ip phones and rtp flow scenarios

[Mohammad Ramadan A.Hafiez]

Hello Bro,

            I have a question simply about when two end points calling each other, how the rtp stream goes in these scenarios:

- we have always learned that when two ip phones say 8861 calling each other, when it commes to the rtp streams between the two phones a two unidirectional sessions are to be created where they can send and receive rtp streams "including the ip phones IPs only" ? Right ? "every ip phone send using network port and receiving using anorther port"

- that supposed to be when they both using the same default g711 codec. what if they are using two different codecs? any change with the stream flow?

- what if these phones are one in the lan side say 8861 and the other phone lets say jabber client in the VPN zone? how is the stream termination in both cases using default g711 codec or differenct codec?

- Regarding the MRA "expressway" I know the streaming goes between  each party and expressway-c "from lan phone to exp-c and from exp-c to jabber vpn client.

Kindly correct me if anything wrong and clarify things. thank you so much Bro,  

[Roger Kallberg]

If the endpoints uses the same codec the RTP stream will be directly between the endpoints regardless if they are on different networks. If the endpoints cannot negotiate a common codec then a transcoder will be invoked in the call, so the RTP stream from each endpoint will go to the xcoder and then to the other endpoint.

[Maren Mahoney]

To add to what @Roger Kallberg said, when Expressway is involved the default behavior is that the RTP stream flows through the Expressway-C. For one internal and one external device, the access to the internal environment for the external device is always the Expressway-C for both signaling and RTP.

RTP flowing through the Expressway-C is true even if both devices are external, where the RTP hairpins through the Expressway-C by default. This can be mitigated by invoking TURN on the Expressway-E or by configuring the environment to use ICE.

Maren (Who is a Sis and not a Bro)

[Mohammad Ramadan A.Hafiez]

thank you @Roger Kallberg thank you @Maren Mahoney so much, was helpful.