[SIPForum-discussion] Question About media attribute within SDP

[Garron, James]

Rashid, assuming that the devices in the flow follow
draft-ietf-sipping-realtimefax-00/01 then I do not think that this
rtpmap should be in issue.  Upon fax detection there should be a
re-INVITE to pass-through mode (or T.38) that would remove this.

 

4.4 Internet telephony gateways and fax pass-through mode 

    

   This section deals real-time facsimile communications over IP using 

   fax pass-through instead of T.38 fax relay.  In this mode, the 

   facsimile communication is handled as a PCM audio call (PCMA/PCMU as 

   specified in ITU-T recommendation G.711).  

    

   The fax pass-through mode is important to prevent call failures, for 

   example in cases when one of the SIP communication peers does not 

   support T.38.  For Internet telephony gateways with support for 

   PCM/G.711 audio but no support for T.38 real-time fax, it is 

   recommended to switch the session to fax pass-through mode.  

   Internet telephony gateways SHOULD handle the fall back mode to fax 

   pass-through by recognizing the SDP T.38 connection and proposing to 

   switch to a new audio connection.  The new audio connection SHOULD 

   have the following characteristics: at a minimum, specify PCM G.711 

   codec, silence suppression OFF. The telephony gateway that 

   originated the failed T.38 re-INVITE SHOULD initiate the subsequent 

   re-INVITE to fax pass-through mode.  

   Refer to section 6.2 for a sample call flow illustrating this 

   scenario.

 

________________________________

From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Shakil, Rashid
Sent: Friday, July 24, 2009 10:21 AM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Question About media attribute within SDP

 

Hello All,

 

For outbound calls the customer's Cisco IAD 2431 generating following
SDP. The SIP peer doesn't like the media parameter "a=rtpmap:19 CN/8000"
in following SDP.  Two quick questions,

 

1.       What "a=rtpmap:19 CN/8000" represent in this SDP

2.       How can it be removed

3.       Is it possible that it cause fax failures ?

 

 

v=0

o=- 2847654225 2847654225 IN IP4 72.245.184.130

s=SIP Call

c=IN IP4 67.102.244.5

t=0 0

m=audio 17746 RTP/AVP 0 101 19

c=IN IP4 67.102.244.5

a=rtpmap:0 PCMU/8000

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-16

a=rtpmap:19 CN/8000

a=ptime:20

 

Thanks in advance for the input

 

Regards,

 

Rashid Shakil

 

 

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