VoLTE tutorial-Voice Over LTE basics and types

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This VoLTE tutorial covers VoLTE (Voice Over LTE) basics, types of VoLTE which include VoLTE using IMS(VOIP) and VoLTE using CSFB(SRVCC).

LTE is all IP based network and mainly designed for internet/data applications. Hence to support voice over LTE there are two options. The first is using VOIP(voice over IP) with the help of IMS(IP Multimedia system) and the the second one is using legacy 2G/3G Circuit Switched Fall Back(CSFB) with the concept called SRVCC. We will see below how these works.


VoLTE using VOIP through IMS

As shown in the figure above in LTE called EUTRAN comprised of more than one eNodeB. eNodeB in LTE has functionalities of RNC and NodeB of 3G. MME,S-GW,HSS and P-GW will form EPC(Evolved Packet Core). eNodeBs and EPC form EPS(Evolved Packet System). LTE supports voice call using VOIP using IMS. If it is VOIP to VOIP call, it will remain within LTE network and supported using VOIP protocols. But if it is VOIP to CS call for GSM/WCDMA network, IMS with the help of application servers and legacy MSC transfers the PS call to legacy networks (2G-GERAN/3G-UTRAN). If it is VOIP to PSTN call, IMS directs through PSTN interface to respective exchanges.

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


When UE moves from LTE network to say legacy networks(GSM/WCDMA), based on measurement report submitted by UE handover is performed and transfer of Voice over IP call to legacy voice call is performed. This to happens UE should support SRVCC. SRVCC stands for Single Radio Voice Call Continuity. It means UE will be connected with sigle radio at a time and hence one single RAT(either LTE or GSM or WCDMA). Hence battery life becomes longer here with SRVCC capable UE.

Useful Links

Other than this volte tutorial, refer wireless tutorials on various IEEE and 3gpp standards with links mentioned on left side panel.

3GPP 23.216 V8.6 SRVCC


3GPP references
LTE tutorial
LTE System Features

RF and Wireless Terminologies