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 8) 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.