Call flow mobile (cell) phone to branch phone using PSTN

[twhittle1]

Hi,

I'm after a bit of clarity, is anyone able to provide me with any :) ?

I'm trying to understand how the actual phone call flows after it's all been setup and brokered by CUCM. I've seen the classic picture showing internal calls where the CUCM server sets the call up and then the RTP stream flows directly between the end points but I get a little hazy when you start adding in extra components such as gateways and external connections:

I get this:

What I'm struggling with is when external networks and gateways are involved.

Attached is a crude drawing (UC1):

This shows a mobile phone calling an extension located at a branch office. I assume the call setup flows through the HQ to the CUCM. CUCM then tells the phone at the branch to ring (black lines) and when the phone picks up the actual RTP call flows out the local PSTN connection to the GSM network and back to the mobile phone - I.E. there is no hairpinning through the HQ and the CUCM server. I assume this only happens if the PSTN gateway is registered with the CUCM server as a gateway of some kind.

2nd (and 3rd) question:

As per crude drawing number 2 (UC2) - If there is no PSTN gateway at the branch site, then calls will be hairpinned back through the HQ? - Black line is setup and red line is RTP call flow.

If the remote device is a phone with jabber software installed is there any way of using the phone's GSM connection to make the call? I.E. Call manager sets up the call in the normal way (black line) but the phone could connect using it's GSM connection directly to the calling mobile? - As in the light blue dashed line?

Can anyone shed any light?

Many Thanks,

Tom

[Chris Deren]

CUCM is never in the RTP path, unless you are using an MTP which could be CUCM itself or hardware/IOS MTP on a router.  The RTP will normally flow between the endpoint and the gateway. 

CUCM's role is merely signaling in a typical voice call.

[twhittle1]

Thanks Chris, I assumed this was the case but just wanted to check.

How about the 2nd question (I appreciate I forgot to upload my second picture). Can a mobile phone with the jabber client installed use it's mobile phone connection as part of the call path? I.E. if a external phone dialed the phone number associated with a jabber client on a mobile phone. Can the call manager "instruct" the mobile to use it's GSM connection (almost using the mobile as a gateway) or would the call have to come back over the data connection be hairpinned through the HQ and then go out the PSTN at the HQ site?

Sorry if I'm not explaining well, I drew a crude diagram previously and promptly forgot to upload it!

Many Thanks,

Tom

[Chris Deren · Accepted solution]

If you are using Jabber, with current implementation it always uses the data connection, so if Jabber is on external network then you are using the mobile carrier's data or external wi-fi to pass the traffic via VPN tunnel and through your network out to voice GW.  Again, this never goes via CUCM.

You should look into deploying MRA (Expressway) rather than VPN if you are on correct versions.  

[twhittle1]

Many thanks Chris,

Regards,

Tom