Re: Implementing app DIAL using ARI

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Jan Svoboda wrote:

I am currently trying to implement asterisk applications using ARI, and
came across few problems trying to recreate the DIAL app.

My current approach is the following:

1. When StasisStart event is received, determine which endpoints to dial
and create new object, representing the DIAL application in my code,
passing it the endpoints to call and the originating channel.

2. Create destination channels from all passed endpoints, listening to
various events.
- if at least one destination channel is created, indicate ring to
originating channel

3. When one of the destination channels answers:
- hangup all other destination channels
- bridge the channel which answered with the originating one

This works and calls can be completed, but the problem is when dialled
destination channel indicates progress and starts early audio. This can
be for example in case of a call to mobile phone, which is switched off.
In this case, there is early audio saying “The called number is
unavailable at the moment.”.

As there is no other indication apart from the audio message, this leads
to the caller hearing only the ringing indication and then the call is
hung up, which makes the misleading impression, that the mobile phone is
switched on, but does not answer the call.

I have checked the source code of the DIAL app in app_dial.c, and
noticed, that this is handled specially when there is only one
destination channel, and that the way it is handled is with
ast_channel_early_bridge(in, c). Using the DIAL app, this works, and I
can hear the early audio.

Not really. What actually happens is that since Dial has control of all channels involved it can create a primitive bridge inside of itself, allowing media to be exchanged back and forth even in an early state. Once answer occurs the normal core bridging is invoked.

Bridging the channel early in the app does not seem to be an option,
since in this case, the channel does not even enter stasis.

Is there any workaround or recommended way to achieve this ? Or is it
something that yet has to be added to ARI ?

This hasn't been added to ARI yet and thus is not achievable. Until the channel is answered you don't have enough control. Ideally what would happen is that an early bridge technology would exist and you could then throw them into a bridge. The early bridge would do the stuff required in an unanswered state and then once answered a smart bridge operation would occur and the bridge would transition to a normal simple bridge or a native RTP bridge. Heck, it would probably even work in early state in normal bridges.

Another problem is that it seems there is no way to indicate and play
early audio to the caller. Any attempt to do so seems to answer the
originating channel and generates ChannelStateChange event with state Up.

This shouldn't happen. Behavior was changed months ago so stuff should not explicitly answer. What operation were you doing?

There is also a problem when playing music on hold to caller. The log
indicates "Started music on hold”, but there is no audio. This also
seems to be related to progress not being indicated (and lack of a way
to do so), because trying it with the PROGRESS app called from the
dialplan before Stasis works.

Can you file an issue for this? The MOH code should be indicating progress.


Joshua Colp
Digium, Inc. | Senior Software Developer
445 Jan Davis Drive NW - Huntsville, AL 35806 - US
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