Voice Over LTE is a collective term for any technology that enables voice call from LTE network. So everybody would mean a different specific technology when they say "Voice Over LTE".

The root of the problem is simple. LTE has no circuit switched domain. Every service in LTE runs over EPS bearers in the packet switched domain, and voice is no exception. But in the first years of LTE, every operator already had a CS network for voice and no IMS network. So each operator needed a way to give voice to an LTE device, and the technologies below are the different answers to that question.

• Which technologies can carry voice for an LTE device ?
• Which technology should an operator use ?
• Reference

Which technologies can carry voice for an LTE device ?

The four technologies differ in one main point. Some of them move the device out of LTE for the call, and some of them keep the call inside LTE. Keep that point in mind while you read the list, because it decides the call setup time and whether data continues during the call.

I am listing up several technologies which are refered to most commonly.

CS Fallback, or CSFB, keeps voice in the legacy CS network. The UE performs a combined EPS/IMSI attach, so the MME also registers the UE in the MSC over the SGs interface. For a mobile originated call, the UE sends an Extended Service Request to the MME. The network then moves the UE to UTRAN or GERAN, and the UE sets up the call there in the normal CS way. For a mobile terminated call, the MSC pages the UE through the MME over SGs. TS 23.272 specifies CSFB, including a variant that falls back to CDMA2000 1xRTT.

SVLTE takes a different path. The device has two radios that transmit at the same time. One radio stays on LTE for data and the other stays on CDMA2000 1x for voice. The network does not need any interworking, so SVLTE worked early on CDMA networks. The cost is on the device side, with two active radios, a higher power consumption and a more complex RF design.

VoLGA reuses the GAN architecture that was defined for WiFi access to the CS core. A VoLGA access network controller sits between the LTE packet core and the MSC. It carries CS signalling and voice over an LTE bearer, so the MSC sees the UE as if it were on a GAN. An industry forum promoted VoLGA, but it did not become a 3GPP work item and it was not deployed widely.

VoLTE over IMS is the only one of the four that is fully packet based. The UE registers in IMS and sets up the call with SIP. The voice travels as RTP packets on a dedicated EPS bearer with QCI 1. When the UE leaves LTE coverage during the call, SRVCC in TS 23.216 hands the call over to the CS domain of UTRAN or GERAN. The MME triggers this through the Sv reference point to an MSC Server enhanced for SRVCC.

• CSFB and SVLTE use the CS core for the call : CSFB moves the UE out of LTE, and SVLTE keeps a second radio on 1x.
• VoLGA carries CS voice over LTE : it needed a new network node and never became a 3GPP work item.
• VoLTE keeps the call in LTE : SIP signalling in IMS, RTP voice on a QCI 1 bearer.
• SRVCC protects a VoLTE call at the LTE coverage edge : the call continues in the CS domain after a handover.

Which technology should an operator use ?

The answer depends on when you ask. The paragraphs below give the view from June 2012, when LTE was new and no operator ran VoLTE yet. After them, you will find what changed in the following years.

Then your question would be which technology should we use ?

There is no solid answer to this question, but one thing for sure would be that the final goal in LTE would be to implement/deploy "VoLTE over IMS".

Are there any network operator who already deployed VoLTE over IMS as of now (June 2012) ? As far as I know, NO. (Please let me know if you see any operator who already started serving this).

As far as I know as of now (June 2012), one of the system operator who is based on C2K network and recently deployed LTE is using SVLTE and most of WCDMA based network operator who just started LTE service are using CS Fallback.

The situation changed only two months later. In August 2012, the first commercial VoLTE services started in South Korea and in the United States. Since then, VoLTE has become the normal voice service for LTE, and CSFB has become the fallback for devices and networks without VoLTE.

Two developments made VoLTE necessary rather than optional. First, many operators have switched off their 2G and 3G networks. CSFB needs a UTRAN or GERAN cell to fall back to, so without a legacy network CSFB has nowhere to go. Second, 5G reuses the same IMS voice architecture. A 5G device makes voice calls with IMS over NR, or the network moves it to LTE for a VoLTE call. Either way, the operator needs a working IMS voice service.

CSFB still has a cost that you can measure in any log. The UE must leave LTE, find a UTRAN or GERAN cell and set up the CS call there. So the call setup takes longer than a VoLTE call, and data usually slows down or stops during the call. After the call, the UE has to return to LTE. A VoLTE call keeps the UE in LTE the whole time.

• In 2012 there was no clear answer : CSFB and SVLTE were the deployed options, and VoLTE was the stated final goal.
• VoLTE became the answer : commercial VoLTE started in August 2012 and became the normal LTE voice service.
• CSFB needs a legacy network : when 2G and 3G are switched off, VoLTE is the only voice option left for an LTE UE.
• CSFB costs time : the move to UTRAN or GERAN adds to the call setup time and interrupts LTE data.

Reference

[1] 3GPP TS 23.272 v20.0.0 - Circuit Switched Fallback in Evolved Packet System, clauses 4 and 6, Annex B

[2] 3GPP TS 23.216 v19.0.0 - Single Radio Voice Call Continuity, clause 4