Voice over Wi-Fi (VoWiFi)

1. Cellular Coverage and VoWiFi

Despite progress from 1G through 4G LTE to 5G, uniform cellular coverage cannot be provided in every location. Radio waves are attenuated, reflected, and absorbed by obstacles, producing weak-signal areas or complete "dead zones" where a call is difficult or impossible.

This problem manifests in many common scenarios that users experience daily:

• Indoor Environments: Modern building materials like concrete, steel, and energy-efficient glass are very effective at blocking cellular signals. A device can therefore show strong service outdoors while a call degrades inside an office basement, a large shopping mall, or an underground subway station.
• Rural and Remote Areas: Building cell towers is expensive. In sparsely populated areas, the economic case for deploying dense cellular infrastructure is weak, leading to vast stretches of land with patchy or non-existent mobile coverage.
• Temporary Network Congestion: Even in areas with good coverage, a sudden influx of users, such as at a concert or a sporting event, can overwhelm the capacity of the local cell tower, leading to failed calls and slow data.

In many such locations another wireless network is available: Wi-Fi. VoWiFi, or Voice over Wi-Fi, carries the operator's voice service through WLAN when the operator, device, and access network support the feature.

2. VoWiFi as an Operator Service

Voice over Wi-Fi allows a user to make and receive calls for a mobile number through Wi-Fi instead of the cellular radio, when the operator provides the service. It differs from an over-the-top (OTT) app such as Skype, WhatsApp, or Facebook Messenger because it uses the operator's profile and IMS infrastructure.

• No Separate App Needed:On a supported device, VoWiFi is a system feature that works with the mobile operator's network. It uses the standard dialer and contact list, so a call can be made without an OTT application.
• Mobile Number: For a VoWiFi call, the recipient normally sees the mobile number assigned by the operator. An incoming call to that number can arrive over Wi-Fi or cellular radio, depending on the service configuration.
• Operator Service:The Wi-Fi segment can use best-effort delivery, but signaling and media are carried through the operator's protected infrastructure. The operator can apply its own QoS, authentication, and session rules.

VoWiFi uses Wi-Fi as non-3GPP accessto the operator's network, rather than as E-UTRAN. The phone and network select access according to operator policy and connection conditions, using LTE or WLAN.

A WLAN can be treated as trusted or untrusted by the EPC. With trusted WLAN access, a TWAN connects to the packet core through S2a, so the UE does not use the same UE-to-ePDG IPsec path. With untrusted WLAN access, the UE establishes an IPsec tunnel to the ePDG over SWu and the ePDG connects to the PGW through S2b. The operator profile decides which access model is available.

3. VoWiFi Architecture

VoWiFi is not a separate voice core. It extends the infrastructure used by VoLTE (Voice over LTE), especially IMS and EPC, with access through WLAN.

EPC and IMS Core

VoLTE calls are managed by a specialized network core called the IP Multimedia Subsystem (IMS), which uses the Session Initiation Protocol (SIP) for call signaling. Call routing, service features, and user authentication are handled in IMS.

VoWiFi uses the same IMS infrastructure for registration and signaling. From the IMS core's perspective the session is similar to VoLTE, but access travels through non-3GPP connectivity and, in the untrusted case, an IP path that may include the public internet. The architecture therefore adds a network element for this entry point.

The ePDG (Evolved Packet Data Gateway)

The ePDG is the gateway between untrusted non-3GPP access and the operator's Evolved Packet Core (EPC). It terminates the protected tunnel from the device and forwards traffic into the packet core.

The ePDG performs several functions:

• Security Termination:WLAN access outside the operator's trusted domain is treated as "untrusted." The phone and ePDG establish a tunnel that protects signaling and media, with the ePDG as the operator-side endpoint.
• IPsec Tunnel Establishment: Security uses the protocol suite called IPsec (Internet Protocol Security). The phone negotiates an IKEv2/IPsec tunnel with the operator's ePDG, and EAP-AKA messages can provide authentication. VoWiFi signaling and media then travel through the tunnel.
• Gateway to the Core Network: Once the traffic has been authenticated and decrypted at the ePDG, it is forwarded to the PGW (Packet Data Network Gateway), the same node that handles data traffic coming from the LTE radio network. From the PGW, the packets travel to the IMS core for call processing just as a VoLTE call would.

The phone selects an ePDG name using operator procedures, starts IKEv2, and carries EAP messages for authentication. After IPsec is established, the ePDG forwards the session to the PGW over S2b and then toward IMS. Separating WLAN access from the core lets the operator use the same IMS registration procedures as for VoLTE.

The S2b interface carries the user-plane bearer between the ePDG and PGW. In a GTP-based deployment this bearer is a GTP tunnel, while other S2b variants use their specified mobility protocol. IMS continues to handle registration and SIP session control. The access protection does not control radio conditions inside the WLAN.

The Wi-Fi radio remains outside the operator's radio control. Delay, jitter, and packet loss between the device and access point can therefore affect a call even when the IPsec tunnel and EPC bearer are functioning correctly.

The operator can also apply admission rules and service policies before a voice bearer is created. These rules determine whether the device stays on WLAN or returns to cellular access when the Wi-Fi path no longer meets the profile.

4. Handover between Wi-Fi and LTE

Profiles that support mobility can hand over a call between VoWiFi and VoLTE. The procedure depends on the operator, device, and radio conditions, so a change of access is not guaranteed to be interruption-free.

Wi-Fi to LTE Handover

A representative scenario is a VoWiFi call at home followed by movement outside the range of the local Wi-Fi network.

1. The phone continuously monitors the strength and quality of the Wi-Fi signal and available cellular LTE signals.
2. As the device moves away from the access point, it detects a weakening Wi-Fi signal and the availability of a stable LTE signal.
3. Based on operator policies, the phone software determines whether a handover is needed to maintain call quality.
4. The phone signals the network over the existing Wi-Fi connection. The network prepares the LTE radio channel and the required QCI 1 voice bearer.
5. Once the LTE path is ready, the network moves the voice packet (RTP) stream from the Wi-Fi/IPsec tunnel to the LTE radio bearer according to the operator procedure.
6. The handover time depends on signaling, authentication, and channel conditions. A prepared procedure limits the interruption, but it cannot guarantee that no interruption occurs.

LTE to Wi-Fi Handover

In the other direction, the phone can prepare Wi-Fi access during a VoLTE call. For example, a user enters an office building where LTE coverage weakens while the Wi-Fi network has suitable quality.

1. The phone, already connected to the office Wi-Fi for data, detects that this access has sufficient quality while the LTE signal deteriorates inside the building.
2. The phone determines that switching to Wi-Fi would provide a better-quality call.
3. It autonomously establishes a secure IPsec tunnel to the ePDG over the Wi-Fi connection in the background, all while the VoLTE call continues over LTE.
4. Once the secure VoWiFi path is ready and authenticated, the phone and network coordinate to switch the voice packet stream from the LTE bearer to the new Wi-Fi IPsec tunnel.
5. After the procedure completes, the unused LTE voice bearer can be released and the call can continue over Wi-Fi. If the procedure fails, the phone remains on the original access or ends the session according to operator policy.

5. VoWiFi Architecture Summary

VoWiFi combines WLAN access with the operator's voice service. It lets a phone use Wi-Fi where cellular radio coverage is insufficient while keeping the operator's IMS procedures.

Key architectural and functional takeaways include:

• Coverage Supplement: VoWiFi can carry voice indoors and in other weak-cellular areas when the Wi-Fi network and operator meet the service requirements.
• Phone Integration: In a supported configuration, the feature uses the native dialer and mobile number without a separate OTT application.
• Architectural Extension of VoLTE:It reuses the operator's existing EPC and IMS core, with the addition of the ePDG to act as a secure gateway for traffic from untrusted Wi-Fi networks.
• Security through IPsec: An IKEv2/IPsec tunnel between the phone and ePDG protects traffic from the Wi-Fi access network, including a public hotspot. The protection boundary is defined by the tunnel endpoints.
• Mobility: Operator profiles can hand over a call between LTE and Wi-Fi as access conditions change. The result depends on support for the procedure at both ends.

An operator that combines VoWiFi with VoLTE can route voice through LTE or WLAN while keeping a common IMS profile. Availability and call quality depend on coverage, delay, and packet loss on the selected access.