VoLTE – Voice Over LTE

Comprehensive VoLTE – Voice Over LTE description with details on end-to-end architecture, call handling, roaming and handover to UMTS and GSM.

LTE as its core is an all IP network in which data connectivity is given more importance than traditional voice and SMS services. But similar to its predecessors LTE eventually require a way to support CS and messaging services which is still used by majorities of the subscribers.

Over the last few years just after LTE specifications were introduced by 3GPP in Release 8, two completely different tracks have emerged in telecommunication industry to support CS and SMS. One of these is Circuit-switched Fall Back and other is IMS powered VoLTE. Both of these technologies are quite contradictory to each other as they solve the same problem with completely different approach.

What is Circuit-switched Fall Back?

Circuit-switched Fall Back solution enables LTE powered UEs to move to CS capable radio technologies like UMTS and GSM to start or receive voice call and remains in the CS domain till call continues. After CS call completes UE moves back immediately to LTE again. Depending upon the radio technology used UE can or cannot support simultaneous voice and packet services. For instance when UE falls back to UMTS from LTE because of voice call, it can continue to use PS service along with voice but in case when LTE capable UE falls back to GSM the PS barrier will be disconnected as long as voice call continues.

As CSBF supports both Voice and SMS, it is considered as the first step before complete LTE deployments.

What is VOLTE (Voice over LTE)?

VoLTE or Voice over Telephony is used to describe the GSMA specification for voice and SMS over IP network i.e. over LTE network and has its origins in the 3GPP IMS-based multimedia telephony (MMTel) solution. When VoLTE standardization work started a set of handful of telecom giants provided their support and created a solution which could use existing 3GPP specification to provide voice and SMS over IP network.

Since then VoLTE has widespread backing in the telecoms industry – more than 40 key players declared their support for it at the 2010 GSMA Mobile World Congress in Barcelona and many more have done so since.

Most of the VoLTE interface requirements came from 3GPP Release 8 but some of the features like emergency voice call over LTE were standardized in Release 9.

VoLTE Network Architecture End-to-End

VoLTE includes voice in full duplex, either in a one-to-one or one-to-many communication format.

VoLTE end-to-end architecture

When VoLTE capable UE is switched on it connects to the LTE/EP network and at the same time set the IMS APN (Access Point Name). IMS APN helps in roaming conditions as well as it is used for signalling bearer to use for SIP signalling. The mobile then initiates the IMS domain-registration process with its MMTel identifier and optionally SMS-over-IP identifier. During this process the device will be checked for authentication and security.

Registration and authentication with IMS

Call Handling

The INVITE signal contains a Session Description Protocol (SDP) that describes preferred media information such as which voice coding standard – Adaptive Multi-Rate Wideband (AMR-WB, used for HD voice) or Adaptive Multi-Rate Narrowband (AMR-NB) – IP addresses and ports to use. The IMS domain passes this information, via standardized interfaces, to the EPC’s Policy and Charging Enforcement Function (PCEF), which in turn performs QoS and charging-rules analysis. A typical outcome of this analysis might be to establish a dedicated EPC and data radio bearer– with a guaranteed bit rate for VoIP media – and to zero-out any data volume charging on this bearer.

Signaling bearer and a dedicated bearer for VoLTE

The LTE radio network uses admission control to ensure that there are sufficient resources and capacity for the network-requested voice bearer. When the bearer has been established, the VoIP packets sent using Real-time Transport Protocol (RTP) are downsized using Robust Header Compression (RoHC) and transported using Radio Link Control (RLC) Unacknowledged Mode (UM) to minimize packet size and thus increase coverage and capacity. Discontinuous Reception (DRX) provides prolonged talk time and battery longevity. End-to-end voice latency is on par with, or even better than, 2G/3G CS.

Cell Edge and Roaming Problems

When a VoLTE capable UE reaches the LTE cell boundary and the signal strength is not so great at that point to prevent VoLTE call from being dropped a handover mechanism is put in place to continue the call as CS call. This facility is called Single Radio Voice Call Continuity (SRVCC) mechanism.

Handover from packet-switched voice to circuit-switched voice

Although CS call still can be handled by facilities like CSFB, STVCC make sure that IMS remain in control during the whole duration of the call.

In case of roaming although the UE is in visitor network but still in case of the voice call the control of IMS services remains with the home operator and the MMTel application server serves the user from the home network also when roaming.

Power Consumption

When LTE and VoLTE where standardized years ago, great care was put into making the system and the service as energy-efficient as possible. Much of the media attention has been on first generation VoLTE implementations based on first generation LTE chipsets. Chipsets developed with a focus on time to market and on plain data services rather than more complex ones.

For “normal” output powers, the radio transmitter circuitry with its power amplifier dominates a smartphone’s power consumption during a voice call. Now, with conservative savings of 20% due to OFDM/multiband optimized circuitry and passives, 15% from SPS and 30% from DRX, a total reduction for the radio circuitry of around 50%. Assuming a conservative ratio of 1.5 between the power consumption of an “app-like” VoLTE client and a deeply embedded one, provides a baseband power saving of roughly 30%.

Comparing with the actual data processing power needs and the actual radio output powers of today’s 3G phones, there is definitely potential for VoLTE phones having significantly longer use time.

Another final interesting observation is that over-the-top services like Skype are forced to run in the power-hungry application processor (since they are apps) and have no help from advanced protocols; consequently they will be increasingly disadvantaged versus VoLTE. Something they are not today.

VoLTE Books and References


References and Specifications

  • GSMA IR.92 IMS Profile for Voice and SMS V4.0
  • GSMA IR.94 IMS Profile for Conversational Video
  • GSMA IR.88 LTE Roaming Guidelines
  • Analysis and Growth Forecasts for Mobile Messaging Markets Worldwide: 5th Edition


  • 3GPP TS 24.341, Support of SMS over IP networks; Stage 3


  • 3GPP TS 24.167 IMS Management Object (IMO); Stage 3


  • 3GPP TS 23.272, Circuit Switched (CS) fallback in Evolved Packet System (EPS); Stage 2


  1. An impressive share! I have just forwarded this onto a coworker who was doing a little research on this.
    And he actually bought me lunch due to the fact that
    I found it for him… lol. So let me reword this…. Thanks for the meal!!
    But yeah, thanx for spending time to discuss this matter here
    on your blog.

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