End-to-end network requirements for 3GPP Voice over Wi-Fi (VoWiFi) service

4G mobile networks were the first to allow packet-switched voice calls, supported by an optional IP-based infrastructure called IP Multimedia Subsystem (IMS). These are part of the Voice over Long Term Evolution (VoLTE) service, called afterwards Voice over New Radio (VoNR) in 5G. In parallel, 3GPP defined the possibility of user terminals accessing either a 4G Evolved Packet Core (EPC) or a 5G Core (5GC) Network also via non-3GPP access. Non-3GPP access represents an alternative to the use of 4G/5G licensed frequencies of the MNO, leveraging Internet connectivity of an Internet Service Provider (ISP) to perform the same IP-based 4G/5G services, and in particular VoLTE/VoNR. Even though, especially for untrusted non-3GPP access, any IP-based access technology is applicable, the initial reference is Wi-Fi access, which is in practice available in all Smartphones. Consequently, the new 3GPP Voice over Wi-Fi (VoWiFi) service was introduced, also called commercially Wi-Fi Calling. VoWiFi is then subject to network conditions not fully under control of the Mobile Network Operator (MNO), not only related just to the Wi-Fi access but to the whole end-to-end data path across shared networks and the Internet. In this article, the VoWiFi technology and protocols are presented in detail, through a review of 3GPP specifications and by performing real calls using a national LTE MNO and a Xiaomi smartphone. The aim of the article is presenting the application traffic pattern and assessing the network conditions under which VoWiFi calls have good quality. These results are useful to define end-to-end Network requirements for assuring a satisfactory VoWiFi service to end users.