Voice over LTE (VoLTE) & IMS: The Complete Technical Guide

Understand how the IP Multimedia Subsystem, an open architecture developed by the third generation partnership project, supports voice over IP and other IP services. Fixed and mobile networks interwork seamlessly.

Explore IMS features and layered architecture that enable scalability and flexibility through diverse application servers; IMS is access-agnostic, backward compatible with PSTN, and built on open standards with vendor-independent interfaces.

Explore the IMS architecture layers, including the application layer delivering service logic via application servers, the control layer managing sessions, and the transport layer connecting end devices with access technologies.

Explore the IMS control layer, the cornerstone that regulates call sessions via the call session control function, coordinates with the application server and HSS, and manages media resources and gateways.

Examine how the proxy call session control function is the first contact with the IMS network, routing SIP requests and ensuring QoS while blocking flooding attacks.

Explain how the interrogating call session control function routes registration requests to the HSS, checks registration status, and forwards calls to the appropriate serving CSCF.

Authenticate devices via HSS keys and register them in the IMS. Coordinate with the proxy call session control function and application servers to enable calls and generate call detail records.

Discover how application servers in the IMS network enable telephony features like call line hiding and call diversion, plus value-added services such as instant messaging, gaming, and video conferencing.

Explore the SIP, Diameter, and RTP protocols used in IMS, showing how SIP establishes calls, Diameter handles authentication and billing with the HSS, and RTP carries voice and video streams.

Explore the session initiation protocol as an application layer, flexible tool to create, modify, and terminate sessions, with SIP carrying SDP to negotiate IP addresses, port numbers, codecs, and bandwidth.

Explore sip services such as user location, user availability, and user capability negotiation via stp protocol during call setup, then examine session setup and session management signaling.

Explore SIP methods driving signaling in VoLTE, including invite to establish, acknowledge to confirm, by to end, cancel to abort, register to share location, and options to query capabilities.

Explain the IMS private identity (IMPI), used for IMS authentication, stored in the HSS and I-SIM, and presented as a network access identifier combining the IMSI and home network.

Explain IMPU as the caller ID linked to IMPI and stored in ISIM, enabling calls; outline SIP URI routing and conversion of a telephone number to a SIP URI.

Understand the service profile in voLTE and ims. See how impu's public identity ties to hss-stored profile downloaded to s-cscf at registration, detailing allowed services, application servers, and charging rules.

The SIP user agent resides in user equipment and transmits and receives SIP messages, enabling telephony; it acts as a client to IMS network and as a server when replying.

Explore IMS procedures for voice over LTE, detailing how the user equipment attaches to the 4G network, establishes data with an EPS bearer, and registers to make IMS calls.

Connect ims to the 4g core with a default eps bearer for signaling and a dedicated bearer for voice, using qci 5 for signaling and qci 1 for voice.

Explain how the 4g core and radio access network connect to the ims network, detailing sgi interface between the pdn gateway and p-csf, sip signaling, and hss/pcrf roles in diameter.

Explains IMS registration, where a user equipment obtains a P-CSCF address and a default EPSB bearer with QCI five, IP assignment via PDN gateway, an app, DHCP, and DNS translation.

Explain how the user equipment registers with the IMS network using SIP messages, diameter authentication with the HSS, and the serving call session control function, including profile data download.

Trace the end-to-end SIP signaling path for IMS call setup. Follow the INVITE through the P-CSCF, S-CSCF, I-CSCF to the HSS and back with 183 progress and SDP negotiation.

Describe end-to-end SIP signaling for IMS call setup, from invite to 200 OK, with STP offer/answer and PRAG, plus EPS bearer establishment via PCRF and PCEF.

Discover how the IMS media bearer tears down during call teardown, triggered by SIP by message and 200 OK, as PCRF and PCEF coordinate with the PDN gateway.

Explain how speech codecs encode and decode voice for voice over IP, compressing data to reduce IMS media bearer bandwidth with adaptive multirate codecs that switch based on link quality.

Explore how VoLTE codecs combine AMR and EVS to deliver voice from narrow to full band, with backward compatibility and robustness against delay jitter and packet loss.

Compare narrowband (300 Hz–3.4 kHz) to wideband (50 Hz–7 kHz), super wideband (50 Hz–14 kHz), and full band (20 Hz–20 kHz). Enable modern codecs to deliver high quality audio.

Explore adaptive multirate AMR codecs, narrowband (300 Hz to 3400 Hz) and wideband (50 Hz to 7 kHz), selecting bit rates based on channel conditions for robust voice.

The EVS codecs operate in interoperable mode for 2G/3G backward compatibility and primary mode for 4G/5G IMS calls, with nine bit rates, three ranges, and channel-aware adaptation.

IMS registration occurs via serving call session control function; MTAS/MMTEL provides advanced voice and video, while Service Centralization and Continuity Server preserves continuity during IMS roaming from 4G to 2G.

The serving call session control function triggers third party registration to application servers like MMTel using user's filter criteria, exchanging SIP messages and activating services after HSS data is retrieved.

An application server subscribes to the registration event package from the serving call session control function using a sip subscribe, receiving 200 ok and notify messages on registration changes.

Explore how VoLTE security rests on mutual authentication, ciphering, and integrity protection. Understand how the IMS ACA procedure enables mutual authentication and generates ciphering and integrity keys for IPSec.

Explore the IMS registration security flow in VoLTE, detailing private and public identities, IP address exchange, HSS interaction, and the authentication vector with RAND, XRES, AUTN, CK, IK.

Understand the media resource function (MRF) as the shared media engine that plays announcements, mixes conference streams, and transcodes codecs. Learn MRFC and MRFP roles coordinating via SIP signaling.

Showcases a VoLTE IMS scenario where an insufficient balance announcement is played after a SIP invite traverses the proxy and serving call session control functions, via MRFC and MRFP.

Show how a VoLTE/IMS network adds a participant to an active call by issuing a SIP invite through the proxy and serving CSCFs, coordinating MRFC and MRFP to establish conference.

Connects two IMS networks at the border to manage signaling. Uses an application layer gateway for SIP/SDP translation across IPv4 and IPv6, NAD translation, and a topology-hiding gateway with firewall.

Identify how the transition gateway manages the user plane between IMS networks by handling RTP media streams, IPv4 to IPv6 interworking, NAT and firewall rules, and QoS/bandwidth enforcement.

Describe IMS and PSTN interworking for voice calls, including breakout and break-in, and how BGCF, MGCF, and signaling gateway translate SIP and ISOOP across IP and circuit-switched domains with RTP.

Explain IMS to PSTN call setup via SIP invite through core call control, gateway control, and SS7 signaling to reserve an IP port and establish an RTP media path.

Explore the core functions of the MMTel application server, including session management for multimedia calls and supplementary services like conference calling, call waiting, and call forwarding with originating identification presentation.

Explain the MMTEL call flow: a SIP invite triggers the serving call session control function to query the MMTEL server for services, then the called side applies barring and forwarding.

Explore how a user equipment customizes services via the direct user interface (UTI), using USSD and XCAP to update MMTEL profiles and authorized features.

Explore how enum and dns route sip messages in volte by translating e.164 numbers to sip addresses and resolving destination host names to ip addresses.

Analyze the end-to-end call flow where ENUM and DNS translate the destination's telephone number to a SIP address, enabling routing via the proxy and serving call session control function.

Explore the access point name (APN) as a logical gateway inside the PDN gateway, anchoring EPS bearers and connecting devices to IMS and Internet networks via DNS-resolved IP addresses.

Explore how two APNs enable VoLTE: the IMS APN carries real-time signaling and voice traffic, while the UT APN manages supplementary services via the MMTEL application server.

Explore local breakout roaming in VoLTE, where signaling routes via IBCF and voice packets break out through transition gateway to home IMS, highlighting dual IMS networks and EPS bearers.

Explains the SA home routed architecture for VoLTE and how voice data is tunneled via the S8 interface using GTB (GPRS tunneling protocol) back to the home IMS network.

Outline the volte call flow: attach to the 4G network, set a default bearer with QCI 9, then IMS bearer QCI 5 and a dedicated voice bearer QCI 1.

Power up selects home or roaming network, builds an RRC connection with the eNodeB, performs LTE authentication, and updates location data via HSS and MME during the attach.

The MME selects a PDN gateway and sends a create session request through the serving gateway to establish the default EPS bearer, with PCRF deciding bearer creation via IPCAN signaling.

Examine how voice over LTE and IMS enable network registration and voice capability exchange, detailing voice domain preference and usage settings during attach and MME signaling.

Establish a default eps bearer with qci 5 for ims registration via a pdn connectivity request to the mme, targeting the ims apn and pcscf discovery.

Shows how the PDN gateway creates a dedicated EPS bearer (QCI=1) through SGW, MME, and eNodeB, attaching the S5/S8 and S1 tunnel endpoints and activating the data radio bearer.

Learn how single radio voice call continuity (SRVCC) hands over a 4G VoLTE call to 2G/3G when coverage drops, with the SCC server in the IMS network.

Explain the SRVCC handover of a VoLTE call from 4g to 2g/3g, including bearer splitting, measurement reports, SIP re-invite, session transfer number, and IMS signaling.

Explore why SRVCC causes high handover delay in VoLTE, due to the signaling anchor at the SCC server in the home network during session transfer and IMS bearer switching.

Learn how enhanced SRVCC for VoLTE uses ATCF as the signaling anchor and ATGW as the media anchor, with IMS registration and ATCF/MME assignment as pre-conditions.

Explore the enhanced SRVCC procedure for VoLTE and IMS, detailing handover from packet switch to circuit switched networks with EPS bearer, E-NodeB, ATCF, ATGW, and MME/MSE coordination.

Explore the evolution of SRVCC in LTE networks across 3GPP releases, from basic SRVCC to alerting and enhanced SRVCC. Learn how release 11 enables video SRVCC and reverse SRVCC.