[SIPForum-discussion] Question About media attribute within SDP

Wed Aug 5 15:01:56 UTC 2009

Chitta,

Thanks for looking into this. The issue is related to the SIP peer signaling. I worked with them and issue is resolved now.

RS

From: Chittaranjan Panda [mailto:cpanda at eng.transcomus.com]
Sent: Wednesday, August 05, 2009 5:40 AM
To: Shakil, Rashid
Cc: 'Garron, James'; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Question About media attribute within SDP

Hi Rashid,

Do you have the tshark trace for this fax failure? I just want to know is it failing at signaling side or media side. If you have the RTP trace then it will be helpfull for further troubleshooting.

Below is the details for your queries.

CN means Comfort Noise.The use of this when both party is in silence mode some RTP stream should flow between them. This kind of Background noise which acts as a Keep alive message.
Normally for CN 13 is used as static payload type and the default packetisation period is 8000. You can map other payload type for this CN.
In your case, 19 payload type is mapped with CN. Hopefully this is not an issue with your fax failure.

Thanks,
-Chitta

Shakil, Rashid wrote:

Thanks for the detailed reply. Your analysis is correct even though I removed the rtpmap from SDP my fax calls are still failing.

From: Garron, James [mailto:jgarron at sonusnet.com]
Sent: Monday, July 27, 2009 10:11 AM
To: Shakil, Rashid; discussion at sipforum.org<mailto:discussion at sipforum.org>
Subject: RE: [SIPForum-discussion] Question About media attribute within SDP

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> [mailto:discussion-bounces at sipforum.org] On Behalf Of Shakil, Rashid
Sent: Friday, July 24, 2009 10:21 AM
To: discussion at sipforum.org<mailto: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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