[SIPForum-discussion] udptl as Media Protocol in SDP

[HM Kias]

Faxes can be transported using ulaw or another noncompressed codec, although
delays and latency can cause problems and failures. Sending faxes directly
over IP networks can be done using the T.38 fax over IP. The MIME type
image/t38 has been defined for the transport of faxes over UDP, using the
UDP Transport Layer (UDPTL). While this approach works, it is better to
transport faxes over RTP. This allows faxes to utilize various RTP
extensions including security, redundancy, and so fourth. Faxes over RTP use
the audio/t38 MIME type.


On Tue, Aug 16, 2011 at 2:45 PM, Mohit Soni <Mohit.Soni at matrixcomsec.com>wrote:

>  Hi friends,****
>
> ** **
>
> I have encountered a method of performing FAX over IP during testing my UA
> with a service provider’s gateway.****
>
> ** **
>
> When I had tried to send FAX from my UA, the service provider gateway sends
> re-INVITE on my UA by setting Media Protocol as “udptl” when Media Type is
> “audio” in SDP body (shown with bold in following trace).****
>
> ** **
>
> ** **
>
> v=0****
>
> o=CiscoSystemsSIP-GW-UserAgent 8963 636 IN IP4 10.8.255.217****
>
> s=SIP Call****
>
> c=IN IP4 10.8.255.217****
>
> t=0 0****
>
> m=audio 19176 *udptl* 0 97****
>
> c=IN IP4 10.8.255.217****
>
> a=rtpmap:0 PCMU/8000****
>
> a=rtpmap:97 telephone-event/8000****
>
> a=fmtp:97 0-16****
>
> a=ptime:20****
>
> ** **
>
> This cause my UA to send 488 as the Media Protocol udptl is not supported
> by my UA.****
>
> ** **
>
> Normally when the Media Type is audio, the Media Protocol is set as RTP/AVP
> but here, it is udptl.****
>
> ** **
>
> Can any one tell me that how this method of FAX works and what kind of
> method is this, i.e., passthrough, T.38 UDPTL or T.38 RTP?****
>
> If there any RFC or draft or specification for performing FAX over IP in
> such a way then please tell me.****
>
> ** **
>
> Thanks,****
>
> Mohit Soni****
>
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-- 
HM Kias
91-9443467600
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