While the UE attached to LTE network, the UE can initiate, or receive a voice or a video call. The UE achieves this, by connecting to an external network, which is the 'IMS' network. We can call this, Voice over LTE (VoLTE).
This is exactly like when you would like to create a call, while you connects to the internet. Once you connects to the internet, you can use 'Skype' as example, to establish and receive your call.
Here, the UE attaches to LTE network, then it connects to the IMS network, in order to establish o receive a call.
The First point in the IMS network that the UE connects to, is called the 'P-CSCF' (Proxy-Call Session Control Function). Once the UE registers to IMS network, the UE will be able to exchange signaling messages with the P-CSCF, in order to establish, or release calls.
The UE and the IMS network are using 'SIP' protocol, in order to exchange signaling messages between each other. First, the UE signals the IMS network in order to initiate a call to another UE. Then the IMS network signals the other UE, in order to receive this call. In turn, both UEs will be able to communicate with each other.
VoLTE call establishment
In order to establish or receive a VoLTE call, two bearers should exist between the UE and the PGW.
- The first one is a 'Default Bearer', which will connect the UE to the IMS network, and passes the signaling messages between the UE, and the IMS network.
- The second one is a 'Dedicated Bearer', which will pass the media streams of the call, between the initiating and receiving UEs.
The Default Bearer is the first Bearer the UE creates with LTE to connect to a certain network.
While The Dedicated Bearer, is a Bearer with special QoS. The UE and the network uses this dedicated bearer, to pass the traffic of the service that needs special QoS requirements.
first the UE creates a 'default bearer' with 'QCI 5'. This default bearer connects the UE to the IMS network. In addition it passes SIP signaling messages, between the UE, and the IMS network.
QCI refers to 'QoS Class Identifier', it is a scalar that determines traffic priority , maximum delay, and maximum packet loss, that the nodes shall provide ,while forwarding the traffic belonging to this bearer. QCI value ranges from QCI 1 to QCI 9.
Then after the UE exchanges signaling messages with the IMS network, the network will create a 'dedicated bearer'. This dedicated bearer will have 'QCI 1' in case of voice calls, or 'QCI 2' in case of video calls. This dedicated bearer will pass the media streams of the call, between the initiating and receiving UEs.
In summary, in order to initiate or receive a voice or a video call while connected to LTE network, the UE first makes a default bearer with QCI 5 that will be pass signaling messages between the UE and the IMS network. Then the network creates a dedicated bearer with QCI 1 or QCI 2 to pass the media stream of the call.