VoLTE Originating Call Flow and Procedure (MO Call)

This page describes VoLTE Originating Call, i.e., VoLTE MO Call flow. The VoLTE call flow procedure covers messages exchanged between the UE (or LTE mobile), LTE/EPC entities, and IMS entities. MO Call stands for Mobile Originating Call.

Introduction

In any cellular network, whether it’s GSM, CDMA, or LTE, there are two main parts: the user part and the network part. The user uses a mobile phone (UE or mobile subscriber unit) to connect with the network.

The system with which the LTE-compliant UE connects initially on the network is known as eNB. eNB is similar to the base station of GSM or eNodeB of CDMA. The call initiated by the UE is known as a mobile originated call, while the call terminated at the UE is known as a mobile terminated call.

Voice over IP (VoIP) signaling is handled using SIP (Session Initiation Protocol).

Figure 1: VoLTE Call Flow State Diagram

Figure 1 depicts the IMS SIP client registration call flow. There are three main elements: P-CSCF, I-CSCF, and S-CSCF. The call flow is self-explanatory with the figure.

The following are the functions of each of these elements:

• P-CSCF: It acts as a gateway for the UE into the home network. It identifies the home IMS network and helps in routing the traffic to/from the home IMS network.
• I-CSCF: It acts as the edge of the home IMS. It interfaces with the P-CSCF in the visited network. It interfaces and queries the HSS and selects the S-CSCF.
• S-CSCF: It handles actual registration requests received from the I-CSCF. It extracts authentication data from the HSS (via the I-CSCF). It authenticates the user after the second registration attempt.

In LTE, the user initiates a VoLTE call through the IMS client on the handset (or UE). Figure 2 depicts the VoLTE call flow with explanation.

Figure 2: VoLTE Call Flow Procedure

• (1): The UE initiates a VoLTE call. Hence, it is known as the originating VoLTE terminal.
• (2): The INVITE message is transmitted to the VoLTE terminating terminal via P-CSCF/S-CSCF/TAS. The message consists of terminal IDs, codes used by the originating terminal.
• (3): The terminating terminal responds with “183 session progress”. This message carries codecs used by the terminating terminal.
• (4): The P-CSCF receives the above message and signals the PCRF to establish a dedicated bearer for voice media.
• (5): The dedicated bearer is established by PGW, SGW, eNB, and the originating terminal. An appropriate QCI is assigned to such a dedicated bearer. A QCI value of 1 is used for conversational speech.
• (6): In parallel to (5), the P-CSCF forwards the “183 session progress” message to the originating terminal. Using this, the originating terminal compares the codecs of the terminating terminal with its own and decides which codec type to be used further.
• (7): The originating terminal sends PRACK to the terminating terminal, which notifies the codec to be used. The terminating terminal responds with “200 OK”.
• (8): This establishes the voice bearer. This is confirmed by the “UPDATE” message initiation from the originating terminal and the response of “200 OK” from the terminating terminal.
• (9): After the confirmation in (8), the terminating terminal starts ringing.
• (10): The terminating terminal transmits a PRACK message which confirms ringing.
• (11): The “Ser” at the terminating terminal answers the call.
• (12): The terminating terminal sends “200 OK” to the originating terminal. This indicates that the call has been answered. This refers to the establishment of the session. Now voice traffic can flow through the established dedicated voice bearer which utilizes a QCI of value 1.