<p>The LTE standard only supports packet switching with its all-IP network. Voice calls in GSM, UMTS and CDMA2000 are <a title="Circuit switching" href="http://en.wikipedia.org/wiki/Circuit_switching">circuit switched</a>, so with the adoption of LTE, carriers will have to re-engineer their voice call network.Three different approaches sprang up:</p>
<p><img src="images/400px-LTE-CSFB-E-UTRAN-UTRAN-GERAN-Interfaces.svg_.png" alt="" /></p>
<ul>
<li>VoLTE (Voice Over LTE): This approach is based on the <a title="IP Multimedia Subsystem" href="http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem">IP Multimedia Subsystem</a> (IMS) network, with specific profiles for control and media planes of  voice service on LTE defined by GSMA in PRD IR.92. This approach results  in the voice service (control and media planes) being delivered as data  flows within the LTE data bearer. This means that there is no  dependency on (or ultimately, requirement for) the legacy Circuit Switch  voice network to be maintained.</li>
</ul>
<ul>
<li>CSFB (Circuit Switched Fallback): In this approach, LTE just  provides data services, and when a voice call is to be initiated or  received, it will fall back to the CS domain. When using this solution,  operators just need to upgrade the MSC instead of deploying the IMS, and  therefore, can provide services quickly. However, the disadvantage is  longer call setup delay.</li>
</ul>
<ul>
<li>SVLTE (Simultaneous Voice and LTE): In this approach, the handset  works simultaneously in the LTE and CS modes, with the LTE mode  providing data services and the CS mode providing the voice service.  This is a solution solely based on the handset, which does not have  special requirements on the network and does not require the deployment  of IMS either. The disadvantage of this solution is that the phone can  become expensive with high power consumption.</li>
</ul>