[VoiceOps] T38 over Sip

Alex Balashov abalashov at evaristesys.com
Fri Aug 28 09:25:10 EDT 2009

Junaid Hattia wrote:

>  if a single page is faxed it works!....if multiple pages are sent, only one
> and a half pages of say three pages are received

Yep, well, that's generally where the other problems I alluded to crop 
up symptomatically.  This is a complex - and, even more importantly in 
the case of T.38 and anything fax-related, a multi-vendor - call path.

1) Where is the fax being sent from?

   a) Is it a Super G3 fax machine?

   b) Have you ruled out the possibility of interference and/or
      poor PSTN service quality on the sending side?

   c) Does wherever you're sending from consistently work fine
      if you send to a conventional fax machine on your side?

2) Have you verified that the switch to T.38/UDPTL actually takes place, 
with a packet capture?  Or does it stay G.711?

It may be that the data is being exchanged using G.711u analog 
pass-through, which is the mode in which things stay if T.38 cannot be 
negotiated between all the media endpoints, and/or if fax tones are not 
properly detected because the sender is SG3, and/or similar situations.

Analog pass-through will work, to some extent, but not over anything 
less than a very, very well-engineered IP network from a QoS standpoint. 
   Even then, with all the media traversal steps involved here, I don't 
know that the reliability issue can be overcome for analog pass-through.

T.30 is exponentially more sensitive to jitter and even the slightest 
bit of packet loss than voice;  a little QoS issue will cause a faint 
(and perhaps not audible) crackle in speech, but it'll cause bits to be 
flipped or missing in modulated digital data.  :-)  And fax was built 
for the PSTN and synchronous transport, not at all designed to be 
resilient to this packetisation business.

-- Alex

Alex Balashov - Principal
Evariste Systems
Web     : http://www.evaristesys.com/
Tel     : (+1) (678) 954-0670
Direct  : (+1) (678) 954-0671

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