[cisco-voip] SIP Load and Re-Invite

[Ryan Ratliff]

If an endpoint could do something about the fax tone then it should be expected to.  That means if you call a 7960 don't expect it to do anything with fax tones.  If you call to a fax machine on the other hand then something needs to be able to detect those tones and switch the call to fax.  I'm not a fax guy but I'm pretty sure in VOIP letting it go in the RTP and relying on the endpoint to do it is about the worst possible way of sending faxes.  With a SIP trunk use something like T.38 where you can send it as a cap in the SDP and use signaling to switch the call over. 

My comment about "in a way we can act on it" means that CUCM does not participate in the RTP stream so sending tones in-band isn't going to do anything unless an endpoint somewhere along the line can intercept them and take some required action.  That action doesn't even have to include telling CUCM (see protocol-based T.38 vs CA-controlled)  This could be a voice gateway with your fax machines attached to FXS ports, an ATA, or some other device. 

What it comes down to is what capabilities does the PSTN provider have and do the devices in your network share those capabilities.   I'm sure Nick or somebody with more fax expertise than I have can jump in too.

-Ryan

On Apr 15, 2011, at 2:57 PM, Mike Lydick wrote:

Thanks Ryan,

I believe we have design incompatibility based on your comment. We are using media flow-around so I guess that would remove the DSP out of the RTP stream so to speak unless we can dynamically allocate one after call setup. I agree the IP phone would not be the correct test so we are moving to a FXS port on our testing. I will removed the media flow-around to see if this helps but flow-around is a requirement as we will have 500 sites and do not intend of pinning all the RTP traffic to the Cube.

I am fussier on the comment about changing codec in a way we can act on it, do you mean a endpoint that can do something with the fax tone. 

Oh sorry for the typo, CNG not CMG...

Best Regards,

Mike Lydick




On Fri, Apr 15, 2011 at 2:49 PM, Ryan Ratliff <rratliff at cisco.com> wrote:
re what the point is in a 79XX phone responding to something like a fax tone since it couldn't do anything with said fax data anyway.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://puck.nether.net/pipermail/cisco-voip/attachments/20110415/e79508f4/attachment.html>