Role of pbx

Hi everyone!

I am new to asterisk and have a confusion about the role of a pbx in a voip setup. What I understand is

  • A traditional PBX (PSTN) performs call routing and is itself involved in the call. i.e the pbx remains in the ‘circuit’ for entire call duration.

  • An IP pbx also routes the calls but isinvolved only in call setup. Once the call is established, the RTP/voice packets bypass the pbx and the two users communicate directly.

Is my notion true?

If this is the case, then asterisk pbx should have nothing to do with the real time protocol (RTP). But the asterisk source code contains the file ‘rtp.c’ which ‘supports RTP and RCTP’. What is its purpose ?

Can anyone clarify this?

Asterisk can support both cases.

Normally asterisk remains in the media stream. This allows asterisk to pick out tones and such and provide features.

Asterisk can also be configured to allow re-invites and in this case the two end points talk directly to each other. Asterisk is out of the media path.


Basically ASTERISK works like a bridge bet. 2 parties. It get the INVITE and the other party ACK bla bla bla and then after session creation it goes out. So need RTP support. otherwise u can also get connected your dialler with asterisk and also need RTP. thats why asterisk has the support.
if you have any other confusion, let me know.
no problem