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VoLTE Originating Call | VoLTE Call Flow Procedure | VoLTE MO Call

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

In any cellular network whether it is GSM, CDMA or LTE there are two main parts viz. user part and network part. The user uses mobile phone or UE or mobile subscriber unit to connect with the network. The system with which LTE compliant UE connects initially on the network is known as eNB. eNB is similar to base station of GSM or eNodeB of CDMA.

The call initiated by UE is known as mobile originated call or mobile originating call. The call terminated at the UE is known as mobile terminated call or mobile terminating call. Voice over IP (VoIP) signaling is handled using SIP (Session Initiation Protocol).

VoLTE Call Flow State Diagram
Figure:1 VoLTE Call Flow State Diagram

The figure-1 depicts IMS SIP client registration call flow. There are three main elements viz. P-CSCF, I-CSCF and S-CSCF. The call flow is self explanatory with the figure-1. Following are the functions of each of these elements.
• P-CSCF: It acts as gateway for UE into home network. It identifies home IMS network and helps in routing the traffic to/from the home IMS network.
• I-CSCF: It acts as edge of home IMS. It interfaces with P-CSCF in visited network. It interfaces and queries HSS and selects the S-CSCF.
• S-CSCF: It handles actual registration request received from I-CSCF. It extracts authentication data from HSS (via I-CSCF). It authenticates user after second registration attempt.

In LTE, user initiates VoLTE call through IMS client on the handset (or UE). Following figure-2 depicts VoLTE call flow with explaination.

VoLTE Originating Call Flow
Figure:2 VoLTE Call Flow Procedure

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

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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