Audio path between two remote phones

[sunstate]

Hello,

In the following scenario, how does the call setup and audio path play out on a call made between two remote phones (someone at B calls someone at C)? Does the call setup go through 3CX and then the audio is direct between the phones, or does the setup and audio go through 3CX? And how does this differ (if at all) when using the tunnel or hardware VPN's?

 

[sunstate]

Ok, finally got this setup in a test environment. According to the packet capture taken at Branch Office B, I think it goes like this:

1. Phone B contacts stun server.
2. Phone B contacts 3CX and requests to setup call.
3. 3CX responds to phone B with the public IP of phone C and the port to use (in this case 11782)
4. Phone B establishes audio directly with phone C over UDP 11782.

Everything seems to be working fine between the two extensions, but I guess I'm just confused on why.

1. How/why did 3CX pick 11782? Randomly?
2. How does the firewall at Branch Office C know to let UDP 11782 through to the phone?
3. When are the ports specified under Settings, Network, Ports used? Only when 3CX is passing audio directly between it and a remote extension or VoIP provider?

 

[leejor]

Phone C (in your case) should be responding , via 3Cx with the port number (11782) it want to receive audio on. 3CX is just passing this on to caller B. Phone C would have to cause that port to be associated with it's IP in the local router/firewall.

 

[sunstate]

Ok, that makes sense.

Now what about if both branch office B and branch office C are running the 3CX SIP Proxy manager and both have tunnels set up back to 3CX. Then when someone at B calls someone at C, what path does the audio take? Does everything go through the 3CX tunnels at that point?

I'm starting to think that the only way to get this to work 100% is a full mesh VPN.

 

[leejor]

I'm starting to think that the only way to get this to work 100% is a full mesh VPN.

What is the original problem? I guess you know about the extension option of "PBX delivers audio" .

 

[sunstate]

There is no problem. I have a potential customer who wants to replace a hosted solution with this 3CX setup. I'm just trying to wrap my head around the various options and why some of them will/won't work.

So what happens in the case where each branch office has a 3CX tunnel back to corporate? Will all the audio between branch offices go through the tunnels? Or will the setup go through the tunnels and the audio direct? If direct, it would be between the machines hosting the proxy manager, correct?

 

[abc123]

Ok, finally got this setup in a test environment. According to the packet capture taken at Branch Office B, I think it goes like this:

1. Phone B contacts stun server.
2. Phone B contacts 3CX and requests to setup call.
3. 3CX responds to phone B with the public IP of phone C and the port to use (in this case 11782)
4. Phone B establishes audio directly with phone C over UDP 11782.

Everything seems to be working fine between the two extensions, but I guess I'm just confused on why.

1. How/why did 3CX pick 11782? Randomly?
2. How does the firewall at Branch Office C know to let UDP 11782 through to the phone?
3. When are the ports specified under Settings, Network, Ports used? Only when 3CX is passing audio directly between it and a remote extension or VoIP provider?

In answer to your question about 11782 and the firewall, it was the firewall (well the NAT part of it) not the phone that gave that port - that is what STUN does - the extension was listening on a different port.

 

[leejor]

So what happens in the case where each branch office has a 3CX tunnel back to corporate? Will all the audio between branch offices go through the tunnels? Or will the setup go through the tunnels and the audio direct? If direct, it would be between the machines hosting the proxy manager, correct?

I would tend to think, but it's just a guess, that because the tunnel is being used, all packets would go back to the 3CX host. I'm suspecting that the proxy servers would not be capable (sophisticated enough) of setting up an audio path between each other, only with the host. Perhaps someone from 3Cx can confirm.

 

[sunstate]

So what happens in the case where each branch office has a 3CX tunnel back to corporate? Will all the audio between branch offices go through the tunnels? Or will the setup go through the tunnels and the audio direct? If direct, it would be between the machines hosting the proxy manager, correct?

I would tend to think, but it's just a guess, that because the tunnel is being used, all packets would go back to the 3CX host. I'm suspecting that the proxy servers would not be capable (sophisticated enough) of setting up an audio path between each other, only with the host. Perhaps someone from 3Cx can confirm.

That's exactly what I'm thinking, but would like comfirmation. I definitely don't want to sell them on that fact and then have it not be true. Thanks for the input.