[SIPForum-discussion] SIP Account

[Maciej Wasiel]

As someone who implements t.38 and fax-passthough (RTP) stacks for a living and tests them against MANY media gateways, I can assure you that RTP (passthrough) fax solutions are VERY inferior to t.38. Packets loss is only one impairment and in many cases it may not be the most significant. The problem stems from the fact that many media gateways treat RTP fax traffic like voice, which is a big no-no for fax. Silence suppression, jitter buffer underruns/overruns, and clock skew can be very damaging to modulated data whereas with voice these impairments can often go unnoticed. When you get into V.34 (SG3) fax, fax passthrough calls are even more likely to experience problems due to the non-packet loss impairments I mentioned above.

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From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Umang
Sent: Thursday, April 29, 2010 3:17 PM
To: Jason Chow
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] SIP Account

I used to work for a very large telecommunications company (ISP).  After months of research, testing, and deliberation, it was decided that Fax would be implemented in in our residential and business networks using just the regular RTP, without any T.38.  I believe T.38 can be used over UDP or TCP, but for our purposes, the extra processing did not matter, our VoIP pipes were big enough that we were fairly confident in having a very minimal, if any, amount of lost packets.

Also, the user agent we were developing had some short comings with responding to T.38 packets, but you might be in a different situation.  Hope this was of some help.

Umang Patel | Communications Research Engineer | Massachusetts Institute of Technology
On Thu, Apr 29, 2010 at 3:34 AM, Jason Chow <myfaxcn at gmail.com<mailto:myfaxcn at gmail.com>> wrote:
Dear Sir, I'm looking for the Service Providers support t38 fax, Can help me to recommend? Thanks!
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Umang
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