Voice over LTE Overview

Voice in 3G

Voice over LTE (VoLTE) is deployed completely different than in voice calls in the 2G/3G networks. Find out how and which new extra features it brings.

2G and 3G have two separate bearers, one for voice a call and another for an internet access. Voice domain or voice bearer is called Circuit Switched – CS domain. Internet access bearer is called Packet Switched – PS domain or PS bearer.

Unlike 2G and 3G, LTE is only packet technology. LTE technology has only one domain – packed switched bearer for internet access. This means that all other services – voice call, video, SMS, internet uses packet switched bearer.

More about the LTE, LTE technology, LTE features and LTE Advanced you can read in the post What is LTE, 4G Technology and LTE Advanced.

Check about the LTE technology in the post LTE Technology – Basic Architecture of the 4G Mobile Internet Access.

Voice Services in LTE

There are two ways to implement the voice over LTE:

  • CS fallback
  • Multimedia telephony

The both solutions are standardized by the 3GPP.

CS Fallback – CSFB

CS fallback is the first phase of evolution to the LTE and voice over LTE. In this scenario the mobile terminal is using the infrastructure of old 2G and 3G network for voice call. Mobile phone call is going over circuit switched bearer in 2G and 3G network. When the call is finished, mobile terminal can use the LTE network for packet services (internet access).

For the CS fallback we need the following:

  • Mobile terminals need to support both LTE and 2G/3G network
  • Overlapped coverage of 2G/3G and LTE network

More about the Circuit switched fallback you can read in the document The first phase of voice evolution for mobile LTE devices.

Multimedia Telephony – MMTel

IP Multimedia Subsystem (IMS) multimedia telephony service (MMTel) enables the following services:


  • Real-time video
  • Text
  • File transfer
  • Sharing of pictures, audio and video clips

What is the most interesting with the MMTel are not all the services. You have the capability to start with the voice call, add the chat, video, add another call, share pictures and video and all these at the same time.

LTE networks today are in the most cases the relatively small islands in the big 2G/3G coverage areas. Outside these islands, where there is no LTE coverage, the mobile users need to have available services both for voice and internet access. This is the reason why the MMTel need to be deployed also in 2G and 3G networks.

Voice over LTE Multimedia telephony (MMTel) can be deployed in two ways:

  • SRVCC – Single Radio Voice Call Continuity
  • ICS – IMS centralized service

The continuity of the MMTel voice service, when you go from the LTE network to the LTE, can be achieved by the Singe Radio Voice Call Continuity (SRVCC).

IMS Centralized Services can be provided, if there is a case of classical CS network without MMTel capabilities.

IMS Centralized Services – ICS

IMS centralized services (ICS) enable the use of IP Multimedia Subsystem – IMS service in 3G networks when using circuit switched CS access for the bearer. ICS provides you the same user experience, as if you were using the LTE access. It enables the MMTel service over the old 3G network with the mobile terminal that supports MMTel/ICS.

Single Radio Voice Call Continuity – SRVCC

I will here show you the example of the Single Radio Voice Call Continuity. MMTel is deployed in the LTE and in the 2G/3G networks. One subscriber is connected to the LTE network and he has established a voice call over MMTel with the subscriber B in 2G/3G network. After call establishment, the Service Centralization and Continuity Application Server (SCC AS) controls the call between the two access domains – LTE and 2G/3G. With the SRVCC it is possible handover from the 2G/3G cell to the LTE cell.

Voice over LTE Summary

In this post I have shown you two ways of deploying the voice over LTE. CS fallback forwards the voice call to the 2G/3G network and MMTel enables the voice service in the LTE network.

If 2G/3G network support MMTel, then it is used the SRVCC or single radio voice call continuity. When 2G/3G network doesn’t support MMTel, then it is used the ICS – IMS centralized service.

Read more about the Voice over LTE in the article Ericsson Mobile Telephony Evolution with VoLTE.

About the 4G internet and what is the real 4G technology read in the article 4G Internet – There is still no Real 4G Technology on the Market.


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