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Re: [Linphone-developers] Using oRTP to send supplemental data besides t

From: Steve Strobel
Subject: Re: [Linphone-developers] Using oRTP to send supplemental data besides the audio
Date: Fri, 19 Dec 2008 09:00:08 -0700

At 04:40 PM 12/18/2008, Vadim Lebedev <address@hidden> wrote:

FMTP is not part of RTP actually.
It is simply an attribute in SDP  packet describing payload-specific
parameters and options which are acceptabel for a give endpoint.

I don't know anything about SDP, but would guess from what you wrote that it is used specifically for establishing a connection and wouldn't be appropriate for passing other data.

BTW how do you establish a session between two endpoints?
If you use SIP, the best approach to send application specific data is
to use SIP MESSAGE, INFO or OPTIONS requests

I am not (yet, at least) using SIP. I took the oRTP send/receive examples and modified them to work with audio fifos that I already had. The VoIP features are only a piece of a larger system used for connecting disparate radio systems together. The code to establish a session just sets the local IP port number and remote IP address/port, plus a few RTP stack settings (scheduling options, etc). I quit streaming RTP data each time a radio unkeys, and detect that loss of data at the other end to signal the corresponding radio there to quit transmitting.

Using SIP at least when I need to pass the extra data along with the audio might be a good option. There are a few things that might cause new complications:

Handling PTT. I need to have a way to signal when a radio keys and unkeys. If SIP already has logic for handling that (such as might be needed for PTT cell phones), this might be easy. Otherwise, I could place and hang up a call each time a radio keys and unkeys. I need to be able to pass data even when no one is talking; I suppose SIP already does that for sending presence information.

Is there a reasonably easy way to use the SIP stack from Linphone as a library? Since the rest of my system is based on using fifos to pass audio streams, I don't think I can just run Linphone directly; it would need to use ALSA or something to handle the audio, and once started, there wouldn't be an easy way to get the extra data I need to pass to and from the Linphone executable. I suppose I could hack Linphone itself to change the audio I/O and adding the data passing stuff.

Thanks for your help thinking through the options,


Steve Strobel
Link Communications, Inc.
1035 Cerise Rd
Billings, MT 59101-7378
(406) 245-5002 ext 102
(406) 245-4889 (fax)

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