VoLTE Deployment Evolution – CS only to Voice over IP in LTE Network

Mobile technology was developed in the beginning for the sole purpose to provide voice service on mobility. Since the beginning voice was the killer application for many years until data became the lead. Today data traffic is the driving force behind most of the new developments in mobile technology.

To provide better data throughput 3GPP offered LTE as the next generation mobile technology with only packet core. By creating a all IP network LTE completely removed the legacy circuit-switched part from it’s network architecture. But despite the fact data traffic is the major factor in today’s mobile network growth voice and other traditional circuit switched services like SMS still have a big role to play.

The primary goal for operators is to provide telco-grade voice services over a data-only LTE network. VoLTE is ready for widespread commercial deployment, operators are faced with the challenge of providing call continuity between LTE and legacy circuit-switched networks.

The deployment strategy for CDMA operators who adapted their next generation mobile network as LTE will be different than operators who migrated their network from legacy 3GPP network to LTE.

VoLTE Deployment Strategy

Simultaneous Voice and LTE (SVLTE)

When LTE is used with legacy 3GPP2 network, SVLTE uses two radios to simultaneously communicate with:

  • 1X circuit switched services like voice call, SMS and emergency call
  • LTE network to get better PS data throughput

Though this approach helps for rapid deployment but using two radio antenna is never a cost effective solution for mobile manufacturers. Also two radio will create some interference which can cause in high output power and in some cases may exceeds the permitted levels. High output power also has direct impact on battery life.

Circuit Switched Fallback (CSFB)

Circuit Switched Fallback is a feasible intermediate solution for early LTE deployment without VoLTE.

With CSFB when a voice call is started in LTE network the UE move to legacy systems like UMTS, GSM or even 1X to start voice call and remain there during the entire duration of the call. After the call is released UE moves back to LTE.

The biggest advantage of CSFB that unlike SVLTE only one antenna can be used. But with this advantage comes some drawbacks as well. When UE is in legacy network during the

voice call it does not have access to fast PS services and in some cases like in case of GSM the complete PS bearer is teared down. So user can not access to PS during the whole duration of voice call. Along with that sometime voice call setup takes more time than expected.

Single Radio Voice Call Continuity(SRVCC)

SRVCC allows a PS/IMS-based (VoLTE) Voice Call to transition to a legacy CS network. Unlike SVLTE and CSFB,SVRCC does enable call continuity. SRVCC uses a single radio, and allows an operator to provide ubiquitous voice coverage, even when LTE coverage is not complete.

However, the signaling required is complicated. The result is that there may be a brief break in audio service when the call is transitioning to the circuit-switched network.

Chapter #4 – VoLTE Call Flow and Procedures

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