There are 2 crucial steps to register to VoLTE (Voice over LTE) services:
Step1: The device must first register with the EPC (Evolved Packet Core). This initial registration to EPC will provide the necessary IP connectivity
- ๐๐ญ๐ญ๐๐๐ก ๐๐๐ช๐ฎ๐๐ฌ๐ญ: The UE (User Equipment) sends an attach request to the MME (Mobility Management Entity).
- ๐๐ฎ๐ญ๐ก๐๐ง๐ญ๐ข๐๐๐ญ๐ข๐จ๐ง: The MME authenticates the UE.
- ๐๐๐ฌ๐ฌ๐ข๐จ๐ง ๐๐ซ๐๐๐ญ๐ข๐จ๐ง: A default bearer is established between the UE and the EPC.
- ๐๐ ๐๐๐๐ซ๐๐ฌ๐ฌ ๐๐ฅ๐ฅ๐จ๐๐๐ญ๐ข๐จ๐ง: The UE receives an IP address.
Step2: Then the device will do another registration to the IMS (IP Multimedia Subsystem) network.
- ๐๐๐ ๐๐๐๐๐๐๐๐ ๐๐๐ฌ๐ฌ๐๐ ๐: The UE sends a SIP REGISTER message to the P-CSCF (Proxy-Call Session Control Function).
- ๐๐ฎ๐ญ๐ก๐๐ง๐ญ๐ข๐๐๐ญ๐ข๐จ๐ง: The P-CSCF forwards this message to the I-CSCF (Interrogating-CSCF) and then to the S-CSCF (Serving-CSCF) for authentication.
- ๐๐๐ ๐ข๐ฌ๐ญ๐ซ๐๐ญ๐ข๐จ๐ง: The S-CSCF completes the registration process by updating the HSS (Home Subscriber Server) with the registration status of UE.
- ๐๐๐ ๐๐ ๐๐๐ฌ๐ฌ๐๐ ๐: The S-CSCF sends a 200 OK response back through the I-CSCF and P-CSCF to the UE, confirming the registration.
Note: As mentioned, this is a very simplified call flow, to easy understanding. For example, PCRF, IMS HSS and others are missing in the block flow diagram, and can be found in complete call flow.
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