IMS Registration Call Flow: VoLTE Registration Call Flow – SIP Registration Call Procedure

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In this article, we will discuss the call flow for IMS registration, focusing specifically on VoLTE UE Attach and SIP IMS Registration. This information is derived from the 3GPP and GSMA documents.

Introduction to VoLTE Registration Call Flows

Before a user can make Voice over LTE (VoLTE) calls, their mobile handset needs to first latch onto the LTE Network and then complete IMS registration. IMS registration enables users to make voice and video calls over VoLTE and use supplementary services. In this article, we will explore the end-to-end signaling process for VoLTE IMS registration and discuss the detailed call flows.

Steps for Using VoLTE Services

To use VoLTE services, users need to go through the following steps:

  1. Attaching to the LTE Network: Users must connect to the LTE Network and activate the Default Internet EPS bearer, which allows them to access the internet.
  2. Activating Default IMS EPS Bearer: This step involves activating the Default IMS EPS bearer, which is used for communicating with the IMS Network.
  3. SIP/IMS Registration: In this step, the User Equipment (UE) directly communicates with the IMS Network for SIP/IMS Registration. The LTE Network acts as a relay for messages between the UE and IMS Network.
  4. Dedicated Bearer Establishment: After IMS Registration, dedicated bearers are established on the fly for Mobile Originated (MO) or Mobile Terminated (MT) calls.

Understanding EPS Bearers

EPS bearers play a crucial role in VoLTE registration call flows. A bearer can be defined as a “pipeline” that connects two or more points in the communication system, allowing data traffic to flow through. In the context of communication technology, EPS bearers are used to establish connections for various purposes.

There are two types of EPS bearers involved in VoLTE registration:

Default EPS Bearer

The Default EPS bearer is established during LTE Attach itself and is allocated an IP address for the UE. It is always used for internet connectivity and has a QCI (Quality of Service Class Identifier) value between 6 to 9. This bearer is also used for SIP signaling connectivity in VoLTE and has a QCI value of 5.

Dedicated EPS Bearer

The Dedicated EPS bearer is established during a call, whether it’s a voice or video call. This bearer doesn’t have its own IP address and uses the IP address of the parent Default Bearer. It is linked to a specific Default EPS bearer and has a specific QoS (Quality of Service). This bearer is used for on-demand bearer services, such as making a voice call (QCI = 1) or a video call (QCI = 2).

Handling Radio Congestion in VoLTE Calls

During radio congestion, it’s important to prioritize critical traffic such as voice and video calls over internet traffic. To achieve this, QCI (Quality of Service Class Identifier) values are used. QCI 5 is used for prioritizing VoLTE IMS signaling, QCI 1 is used for prioritizing VoLTE IMS voice calls, and QCI 6 to 9 is used for internet traffic (low priority). By assigning appropriate QCI values, the network can ensure seamless access to critical traffic even during congestion.

VoLTE UE Attach and IMS Registration

The UE attaches to the 4G network and establishes a PDN (Packet Data Network) connection with the default APN, which is the Internet APN. Additionally, the UE attempts to establish a PDN connection with the IMS APN for IMS registration. The IMS APN has a default EPS bearer with a QCI value of 5, specifically used for SIP signaling. After completing the PDN connection and establishing the default EPS bearer, the UE can communicate with the IMS Core for VoLTE services.

SIP IMS Registration Call Flow

The SIP IMS registration process involves multiple steps and SIP communications. In this section, we will explore the call flow for SIP IMS registration.

  1. Initial Registration Attempt: The VoLTE UE initiates a SIP REGISTER request to the P-CSCF (Proxy-Call Session Control Function) using the P-CSCF IP address obtained during LTE Attach.
  2. Forwarding SIP REGISTER: The P-CSCF inserts a Path header with a SIP-URI identifying the P-CSCF and forwards the SIP REGISTER request to the I-CSCF (Interrogating-Call Session Control Function).
  3. Authentication and Authorization: The I-CSCF queries the HSS (Home Subscriber Server) for user authentication and authorization. The HSS validates the user’s identities and retrieves authentication and S-CSCF (Serving-Call Session Control Function) information.
  4. Selecting S-CSCF: Based on the HSS response, the I-CSCF selects an appropriate S-CSCF for the public user identity.
  5. Multimedia Authentication: The S-CSCF initiates a Multimedia Authentication Request (MAR) to the HSS for IMS-AKA related security.
  6. Completing SIP Registration: The S-CSCF forwards the SIP REGISTER request to the selected S-CSCF, completing the IMS registration process.


In this article, we have discussed the call flow for IMS registration in VoLTE, focusing on the SIP IMS registration procedure. We covered the steps involved in using VoLTE services, the role of EPS bearers, handling radio congestion, and the detailed call flow for SIP IMS registration. Understanding these call flows is crucial for network engineers and individuals interested in VoLTE technology.