VoLTE UE Attach and Registration Procedures

This article explains the VoLTE UE attach and registration procedures, outlining the message flow between the User Equipment (UE) and the IP Multimedia Subsystem (IMS).

Introduction

VoLTE stands for Voice over LTE. LTE is designed for packet switching and operates on an all-IP network. To support voice services over LTE, two main methods are used: IMS and Circuit Switched Fallback (CSFB).

VoLTE UE Attach Procedure

Figure 1: VoLTE UE Attach Procedure

Figure 1 illustrates the VoLTE UE attach procedure. Let’s examine the message flow between the LTE UE and other LTE system entities:

1. ATTACH REQUEST: When the UE is powered on, it sends an “ATTACH REQUEST” message to the Mobility Management Entity (MME).
2. Location Registration: The MME executes location registration with the Home Subscriber Server (HSS). It also retrieves the user profile of the VoLTE subscriber, including the VoLTE Access Point Name (APN), from the HSS. The APN helps the MME determine the Packet Data Network Gateway (PGW).
3. Bearer Establishment Request: The MME transmits a request to the PGW to establish a VoLTE bearer.
4. IP Address Assignment and P-CSCF Determination: The PGW assigns an IP address to the UE and determines the Proxy-Call Session Control Function (P-CSCF) to be used for the call.
5. P-CSCF Address Communication: The PGW sets the P-CSCF address in the protocol configuration option and communicates it to the Serving Gateway (SGW) in a bearer response. The SGW then passes this address to the MME.
6. ATTACH COMPLETE: The MME transmits an “ATTACH COMPLETE” message to the UE, which includes the P-CSCF address.

This completes the LTE UE attach procedure, paving the way for the LTE UE registration procedure. The UE’s IMS client can now initiate the IMS call flow.

VoLTE UE Registration Procedure

Figure 2: VoLTE UE Registration procedure

Figure 2 depicts the VoLTE UE registration procedure. The P-CSCF, Interrogating-CSCF (I-CSCF), and Serving-CSCF (S-CSCF) elements are crucial during VoLTE UE registration.