VoLTE UE Attach Procedure | VoLTE UE registration procedure

This page describes VoLTE UE Attach Procedure and VoLTE UE registration procedure. VoLTE UE Attach and registration procedures cover message flow between UE and IMS.

The VoLTE stands for Voice over LTE. Actually LTE is designed for packet switching and it is all IP network. In order to support voice over LTE there are two methods viz. IMS and CSFB. Refer VoLTE basics>> and CSFB benefits>>.

VoLTE UE Attach Procedure

VoLTE UE Attach Procedure
Figure:1 VoLTE UE Attach Procedure

The figure-1 depicts VoLTE UE Attach Procedure. Let us understand message flow between LTE UE and other LTE system entities.
➨When UE is powered ON, it sends "ATTACH REQUEST" message to MME.
➨MME executes location registration with HSS. It also fetches user profile of VoLTE subscriber in addition to VoLTE APN (i.e. Access Point Name) from HSS. Based on APN, MME determines PGW.
➨MME transmits request to PGW in order to establish VoLTE bearer.
➨PGW assigns IP address for UE and determinew P-CSCF to be used for the call.
➨PGW sets P-CSCF address in protocol configuration option and communicates the same to SGW in bearer response. SGW passes the same address to MME entity.
➨MME transmits "ATTACH COMPLETE" to UE which contains P-CSCF address. This completes the LTE UE attach procedure and begins the LTE UE registration procedure. Now UE IMS client starts IMS call flow. Let is understand IMS SIP client registration call flow as described below.

VoLTE UE Registration Procedure

VoLTE UE registration procedure
Figure:2 VoLTE UE Registration procedure

The figure-2 depicts VoLTE UE registration procedure. Refer which explains IMS SIP client registration call flow. The three elements viz. P-CSCF, I-CSCF and S-CSCF are used during VoLTE UE registration. Also refer VoLTE originating call flow procedure>> which describes call flow state diagram.

LTE Tutorial related links

What is LTE?  Terminology  3GPP ref   LTE tutorial  LTE Features  UE categories  LTE Bands  EARFCN vs frequency   LTE frame  FDD vs TDD   PHY  protocol stack  throughput  VoLTE  CA
UE cell search  UE network entry  Timers  PSS vs SSS  LTE RS DMRS SRS  LTE Channels  BW vs FS vs RB  Zadoff chu sequence  LTE Advanced   eNodeB Measurements  UE Measurements  SC-FDMA Vs OFDMA   LTE Security  

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