> ------------------------------------------------
>
> The sentence that begins, "On networks that have a packet loss  
> rate..." is simply nonsense, first-off, and what truth it is  
> actually hinting at is as relevant to non-ECM faxing as it is to  
> ECM faxing.
>
> The intent of that sentence is probably directed towards the  
> situation where you'd have a fax machine plugged into an ATA that  
> is communicating to your LAN-hosted PBX that has PSTN connections.   
> So let me talk about that situation...
>
> In order to experience packet-loss or some other kind of jitter on  
> a LAN you either have problems with the LAN or your LAN bandwidth  
> is seriously stretched.  VoIP packets running on a LAN are *far*  
> more reliable than VoIP packets running over the internet.  So if  
> you truly are experiencing 2% packet loss on a LAN between the ATA  
> and the PBX, then there are network issues that need to be  
> resolved.  In truth, most of the issues involved with using a fax  
> machine on an ATA are not going to be network-related... but  
> instead they're going to deal with issues in the ATA and in the PBX  
> themselves when handling audio like fax that reliably cannot be  
> corrupted (jitter buffers, echo cancellation, proper function, etc.).
>
> The typical "packet loss" issues that Asterisk users see with fax  
> are not network related, but rather deal with issues in the zaptel  
> hardware or driver.  This has nothing to do with whether or not ECM  
> is being used.  These issues generally cause premature carrier loss  
> detection to occur by the fax receiver (meaning that the missing  
> audio became silence somewhere along the audio path).  Fax protocol  
> uses carrier loss as a way to indicate end-of-signal or end-of- 
> message.  If carrier loss is detected prematurely then the  
> connectivity between endpoints is put at risk and recovery largly  
> will depend on the receiver's tolerance for that situation.
>
> In fact, in these kinds of situations using ECM on a well-tolerant  
> receiver may actually prove to be the only way that reliable fax  
> reception can occur.  Whether or not that is the case depends upon  
> the timings of the "packet loss" the effect of that packet loss on  
> the receiver, and the data format used for the image.  By disabling  
> ECM you limit data format types to MH and MR - which are both image  
> format types that are generally rather tolerant of data  
> corruption.  So if the "packet loss" translates into image data  
> corruption and not premature carrier loss and if the amount of data  
> corruption is negligible to the human eye, then it may appear that  
> disabling ECM will help.
>
> The chances that enabling ECM on an ATA-connected receiver will  
> cause fax failures are pretty slim, in my expectation... and if it  
> actually did, then I would suspect more fault lies on the ECM  
> implementation on the receiver than in the nature of the ECM  
> protocol itself.
>
> Lee.
>
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