Voice Over LTE tutorial: Features Approaches CSFB Volga MMTEL OTT

Introduction to Voice Over LTE

You probably own a new 4G LTE smart phone like Driod, HTC thunderbolt, Goolge Nexus but do you realize that voice service on it might still be 3G. This Voice Over LTE tutorial explains various mechanisms used to deliver voice over LTE networks today and in future.

LTE features that help explain Voice over LTE implementation

  • The LTE technology is designed for IP-based services.
  • It has no support for dedicated channels for voice calls (as used in circuit switching technology).
  • It is a packet-only access technology with really no connection to the circuit-switched mobile core network. This makes it quite different from  GSM, WCDMA and CDMA technologies that support both circuit-switched and packet-switched services.


Voice Over LTE Approach

Voice Over LTE tutorial: Features Approaches CSFB Volga MMTEL OTTIn the initial stages of LTE deployment CSPs cannot provide the Quality of Service (QoS) necessary for voice service. Therefore the adoption of LTE for voice will happen in a few stages rather than all at once. There are four approaches that are discussed for voice service support :

Circuit Switched Fallback (CSFB) (An interim solution until full voice over LTE support)

CSFB or Circuit Switched Fallback , which provides support for voice and SMS, is the 3GPP standard solution for early LTE deployment stages. Through CSFB, the terminal is directed to GSM, WCDMA and CDMA technologies to initiate or take a voice call.

CSFB or Circuit Switched Fallback features

  • In CSFB the call remains in the CS domain until it is completed. So it is not truly a voice Over LTE implementation.
  • IMS is not part of the solution and voice calls are not even served over LTE.
  • CSFB solution relies on a temporary inter-system that switches between LTE and a system where CS voice calls can be served.
  • Solution is based on the fact that LTE terminals register in the circuit switched domain when powered.
  • This solution for Voice Over LTE support is currently recommended by the Next Generation Mobile Networks (NGMN) Alliance as a minimum roaming requirement for LTE terminal vendors and LTE operators that provide a CS voice service over GSM, WCDMA and CDMA.
  • This will be phase one deployment however it will probably be retained in later phases (after IMS voice is introduced), because CSFB is the industry choice for supporting inbound LTE roamers without IMS services or without IMS roaming capabilities.

Voice over LTE via Generic Access (VoLGA) – A Circuit Switched over Packet Switched paradigm ( An interim solution until full voice over LTE support)

VoLGA is an alternative in the CSFB and hence not a true voice over LTE implementation. VoLGA makes traditional GSM/UMTS circuit switched (CS) services available to UEs accessing the EPS network via E-UTRAN.

Voice over LTE via Generic Access (VoLGA) features

  • It has very low impact on the existing CS core, but does not offer the possibility of evolving toward a full multimedia service experience.
  • The MMTel-based alternative has been chosen by GSMA as the unified solution for voice over LTE  and  SMS over LTE, termed GSMA VoLTE, and has wide backing in the industry. GSMA VoLTE intends to bring interoperable, high quality SMS and Voice over LTE.
  • The VoLGA solution is not standardized by 3GPP, and after GSMA adopted the One Voice initiative for GSMA VoLTE, VoLGA has been constantly loosing traction in the industry.

Voice Over LTE via Over-the-top (OTT) applications

OTT involves solutions like Google Talk and Skype. Penetration of these services are expected to increase and this will increase greater competition. Currently OTT solutions cannot deliver a good user experience in current non-continuous LTE coverage. Therefore the adoption of OTT services will rely on extent of broadband coverage on smart phones in coming future.

Voice Over LTE via IMS infrastructure : IMS Multimedia Telephony Service (MMTel) applications

The 3GPP MMTel solution gives operators the possibility to evolve their telephony service by incorporating the multimedia feature-richness needed to compete with OTTs. This allows a true voice Over LTE implementation when compared to other interim voice over LTE approaches listed above.

IMS Multimedia Telephony Service (MMTel) features

  • Even though MMTel has been designed to support voice calls using IMS, MMTel can provide more than telephony in that other media like video can be added to the voice call, turning this into a complete multimedia session rather than just Voice Over LTE solution.
  • It can provide high-quality, guaranteed end-to-end QoS along with regulatory services support (such as emergency calls). The latter may not be an option with OTTs.

List of supplementary services for MMTel include :

  • Communications Diversion (CDIV)
  • Communication Hold (HOLD)
  • Conference (CONF)
  • Message Waiting Indication (MWI)
  • Originating Identity Presentation (OIP) / Originating Identity Restriction (OIR)
  • Terminating Identity Presentation (TIP) / Terminating Identity Restriction (TIR)
  • Call Barring (CB)
  • Explicit Call Transfer (ECT)
  • Communication Wait (CW)
  • Completion of Communications to Busy Subscriber (CCBS)
  • Completion of Communications by No Reply (CCNR)
  • Malicious Communication Identification (MCID)
  • Customized Alerting Tone (CAT)
  • Closed User Group (CUG)
  • Personal Network Management (PNM)
  • Customized Ringing Signal (CRS)
  • Reverse Charging
  • Advice of Charge (AoC)

In future operators would also need to provide:

  1. An MMTel voice service to LTE subscribers outside LTE coverage areas.
  2. Handover of MMTel calls from LTE to WCDMA,GSM when subscribers move out of LTE coverage.

This functionality is achieved via IMS Centralized Services and Single Radio Voice Call Continuity (SRVCC) when roaming.

Refer 3GPP TS32.275 for MMTel charging for online and offline services.

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