Call Flow Explained (MO & MT) - 2Greet
2Greet provides voice services over Any IP network, whether we are talking about 3G, 4G, WiFi or future 5G. It can be integrated into the mobile operator’s network with no special configuration.
For example, a Mobile Operator that has a 4G network can use this service to provide voice over LTE for its subscribers without any changes to the current network.
Let’s talk about how calls are made with 2Greet!
With a mobile originated call, it’s simple.
We have the phone with the 2Greet App on it.
It makes the call through the radio network and goes to the data Network. It could be any IP data.
After the connection is established, it goes to the 2Greet function. If data is unavailable or the request fails the call falls back to the primary SIM.
2Greet continues to the Operators’ routing rules over SIP.
The next step is proceeding to the interworking MSC that goes to the phone network.
Basically, the dialer intercepts the request from the keyboard, or from the contacts list and makes an HTTPS request to the function, that establishes an SRTP connection for the voice.
It’s that simple!
For mobile terminated calls, things get more interesting.
The call comes from the PSTN and arrives in the GMSC. Because it comes from outside, it has to make sure the subscriber belongs to this network.
So, the GMSC requires the HLR a routing.
The only thing is, the HLR has a record that notifies this particular user has a CAMEL subscription.
So, the HLR returns to the GMSC with the required information, the CAMEL subscription.
The GMSC will make a CAMEL request to the 2Greet function instead of proceeding with the call. In this case, it accesses a service control function (SCF).
Now, 2Greet figures out “Okay, this number has been registered”, and it sends out a request to Google’s Firebase Cloud Messaging Server;
FCM is the guy that sends notifications to all the Android devices.
The Firebase service sends the request to the phone through the internet and wakes it up.
The phone does an HTTPS request that goes through the internet, and next within the 2Greet function that says to the SCF “I want to pick up this call”, and then the message is returned to the GMSC saying “I want to divert this call to a landing number”.
At this moment, the GMSC sends a SIP message to the function, the function identifies and joins the call legs, and we have the call ringing.
If the phone cannot be notified or it does not pick up the call the SCF orders the call to continue so it falls back to the visited MSC.
That’s it! This is how 2Greet works!
If you want to test 2Greet you can sign up to our Demo.
Thank you!
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