[SIPForum-discussion] SIP Faxing

Greg Settle gsettle at opentext.com
Tue Aug 9 16:52:47 UTC 2011

What is the topology?

SIP trunk - SBC/gateway - fax server?  If yes, who is the SIP trunk
provider?  Do they support T.38 FoIP?
T1/PRI - gateway - fax server?  If yes, which gateway(s) is involved?
Support for T.38?

Assuming a fax server is involved?  If yes, which fax server?

Generally, if T.38 FoIP is supported among all endpoints, and proper
testing was done to verify configuration among the components, SIP
faxing failure rates should remain low.  15% failure rate is high, and
points to weak links in the topology where FoIP standards (T.38) are not
being met.  Attached is a document which may assist.

-----Original Message-----
From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Steve Underwood
Sent: Monday, August 08, 2011 9:47 AM
To: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] SIP Faxing

On 08/04/2011 09:40 PM, Melissa Parsons wrote:
> Hello,
> Can anyone tell me what an acceptable failure rate would be when 
> faxing over SIP? Currently we are averaging around 15%. This number is

> high I would expect it to be somewhat higher than traditional faxing 
> but not this high.
> Thank you,
> *Melissa Parsons*|Enterprise Systems Engineer
> *MarineMax, Inc.* ( 727.531.1700 office | (727.524-3954 fax
> 18167 US Hwy. 19 N. Clearwater, Florida 33764
That's vague. Are you using A-law or u-law? Are you using G.726? Are you

using T.38? Pretty much anything else you might use will give 0 success 
rate, so I assume its one of those. The failure rate you get will depend

on many factors. A high packet loss rate is a disaster for FAX. High 
jitter levels for the packet delivery time can be too. T.38 
implementations are quite variable in their behaviour, and not always 
that compatible. Many quirks in the SIP signalling arrangements exist in

various implementations, too. At the end of the day you'll get somewhere

between 0 and 100% success, depending on all these factors. Between 
servers in large data centres, connected straight on to the internet's 
backbone you might be able to send FAXes all week without an error. In 
really bad tributaries of the internet you might get a near 100% failure


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