[iaxmodem-users] Robustness of V.17 (no ECM) over VoIP?

[Gert D. <ge...@gr...>]

Good morning,

I am trying to understand something - or, more precisely, I have a theory
how the world should work, but observation conflicts with the theory.  I'm 
sending this *here* because I assume that this is the last bastion of
"in-depth fax signal understanding"...

Theory first: 

  "14400 fax is more robust than 9600 fax over VoIP, because typical
   problems with VoIP (packet loss) should apply in a statistical manner,
   and since the 9600 bps call takes *longer*, statistics should make
   packet losses hit more often"  (without ECM).

Experiment 1:

  freeswitch/spandsp --(SIP, G.711) --> 3CX PBX --(SIP)--> Telco
    --> same Telco --(ISDN)--> old Ackermann PBX --(analog)--> ZyXEL 1496

  What I observe is that pages sent with 14400/v.17 have a significant 
  number of lines missing.  Like, 2380 lines sent, 1600 lines received(!).

  If I send with V.17 turned off, using 9600 bps, the received pages
  are fine.


Experiment 2: 

  same SIP and ISDN path, just using a USR I-Modem as receiver, also no ECM

  freeswitch/spandsp --(SIP, G.711) --> 3CX PBX --(SIP)--> Telco
    --> same Telco --(ISDN)--> USR I-Modem

  *most* pages are perfect with V.17/14400 (one page in a 11-page fax is 
  missing some lines).


I can't remember ever having such massive problems with the ZyXEL 1496
on the old PBX - usually the 1496s have been my workhorses, and always work.


So - after this introduction, now the questions :-)

 - should 9600 be more robust than 14400 over VoIP?
   - why (either way)?

 - what are the effects of VoIP mistreatment of "fax audio"?

   I can see "packet loss" leading to "missing lines" or "prematurely
   ending pages", and "wrong codec negotiated" leading to "fax is
   compressed to death, and not working at all anymore"


What I already know is "with ECM, things usually work" :-)  (even with
higher bit rates, though I can't use them with freeswitch/spandsp).

thanks,

gert

-- 
"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             ge...@gr...