Over the past one year, long term evolution (LTE) has witnessed significant growth. Network penetration has increased and subscriptions have cro­ssed 200 million. Although providing voice services through this all-IP technology has been a major challenge, now, with the growth in LTE coverage, operators are increasingly providing these services over their LTE networks.

Voice implementation may take different paths based on factors such as spectrum availability, operators’ voice strategies and market conditions. Hence, operators that have pure LTE-based networks will offer only voice over LTE (VoLTE) services (Reliance Jio Infocomm Limited for instance). Others, like Bharti Airtel, Vodafone India and Idea Cellular, which have multiple radio access technologies in their networks such as GSM, CDMA, WCDMA and LTE, have the option to provide voice services using either circuit switched fall back (CSFB) or VoLTE, or a combination of both.

Unlike CSFB, packet-based technologies (VoLTE) may not have a dedicated path for voice packets. Voice is a real-time service with limited scope for delay and thus requires a robust underlying radio network to ensure an optimal user experience. VoLTE requires end-to-end quality of service (QoS) to be supported at all layers, from the device through the radio network up to the core network, and including interaction with the IP multimedia subsystem (IMS) core.

To this end, the Telecom Regulatory Authority of India’s (TRAI) recent consultation paper, “Voice Services to LTE Users (including VoLTE and CSFB)”, seeks industry suggestions on identifying key performance indicators for ensuring QoS, and measuring and evaluating the same.

Voice service for LTE users via VoLTE

While VoLTE has the advantages of flexibility and evolution to provide better and more enriched communication services, it may suffer from variable latency and possible IP impairments or interoperability issues. These impairments can come from any part of the chain of subsystems from the radio network to application servers, including intermediate subsystems like packet core network, IMS and transport networks. VoLTE interoperability issues may also be related to the evolved packet system bearer set-up, proprietary simplification of session initiation protocol call flows, operator-specific IP headers, and different security configurations like IPsec and AKA. Second, as in the case of other mobile technologies, there can be several instances of voice call drops/session drops or call set-up delays in LTE too. To overcome these problems, various timers (such as T304, T301 and T311) and constants (such as N310 and N311) have been defin­ed by 3GPP. Optimisation of such timers and constants is essential from the user experience perspective as inappropriate value configuration of these in the network may lead to situations like frequent call drops, longer call set-up delay, silence periods or voice muting.

Voice service for LTE users via CSFB

CSFB is used to provide voice services to LTE users without IMS by redirecting the user device to 2G or 3G networks in the case of mobile originated or terminated voice calls. However, there are several issues and concerns when offering voice service via CSFB. A delay in the call set-up can be experienced from the time an extended service request message is sent until the message alert is received. Ano­ther issue is how fast users can camp back to LTE after the voice call is released. This is network deployment specific and a cell reselection procedure or a network-based fast return to LTE may be deployed. Further, CSFB requires additional steps to set up a call and this causes extra delays.

The International Telecommunication Union has recommended some quality indicators such as registration success rate, post-dialling delay (PDD), voice quality and call drop rate, which can reflect the poor customer experience. Meanwhile, some of the parameters identified by the GSMA for CSFB are set-up time tele­phony (equivalent to PDD), CSFB return to LTE success ratio, speech quality on call basis, and call completion ratio circuit switched telephony.

Issues for consultation

  • Whether the prescribed QoS parameters, as per the existing regulations, are sufficient to effectively monitor the QoS of VoLTE/CSFB calls.
  • How to define an instance of silence/ voice mute and how many such instances may be acceptable during a voice call.
  • Whether existing parameters like packet loss, jitter, latency and end-to-end delay are sufficient to identify or measure silence/voice mute, or some other parameters need to be factored in.
  • Whether a certain range of timers and constants needs to be prescribed, which may affect the VoLTE call quality assessment.
  • Which parameters should be introduced to measure the performance of voice via CSFB.

Conclusion

TRAI’s consultation on fixing QoS norms for VoLTE is very timely as VoLTE is set to become the new voice platform in India. Going forward, VoLTE QoS monitoring and reporting at critical network points (end-user part/access part/core part) will help the regulator and service providers to troubleshoot the concerns related to the customer experience.