SIP/RTP with SBC

[cct]

Hi Guys,

Totally new to SIP so go easy.

Can someone give me a basis 101 of how the SIP traffic works?

I've got a cloud hosted 3CX with an SBC onsite.

Does the RTP traffic go through SBC to the 3CX or does 3CX set up the call and then the handset sends and receives the RTP directly to/from provider?

thanks

 

[JohnS_3CX]

Hello,

Depends on a few things, but generally speaking your external calls will route via the SBC to the PBX and for your internal calls the RTP will remain local between the endpoints.

The audio does not go to the provider directly from your phones though, only the PBX is connected with the provider, and your phones reach the PBX only via your SBC.

 

[Marari]

Hi Guys,

Totally new to SIP so go easy.

Can someone give me a basis 101 of how the SIP traffic works?

I've got a cloud hosted 3CX with an SBC onsite.

Does the RTP traffic go through SBC to the 3CX or does 3CX set up the call and then the handset sends and receives the RTP directly to/from provider?

thanks

To answer your question about "What is SIP?" and how it works ...
https://www.softwareadvice.com/resources/what-is-sip/

Also - look at this:

https://www.3cx.com/blog/voip-howto/sip-overview/

Does that answer your questions?

 

[cct]

Thanks for that, ive had a read and watched the video.

Im more curious on how the RTP traffic works.

I know internally the RTP is endpoint to endpoint but how does it travel when it's an external call?

Assuming 3CX Ext is set to PBX Delivers Audio Off.

When the call is set up does the RTP go back and forth via the voip provider?

Im just a bit confused on where RTP flows.

for example, i've got VOIP Provider, 3CX Cloud, EXT behind an SBC.

I can see RTP stream from VOIP Prov to 3CX Cloud then to EXT

whats the difference in flow if PBX delivers audio?

 

[JohnS_3CX]

So on the extension level, "PBX Delivers audio" applies for internal calls

Now when you initiate an outgoing call though, you are doing something different (very simplified here):

Ext: Hey PBX, i wish to contact 123456789

PBX: Ok I will Try

PBX: Hey Provider, I wish to contact 123456789

Prov: Hi PBX, Ok I will try, its ringing, and you are now connected, I will send the audio to you from him

PBX: Hey Ext, I will forward audio for you from the provider, who is now talking with 123456789

Your phone, and the other phone, do not speak directly between them and rely on the provider and PBX sort of doing the work for you in the middle in a very simplified sense.

We recommend leaving on PBX Delivers audio (on Trunk Settings) to avoid certain problematic cases or incompatibilities between codecs, by letting the PBX handle things for you.

 

[Lee Cramman]

Everything passes through SBC.

It helps to think of your link between the SBC and PBX as a VPN.

 

[cct]

thanks guys, very helpful and thanks John for that very simplified explanation that's helped massively.