The Benefits and Challenges of VoWiFi Calling

The Benefits and Challenges of VoWiFi Calling

We have experienced for quite some time the offload of mobile data to WiFI. With VoWiFi calling, not only will users save money on international calling while traveling, but they will also experience the benefit of an enhanced call experience with the extension of VoLTE coverage to indoor. At the same time, VoWiFi calling also offers operators an opportunity to combat OTTs. With WiFi coverage, voice and messaging can be provided at lower costs, resulting in reduced spending for OTT. Additionally, unlike OTTs, operators can offer seamless call transfer support between Wi-Fi and LTE (using VoLTE) with specific device support.

Now think about how although there are a multitude of reasons why and when to introduce VoWifi, all operators will ultimately offer VoWiFi calling services in additon to their current solution offerings, such as CS or VoLTE. Some front-runners like T-Mobile and EE are offering the service as a way to attract more subscribers. Others, such as Sprint, are offering it as an alternative solution during the transition to VoLTE. Similar announcements have been made by Verizon and AT&T (US), Vodafone UK, and Rogers Canada. All benefits, however, come with a pricetag. Let’s take a look at a few of expected challenges of VoWiFi, how they may affect subscriber satisfaction, how they can be managed, and how we at Ascom address these challenges.

Today’s VoWiFi is possible due to 3GPP Release 12 mobility solution with IP address preservation between cellular and WiFi accesses, enabling real-time service delivery and consequently offering operators the opportunity of extending their voice service coverage when migrating from cellular to WiFi. Important challenges can be addressed by utilizing drive testing solutions on the device side, as well as passive and active monitoring at various interfaces and network elements (e.g. S2a interface with Trusted WLAN Access Gate or S2b with enhanced Packet Data Gateway for untrusted WiFi solutions) of WiFi and cellular networks,  and by identifying VoWiFi QoE problems that can be related to VoWiFi service accessibility, continuity, retainability and voice quality (MOS).

The overall VoWiFi service quality is first and foremost determined by an optimal cellular – WiFi integration secured by an intelligent network selection performed by the Access Network Discovery and Selection Function (ANDSF) entity. Due to its real time and high quality demands, the voice service case is much more sensitive to improper network selection, such as networks without comparative evaluation of the existing cellular or strong WiFi network, networks that are heavily loaded, networks with lower backend capabilities, or networks expressing ping-pong behavior), all which can result in the degradation of the user’s perceived voice QoE. Passive and active monitoring allow the detection and troubleshooting of improper network selections as well as their overall impact of perceived QoE if voice quality (MOS like) evaluation is enabled. VoWiFi service monitoring solutions help optimize the performance of various signaling procedures required by the IP address preservation-based service continuity towards high voice QoE. Monitoring solutions running MOS-like evaluations can also estimate the perceived impact of possible longer voice interruptions than the acceptable limits, as caused by signaling procedures affecting the duration of the handover to/from cellular/WiFi.

As the most sensitive service to real-time QoS/QoE continuity, VoWiFI requires that QoS admission parameters of trusted WiFi access to be mapped to cellular parameters for voice-dedicated traffic and default signaling barriers during the migration scenarios from/to WiFi/cellular or calls initiated on WiFi access. Inaccurate mapping at TWAG (or ePDG) and/or device-causing budget delays, packet error losses higher than the maximum acceptable thresholds, as well as annoyingly perceived variable delays generated by extensive additional signaling between TWAG (or ePDG) and devices can be accounted for using passive and active monitoring and troubleshooting. VoWiFi service quality is also determined by the performance of the UE’s VoWiFi clients, and therefore it is highly UE dependent. Poor performing error concealment algorithms implemented in VoWiFi client that are generated too frequently or for too long, with periods of voice stretching or compression, can result in annoying perceived voice quality. In addition, poor performing ANDSF UE-based algorithms using UE measurements of WLAN connectivity, as well as of cellular network conditions, can negatively impact the decision-making process of when to connect to the WiFi access network, therefore impacting the voice service continuity while migrating to/from WiFi/cellular.

Another factor impacting the handover decision is the device’s battery. In particular, scenarios with connectivity to untrusted WiFi access (when two levels of encryption are used) are prone to poorer measurements’ accuracy and consequently poor handover decisions. Monitoring and reporting information on the device’s battery status can be used in troubleshooting handover behavior.

Ascom’s solutions address passive/active monitoring and drive testing. Using Ascom’s solutions, operators can proactively optimize the cellular-integration WiFi as well as troubleshooting VoWiFI service problems towards high voice QOE. Drive testing solutions complete the end-to-end VoWiFi QoE evaluation by analzying problems related either to the device itself or to cellular or WiFi access.

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