VoWiFi & WiFi Calling: Architecture, Call Flows & IMS Core

Trace the shift from 2G/3G circuit-switched calls to VoLTE over 4G and VoWiFi, and compare coverage, reliability, and quality across legacy, VoLTE, and VoWiFi.

Compare VoWiFi, VoLTE, and 3G voice quality and performance, emphasizing wifi stability, router power, and distance factors. Note that VoLTE and VoWiFi support HD calls, while 3G does not.

Compare VoWiFi, VoLTE, and 3G by coverage and availability: VoWiFi works where cellular signals are weak, VoLTE requires 4G, and 3G offers the broadest coverage.

Compare voice over wifi architecture with legacy 3g and volte: wifi router to epdg and ims via proxy cscf, versus enode b in 4g and rnc in 3g.

Identify the prerequisites for voice over WiFi: a compatible device with supported iOS or Android versions, a reliable IPv6 WiFi network with QoS, and carrier, SIM, and provisioning readiness.

Compare volte and wifi calling, detailing sip invite flow through epc, gateway, and ims with cscf and pgw; explain when to use wifi calling with home wifi.

Improve indoor coverage and call quality with voice over Wi‑Fi, enabling calls where cellular coverage is weak, while offloading network load and enabling seamless Wi‑Fi to VoLTE handover.

Explore VoWiFi deployment challenges: quality of service, handover from wifi to VoLTE based on receive signal strength, capacity planning, and end-to-end security with IPsec.

Examine voice over Wi-Fi architecture by contrasting 3GPP access through the E node B with non-3GPP Wi-Fi access, and distinguish trusted versus untrusted Wi-Fi provided by different operators.

Outline the UE requirements for voice over Wi-Fi, including an active IMS SIM, 4G VoLTE support, LTE and Wi-Fi radio features, and SIP codecs for seamless IMS calls.

Explore how the wifi access point forms the first connection layer in VoWiFi, addressing untrusted entry points and securing data with an IPsec tunnel to the EPG.

The ePDG secures voice over Wi-Fi with an IPsec tunnel between the user and core, authenticating via AAA and HSS on an untrusted network.

Explore how IPSec encrypts and ensures mutual authentication between user equipment and the mobile core for VoWiFi signaling and traffic over untrusted Wi-Fi.

Describe how an IPsec tunnel is established between the user and the EPG through mutual authentication, security proposals, and negotiated encryption of headers and payload for voice over wi-fi.

The packet gateway serves as the anchor between packet core and the IMS network, handling signaling and voice traffic for VoWiFi, allocating IP addresses and enabling session continuity during handovers.

Discover how the PCRF governs IMS traffic in VoWiFi by establishing default and dedicated bearers, triggering QoS parameters, and coordinating with the PGW for secure IPsec tunnels.

Explore how the AAA server handles authentication, authorization, and accounting for voice over wifi, using IMSI verification against the HSS and billing via CDRs.

Operate the P-CSCF as the first IMS entry point, routing SIP signaling, handling registration, and guiding toward interrogating or serving CSCFs for VoWiFi call setup with QoS control.

Explore how the I-CSCF in IMS locates the appropriate serving CSCF and enables inter-operator SIP signaling for VoWiFi, leveraging HSS queries and cross-network routing.

The serving cscf acts as a registrar and authenticates IMS users, coordinating with MTAS to determine originating and terminating services and whether voice calls are allowed.

The home subscriber server (HSS) is the master database for user profiles, handling AAA authentication, profile management, and voice over wifi service authorization and roaming decisions.

Differentiate EMS (IMSI) and the SIP URI for identity handling in VoWiFi, showing how SIP URI registers to the IMS network and targets calls through CSCF and HSS.

Explain how a fully qualified domain name (FQDN) enables DNS-based routing for VoWiFi, guiding EPG discovery, IPsec establishment, and IMS SIP registration via DNS and proxy CSF.

Explore how the APN enables voice over Wi-Fi by provisioning an IMS APN from the PGW, granting an IMS IP and enabling IMS registration and calls.

Explore how VoWiFi bearers transfer signaling and voice traffic via an IPsec tunnel, with the default bearer (qci 5) for IMS signaling and the dedicated bearer (qci 1) for voice.

Understand how voice over wifi secures signaling with a mandatory IPsec tunnel, established after wifi connection and EAP-AKA authentication using IKEv2 messages, leading to IP allocation and IMS access.

Establish the ipsec tunnel and create the default bearer to enable sip registration and ims core access via the pgw and edg, with qos on the ims apn.

Establish a dedicated bearer for VoWiFi calls to guarantee QoS, with QCI 1, minimal delay, jitter, and packet loss, triggered by SIP invite signaling.

Explore how the evolved packet gateway maps default and dedicated bearers through the ipsec tunnel, using qci, tft, and s2b interfaces, to route voice and signaling traffic.

Compare VoLTE and VoWiFi bearers by architecture and tunnel use, detailing QCI 5 signaling, QCI 1 voice, QCI 2 video, and IPsec tunnels in VoWiFi.

Understand voice over wifi interfaces and protocols, where interfaces are links between nodes and gps tunneling protocol, diameter, and ikev2 govern connections among hss, pgw, aaa, and the user.

Explore the swu interface in vo wifi, establishing an ipsec tunnel with ikev2 between user equipment and epg on untrusted networks for ims signaling and voice traffic.

Explain the S-2b interface between the PPG and EPG, detailing GTP-C/U roles for signaling and user traffic, IPsec tunneling, SIP registration, and default and dedicated bearers for IMS voice.

Explain how the S interface uses the diameter protocol to exchange authentication signals between EPG and AAA, and how SWx, MA, SAR, and S6b support handover with the HSS.

Learn how a VoWiFi user discovers the ePDG through a multi-level DNS path and selects an EPG to initiate IPsec security with the internet key exchange version two.

Explore the detailed ePDG discovery signaling flow in VoWiFi, tracing DNS lookups from the user’s ISP DNS to root DNS, operator DNS, and EPG selection for authentication.

Describe the initial UE to ePDG authentication and IPsec tunnel setup, detailing EAP exchanges, AAA and Diameter interactions, IMS APN authentication, and default bearer creation.

Explains the mobile-originated VoWiFi call signaling flow from handset to IMS, detailing the SIP invite path, IPsec tunnels, and key nodes like EPG, EPC, proxy CSCF, BGF, and DNS-based routing.

Explain the mobile-originated VoWiFi call flow on M0. Trace how the interrogating cscf selects the serving cscf via DNS and advances 183 session progress to 180 ringing and bearers.