VoLTE Tutorial: Voice Over LTE Basics and Types

This tutorial covers the fundamentals of VoLTE (Voice Over LTE), including the different types: VoLTE using IMS (VOIP) and VoLTE using CSFB (SRVCC).

LTE is an all-IP based network primarily designed for internet and data applications. Therefore, there are two main ways to support voice calls over LTE. The first is using VOIP (Voice over IP) with the help of IMS (IP Multimedia Subsystem). The second is using legacy 2G/3G Circuit Switched Fallback (CSFB) with a concept called SRVCC. Let’s explore how these methods work.

VoLTE using IMS (VOIP)

Image alt: VoLTE using VOIP through IMS

As shown in the figure above, the LTE network, formally called EUTRAN, comprises one or more eNodeB base stations. An eNodeB in LTE combines the functionalities of the RNC and NodeB found in 3G networks. The MME, S-GW, HSS, and P-GW together form the EPC (Evolved Packet Core). eNodeBs and the EPC together make up the EPS (Evolved Packet System).

LTE supports voice calls using VOIP through IMS. If the call is VOIP to VOIP, it remains entirely within the LTE network, supported by VOIP protocols. However, if it’s a VOIP call to a CS (Circuit Switched) call destined for a GSM/WCDMA network, IMS, with the help of application servers and the legacy MSC, transfers the PS (Packet Switched) call to the legacy networks (2G-GERAN/3G-UTRAN). For VOIP calls to a PSTN (Public Switched Telephone Network) line, IMS directs the call through the PSTN interface to the appropriate exchanges.

VoLTE using 2G/3G CSFB (using SRVCC)

Image alt: VoLTE using CSFB,SRVCC

When a UE (User Equipment, i.e., your phone) moves from an LTE network to a legacy network (e.g., GSM/WCDMA), a handover occurs based on measurement reports submitted by the UE. This involves transferring the Voice over IP call to a legacy voice call. For this to happen, the UE must support SRVCC.

SRVCC stands for Single Radio Voice Call Continuity. This means the UE is only connected to a single radio technology at any given time, either LTE, GSM, or WCDMA. Because of this single radio connection, battery life is typically better on SRVCC-capable UEs.