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What is Voice over Wi-Fi (VoWiFi)?

Voice over Wi-Fi (VoWiFi) is a term typically employed to describe the delivery of commercial telephony services using Voice over IP (VoIP) technologies from mobile devices connected across Wi-Fi. This is typically counter to alternatives, predominantly Voice over LTE (VoLTE), in which a mobile network operator’s (MNO’s) licensed spectrum (i.e. 4G LTE) is used to carry packetized voice. Broadly speaking, VoWiFi terminology is assigned to all core IMS services accessed from unlicensed spectrum and across untrusted access infrastructures, such as public Wi-Fi access points. VoWiFi is used synonymously to describe the GSMAs permanent reference document IR.51: IMS Profile for Voice, Video and SMS over untrusted Wi-Fi access.

While over-the-top (OTT) mobile communications services may also employ Wi-Fi, where possible, to prevent using a subscriber’s (licensed) data plan, this is not typically referred to as VoWiFi as these services are not tightly integrated with a MNOs global service offering and infrastructure.  

Employing VoWiFi enables MNOs (or MVNOs) to quickly and easily extend their coverage or service range without costly radio access network (RAN) infrastructure build-outs, investing in new licensed spectrum or engaging in complex roaming agreements. Following the 3GPP reference architecture defined within the 3GPP’s TS 23.402 (Architecture enhancements for non-3GPP [IP] accesses), connectivity from the mobile handset to the evolved packet core (EPC) is over the S2b reference interface via an enhanced packet data gateway (ePDG) function. The specification covers policy control, charging, authentication and handover between untrusted and trusted access infrastructures for both roaming and non-roaming scenarios.


TS-23-402 architecture enhancements for non-3GPP (non-roaming) access

Outining a profile for supporting voice, video and SMS, IR.51 extends the requirements, detailed within IR.92 (Voice over LTE / VoLTE) and IR.94 (Video over LTE / ViLTE) to include untrusted Wi-Fi access. The PRD also stipulates the use of (dual-radio) circuit switch (CS) call continuity and fallback (VCC), per 3GPP TS 23.237. IR.51 also specifies the basic Wi-Fi radio, IMS and EPC capabilities along with describing functionality that is applicable across the protocol stack and subsystems.  The VoWiFi PRD also identifies the necessary information a mobile device requires to connect to, and obtain services from, an IMS core. Furthermore, the specification outlines the subset of required telephony supplementary services and the supported

What is Voice over LTE (VoLTE)?

Outlined in GSMA standards, Voice over Long Term Evolution (Voice over LTE or VoLTE) is the practice of packetizing voice over the Internet Protocol (VoIP) and transporting both the signaling and media components over a 4G LTE packet switched (PS) data path. This is in contrast to delivering voice using circuit switch (CS) methodologies, which requires 4G handsets to employ a secondary 3G radio and network operators to continue supporting this inefficient access infrastructure and licensed spectrum.

Originating in 2010 and currently at version 9.0, the GSM Association’s Permanent Reference Document (PRD) IR.92 describes the application of the IP Multimedia Subsystem (IMS) for the transport of voice and short message service (SMS) traffic (SM-over-IP).  IR.92 identifies a minimum subset of mandatory 3GPP (release 😎 standard features that a mobile endpoints — or user equipment (UE)  are required to implement in order to guarantee interoperability in all-packet infrastructures. Along with defining basic IMS capabilities and the requirements of the Radio Access Network (RAN) and the Evolved Packet Core (EPC), this includes the 19 mandatory supplementary services which must be supported by the signaling core and the Telephony Application Server (TAS).

In addition, IR.92 defines the process by which calls fallback to circuit switched voice in the event that the UE roams out of a 4G LTE coverage area.


GSMA PRD IR.92 Depiction of UE and Network Protocol Stacks in IMS Profile for Voice

With an IMS core as its foundation, VoLTE employs the Session Initiation Protocol (SIP) for registration, authentication addressing, call establishment and call termination. The Session Description Protocol (SDP) is employed for RTP media and bandwidth negotiation. As a conversational voice service, VoLTE demands the use of the LTE QoS Class Identifier (QCI) number 1, which provides a guaranteed bit rate (GBR) with sub 100ms packet delay and an error rate not exceeding 10-2. The SIP MESSAGE header is also employed to support SM-over-IP, with IR.92 referencing the applicable 3GPP specifications.

IR.92 and VoLTE is often used synonymously with IR.94 and ViLTE, which is the GSMA PRD describing the transport of conversational video over IMS. There are also nine additional PRDs which are related to VoLTE.

VoIP - Voice over Internet Protocol

Voice over Internet Protocol is a category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls by sending voice data in packets using IP rather than by traditional circuit transmissions of the PSTN.

Advantages of VoIP

One advantage of VoIP is that the telephone calls over the Internet do not incur a surcharge beyond what the user is paying for Internet access, much in the same way that the user doesn't pay for sending individual emails over the Internet.There are many Internet telephony applications available. Some, like CoolTalk and NetMeeting, come bundled with popular Web browsers. Others are stand-alone products. VoIP also is referred to as Internet telephonyIP telephony, or Voice over the Internet (VOI)