Simplified VoLTE registration procedure

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

  1. ๐€๐ญ๐ญ๐š๐œ๐ก ๐‘๐ž๐ช๐ฎ๐ž๐ฌ๐ญ: The UE (User Equipment) sends an attach request to the MME (Mobility Management Entity).
  2. ๐€๐ฎ๐ญ๐ก๐ž๐ง๐ญ๐ข๐œ๐š๐ญ๐ข๐จ๐ง: The MME authenticates the UE.
  3. ๐’๐ž๐ฌ๐ฌ๐ข๐จ๐ง ๐‚๐ซ๐ž๐š๐ญ๐ข๐จ๐ง: A default bearer is established between the UE and the EPC.
  4. ๐ˆ๐ ๐€๐๐๐ซ๐ž๐ฌ๐ฌ ๐€๐ฅ๐ฅ๐จ๐œ๐š๐ญ๐ข๐จ๐ง: The UE receives an IP address.

Step2: Then the device will do another registration to the IMS (IP Multimedia Subsystem) network.

  1. ๐’๐ˆ๐ ๐‘๐„๐†๐ˆ๐’๐“๐„๐‘ ๐Œ๐ž๐ฌ๐ฌ๐š๐ ๐ž: The UE sends a SIP REGISTER message to the P-CSCF (Proxy-Call Session Control Function).
  2. ๐€๐ฎ๐ญ๐ก๐ž๐ง๐ญ๐ข๐œ๐š๐ญ๐ข๐จ๐ง: The P-CSCF forwards this message to the I-CSCF (Interrogating-CSCF) and then to the S-CSCF (Serving-CSCF) for authentication.
  3. ๐‘๐ž๐ ๐ข๐ฌ๐ญ๐ซ๐š๐ญ๐ข๐จ๐ง: The S-CSCF completes the registration process by updating the HSS (Home Subscriber Server) with the registration status of UE.
  4. ๐Ÿ๐ŸŽ๐ŸŽ ๐Ž๐Š ๐Œ๐ž๐ฌ๐ฌ๐š๐ ๐ž: The S-CSCF sends a 200 OK response back through the I-CSCF and P-CSCF to the UE, confirming the registration.

:warning: 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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