Voice over LTE – An Overview and Implementing Stages

Mobile broadband is exploding and LTE (Long Term Evolution) deployment is gaining momentum with mass-market reach expected by 2011. According to a white paper by Ericsson, GSMA VoLTE (Voice over LTE) is the preferred industry choice for mass-market voice services over LTE. It offers an optimal evolution path towards full multimedia services. GSMA VoLTE is based on existing 3GPP MMTel standards.

With the GSMA VoLTE solution, operators can easily evolve their services toward fully fledged multimedia. Users will benefit from richer multimedia services, available from any device, any place and any operator, combining mobility with service continuity. GSMA VoLTE will fully support the evolution of communication services as desired by operators and users, today and in the future.

Here is the summary from the Ericsson white paper.

LTE, the technology behind pre-4G, is rapidly gaining market following the success of mobile broadband services. The high speed broadband services however possess threat to traditional SMS and voice revenues, which represent almost 70% of mobile operator revenues. So finding a solution for maintaining and growing current revenue models, even after deploying high speed communications is critical to mobile operators.

In this context, an imminent need exists for incorporating global voice and messaging solutions in the new LTE platform also, which offers higher capacities and lower latencies. The technology should be made advantageous for both operators as well as the end users (subscribers).

The challenge is that in the new LTE Radio and Evolved Packet Core (EPC) architecture, circuit-switched (CS) domain is not available to handle voice calls as being done in 2G/3G. So a solution for voice over LTE is required. For this, four possibilities are being identified by the industry.

  1. Circuit Switched Fallback to 2G/3G CS
  2. IMS (IP Multimedia Subsystem) – based Multimedia Telephony (MMTel) over LTE
  3. Over-the-top (OTT) eg: Skype
  4. Circuit Switched over Packet Switched (CS over PS)

In the four tracks, Over the top (OTT) and MMTeL based tracks can offer permanent solutions for mature LTE networks. Here we take a look at each of the four technologies mentioned above.

Circuit Switched Fallback (CSFB)

Circuit Switched Fallback is ideal for early LTE deployment stages where no IMS (framework for delivering IP based multimedia services) voice service is available. CSFB allows the terminal to contact WCDMA/GSM to initiate or take a voice call. The Next Generation Mobile Networks (NGMN) Alliance recommends CSFB as the minimum roaming requirement for LTE terminal vendors and LTE operators who provide a CS voice service over WCDMA/GSM.

CSFB provides support for voice and SMS and is considered as an intermediate step towards fully fledged multimedia communication services.


The 3GPP MMTel offers the possibility for operators to incorporate multimedia features needed to compete with OTT solutions such as Skype and Google Talk. MMTel can take advantage of Mobile Subscriber Integrated Services Digital Network number (MSISDN) which is the world’s biggest mobile user community. MMTel also supports classic telecommunication values such as high quality, guaranteed end-to-end QoS, regulatory services support (eg: emergency calls) and global reach.

Over The Top (OTT)

The higher smartphone penetration is propelling the implementation of OTT solutions such as Skype and Google Talk. However, the performance of OTT solutions degrade in non-continuous LTE coverage areas and hence they have to depend on mobile broadband coverage. So to take full advantage of OTT for Voice communication, all operators should co-operate to consolidate a global voice over LTE solution.

Circuit Switched (CS) over Packet Switched (PS)

The best-known CS over PS alternative is VoLGA, specified by the VoLGA forum. VoLGA technology can enable mobile subscribers to receive consistent set of voice, SMS (and other circuit- switched) services as they transit between GSM, UMTS and LTE networks. However VoLGA is not standardized by 3GPP, the telecommunication association which provides technical specifications and technical reports for 3G mobile systems. VoLGA also lost traction in industry when GSMA adopted the One Voice initiative GSMA VoLTE.

GSMA VoLTE is a MMTel-based alternative, which is based on established standards and having support from more than 40 key players in the industry. So operators can easily evolve their voice services towards a rich multimedia offering for the growth of their communication business.

GSMA VoLTE will bring high quality voice and SMS service over LTE, and will also facilitate development of interconnect and international roaming agreements among LTE operators. The GSMA collaboration also widens industry support and brings economy of scale for terminals and network infrastructure for mass-market uptake.

Implementing stages of Voice over LTE

It is more likely that GSMA VoLTE will become the industry standard for voice and SMS over LTE. However to introduce GSMA VoLTE for Voice over LTE deployment, factors such as LTE coverage and deployment strategy are to be considered.

Operators having less LTE coverage area will probably deploy Circuit Switched Fallback (CSFB) as it can avoid frequent call handovers between the CS and VoIP domain. Operators who have more competitive LTE coverage plans are the ones who are likely to introduce GSMA VoLTE as it offers richer multimedia services along with mobile convergence.

For simplicity, the expansion plans for an entry level LTE operator can be classified in three categories

Phase 1 – Spotty LTE Coverage

During Phase 1 which can be called the first expansion phase, the operator starts by providing LTE dongles in laptops. These dongles are required to provide SMS support for configuration as well as to give information relating roaming cost alerts.

The non continous LTE coverage may prompt operator to use 3GPP SMS over SGs in initial stages. But once LTE smartphones are launched, they can introduce Circuit Switched Fallback which can reuse existing WCDMA/GSM CS voice services to initiate or take a voice call.

Phase 2 – Areas with Continuous LTE Coverage

Phase 2 involves LTE network expansion for covering big cities and densely populated areas. In this phase, operators can introduce GSMA VoLTE for LTE subscribers, thereby providing them with an IMS/MMTel-based service which supports full multimedia offerings and mobile convergence scenarios.

For LTE subscribers outside the LTE coverage area, MMTel voice service must be provided. The system should also support handover of MMTel calls from LTE to WCDMA/GSM when the subscriber moves out of LTE coverage area.

MMTel based voice can be called as an evolutionary step towards multimedia services that provides full service coverage even if LTE network is not yet perfected.

Phase 3 – Full LTE coverage phase

In phase 3, operators can complete LTE coverage or can introduce 3GPP/3GPP2 PS technology such as High Speed Packet Access (HSPA) or Evolution-Data Optimized (1xEVDO) for remaining areas not covered under LTE.


  • 1xEVDO – Evolution-Data Optimized
  • 3GPP – 3rd Generation Partnership Project
  • CS – Circuit Switched
  • HSPA – High Speed Packet Access
  • IMS – IP Multimedia Subsystem
  • MMTel – Multimedia Telephony
  • QoS – Quality of Service
  • UMA – Universal Mobile Access
  • UMA/GAN – UMA Generic Access Network
  • VoLGA – Voice over LTE via Generic Access
  • VoLTE – Voice over LTE
  • WCDMA – Wideband Code Division Multiple Access

[via 3G and 4G Wireless Blog]

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