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.
Introduction:
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
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
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