How to make a VOLTE call

VOTE basic process
Mobile communication is a huge system project. To make our mobile phones and mobile phones make smooth phone calls and access the Internet, few of us know how ordinary mobile users communicate with mobile phones. For this reason, we This lovely group of engineers claiming to be communications migrant workers also spent a lot of effort on building bridges.

Let ’s just talk about a common process
1 Cell phone brother turned on, plug in the phone card you just bought (Isn’t this nonsense, hehe)
2 Mobile phone brother searches the network based on the contract information of the mobile phone card
3 Wow, find the network, come, I want to log in, register with the network mom mobile phone
4 Registration is successful, what default bearers, dedicated bearers, etc. are starting to be established?
5 The matchmaking is successful, I have to tell the network mom, my name is XXX, I will be GSM900 1800, I will be LTE FDD, TDD, and UMTS FDD, and I will also be VOLTE. Tell the internet mom where you are
6 Alright, explain clearly, it ’s okay to report to the network periodically where I am
7 You have to go out to play, you have to tell your base station brother here, he will pass your information to the network mom, guide you to switch, how to go, and you must report the information in time to avoid losing the connection
8 I want to call, someone calls me, network mom and base station brother, they will dispatch and call the younger brother in time.
9 Well, shut down at night to sleep, report the last position, go offline.
Above, it looks relatively simple, but the system engineering is more rigorous.

1. Access network selected by the terminal after the UE is turned on
   For a terminal, only one network (IMS network or CS network) can be accessed at the same time when the terminal is powered on, regardless of whether it is a Single Radio terminal or a CSFB terminal. Therefore, the terminal needs to select the attached network according to its own settings. Connect to the network and go for the first choice.
   At the same time, some network settings will also affect the terminal’s choice of network, such as the following:
   IMS / PS voice and CS voice priority
   CS Voice only
   IMS PS Voice only
   prefer CS Voice with IMS PS Voice as secondary
   prefer IMS PS Voice with CS Voice as secondary
   However, only CSFB network terminals are supported. The above voice can only be CS Voice only

Voice and data priorities are:

Voice centric
Data centric

2) Access the LTE network for EPC authentication and registration, that is, network attachment

1 MME authenticates terminal USIM
2 Get mobile phone contract information from HSS
3 The MME selects the S / P-GW according to the APN configuration, and the S / P-GW establishes the default bearer. At this time, the default bearer for our Internet access is established.
4 The S / P-GW assigns an IP address to the UE. The MME informs the terminal of the UE IP address, P-CSCF address, and other IMS signaling related parameters through the Attach Accept message. At this time, the SIP signaling bearer is established. So far, we and the IMS network default The bearer is set up. Here you simply think that my voice channel has been established (this terminal needs to support it), but whether it can be used requires IMS authentication and registration.

 

3) Perform IMS authentication and registration
Here are a few keywords to know. The IMS network is a dual-signature, third-party registered network.
1 The UE initiates a registration process, reads the USIM card information, and derives a T-IMPU based on the IMSI; the CSCF obtains authentication information from the IMS-HSS and sends it to the UE; the UE authenticates the network layer according to the authentication information.
2 The UE initiates the second register information, which carries the terminal authentication information; the CSCF authenticates the UE. After CSCF authentication is passed, it downloads user subscription information from HSS and returns registration success information to the terminal.
3 Third-party registration: CSCF initiates third-party registration with TAS on behalf of the UE. TAS downloads mobile phone contract information from HSS through the Sh interface.

By now, mobile phones can use VOLTE normally.

4) VOLTE communication process

1 When the UE initiates a call in the LTE coverage area, it will be sent to the IMS through the EPC network, and the calling side TAS will provide the calling service for the UE.
2  After the calling party’s S-CSCF completes the calling party’s TAS trigger, it queries ENUM to obtain the service domain of the called party, thereby connecting the call to the called party’s IMS network.
3 The TAS in the called-side IMS network provides the called service for the called UE, then completes the called access domain selection (T-ADS), and determines the access network (2G / 3G CS or LTE) where the called UE resides. .
4 Finally, the called S-CSCF connects the call to the called UE according to the domain selection result.
5 When both the calling and called UEs are in the LTE coverage area, the media plane path is directly connected via two SBCs.

 

5) Domain selection in VOLTE communication process
What is domain election?
Literally, it is not difficult to know that domains are different service bearer networks, such as PS domain and CS domain. Since VoLTE-enabled UEs can have multiple modes, they can be attached to different networks under different signal strength coverage. Attach to 2G / 3G networks, sometimes to LTE networks, or even to two networks at the same time. Therefore, a VoLTE-enabled UE must choose to access one of the networks for a voice call when calling, and the process of selecting an access network is This is called domain selection.
The domain selection function on the network side only involves Single Radio terminals. Only CSFB network terminals are supported. The voice can only be CS Voice only.
The purpose of the domain selection is to maintain the continuity of the call. Should the caller be in the IMS domain or the CS domain, should the VOLTE call be made, or should CSFB or Srvcc return to 3G.

How to complete domain selection?

When the user is the calling party, the UE completes the domain selection based on the saved registered network information.
As the called party, the network side queries the fusion HLR / HSS to obtain the registered network information to complete the domain selection

T-ADS implements called domain selection

T-ADS (Terminating Access Domain Selection)

The T-ADS function is integrated in the MMTel AS. It queries the integrated HLR / HSS to obtain information such as the UE terminal type and access domain, and then completes the domain selection according to the operator’s policy.

 

5) VOLTE Voice Continuous Retention Solution (SRVCC)

VOTE’s voice solutions now use Esrvcc for most operators. Under what circumstances will ESRVCC switchover be triggered?

When users are in the LTE coverage area, they provide voice over LTE / IMS. When the user moves out of the LTE hotspot coverage area and enters the 2G / 3G coverage area, it can be understood here that 2G and 3G signals are stronger than 4G. How do you ensure the continuity of the user’s voice service?

Because the voice service is a real-time service, how to ensure that the user’s service interruption time is minimal when moving across this network? In this case, the ESRVCC scheme can well guarantee business continuity.

The implementation process of eSrvcc is shown in the figure below.

to sum up

At this point, we have briefly gone through the related VOLTE interaction process. Of course, the VOLTE communication process is not as simple as the above. In the communication process, it will involve VOLTE to VOLTE users, VOLTE to CS users, CS to VOLTE Users, and whether to rebuild CSFB, bit rate negotiation, etc., but no matter how complicated, the basic factors are as described above.