Introduction to IP Multimedia Subsystem (IMS)

Background: LTE is a All IP network and hence, the voice need to be delivered using different way (Circuit Switched

voice not possible). 3GPP only specified the ingredient for IMS based VoLTE implementation. It is up to vendors/operator on

how to implement it. The VoLTE was not fully standardized. Alternatives emerged:

CSFB VoLGA IMS based

November 2009, One Voice initiative (By AT&T, Orange, Telefonica, TeliaSonera, Verizon, Vodafone, Alcatel-Lucent, Ericsson, Nokia Siemens Networks, Nokia, Samsung and Sony Ericsson)

Feb 2010, GSMA permanent reference document (IR.92) on IMS profile for voice and short message service (SMS)

RCS - a GSMA program for the creation of inter-operator communication services based on IMS (marketed by the GSMA under the brand name joyn™).

IMS Introduction: The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem (IMS) is an architectural

framework for delivering IP multimedia services. It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), as a part of the vision for evolving mobile networks beyond GSM.

By using a new, wideband codecs can provide higher voice quality and enhance the user experience (HD Voice) Wideband Codec: Frequency range 80 Hz to 14 kHz Narrowband Codec: Frequency range 300 Hz to 3.4 kHz

IMS Based VoLTE Architecture:

4 Main Parts: 1) VoLTE UE: LTE Capable, Embedded IMS Stack and IMS Application needed 2) RAN: E-UTRAN 3) Core Network: EPC 4) IMS CN: Service layer for Multimedia Telephony

Proxy – Call Session Control Function (P-CSCF): First point of contact for session signaling from

VoLTE UE. SIP Proxy: Forward SIP message between UE and

IMS Core Incorporate with PCRF for PCC Can be implemented in Access Session Border

Gateway (SBC) May be also incorporate with IMS-ALG and IMS-

AGW

Interrogating Call Session Control Function (I-CSCF): Contact point within operator's network for all

connections destined to a user of that network. On IMS registration, it interrogates the HSS to

determine which suitable S-CSCF to route the request for registration.

For mobile terminating calls, it interrogates the HSS to determine which S-CSCF the user is registered on.

Serving – Call Session Control Function (S-CSCF): Session set-up, session tear-down, session control

and routing functions; Generates records for billing; Queries the HSS for the applicable subscriber

profiles and handles calls (IFCs); Invokes Application Servers based on IFCs received

from the HSS; SIP registrar for VoLTE UEs that the HSS and I-CSCF

assign to it. *IFC - Initial Filter Criteria is an XML-based format used for describing control logic. Telephony Application Servers (TAS): Providing support for a minimum set of mandatory

Multimedia Telephony (MMTel) services (Examples: voice, real-time video, text, file transfer and sharing of pictures, audio and video clips).

Media Resource Function (MRF): Use by IMS AS and I/S-CSCFs, to provide media

plane processing Examples: multiparty conferencing, network

announcements/tones, etc. Interconnection Border Control Function/Transition Gateway (IBCF/TrGW): As the control (IBCF)/media (TrGW) plane at the

network interconnect point to other PMNs May be implemented in Interconnect SBC

Media Gateway Control Function / IMS Media Gateway (MGCF/IMS-MGW): As the control (MGCF)/media (GW) plane

interworking at the network interconnect point to circuit-switched networks

Breakout Gateway Control Function (BGCF): Determining the next hop for routing of SIP

messages for end users located outside of the IMS core (CS, other IMS);

For CS Domain terminations, the BGCF determines the network in which CS domain breakout is to occur and selects the appropriate MGCF;

For terminations in peer IMS networks, the BGCF selects the appropriate IBCF to handle the interconnect to the peer IMS domain.

Electronic Numbering (ENUM): This functionality enables translation of E.164

numbers to SIP URIs using DNS to enable message routing of IMS sessions.

*URI (Uniform Resource Identifier) name@domain.com

IP Exchange (IPX): for the exchange of IP based traffic between customers of separate mobile

IMS Based VoLTE: Call Flows

UE Send “SIP Register” to P-CSCF (Obtained during

attach procedure). The message will be

forwarded to I-CSCF.

UE Attach Procedure with LTE/EPC:

Default bearer to IMS-APN established and IP

address allocated

I-CSCF queries HSS for Authorization and select appropriate S-CSCF. “SIP

Request forward to S-CSCF.

UE <-> S-CSCF Registration:

• Exchange security procedure

• Bind UE to contact list

VoLTE UE originates voice call:

Sending “SIP INVITE” to P-CSCF

P-CSCF forwards “SIP INVITE” to S-CSCF (Identified during attach procedure)

S-CSCF routes the “SIP INVITE” to TAS or MRF

for supplementary services

S-CSCF queries ENUM/DNS

Route the “SIP INVITE” to I-

CSCF to determine the

S-CSCF of terminating

end

S-CSCF invoke BGCF to determine

the target MGCF to

break out to the CS

network.

Local VoLTE Call

MGCF/MGW performs

transcoding and

internetwork with CS

Network

S-CSCF invoke BGCF to determine

the target IBCF to break

out to the Peer IMS Network

IBCF controls one or more

TrGWs to provide

appropriate resources in the media

plane.

Peer Network VoLTE Call

CS Call

IMS Based VoLTE: Deployment Challenges

Integration, Upgrade or expansion of existing network elements to support IMS Based VoLTE: - Integration to CS Core - HSS - PCRF - HSS - Charging and Billing system Inter-Operator Interconnect: - Global ENUM needed - GSMA Pathfinder solution Limited LTE Coverage. Alternative solution needed for evolution: - CSFB - SRVCC Limited IMS VoLTE supported devices Customers are expecting richer functionality to match OTT Apps (Whatapps, WeChat, Facebook Messenger)

IMS Based VoLTE: Deployment Phases

2014 - 2015 2016 - 2018 Beyond 2018

IMS Based VoLTE: Early Adopters

Operator Country Region

HKT-PCCW Hong Kong Asia

3 HK Hong Kong Asia

CSL Hong Kong Asia

NTT DoCoMo Japan Asia

SingTel Singapore Asia

StarHub Singapore Asia

KT South Korea Asia

LG U+ South Korea Asia

SK Telecom South Korea Asia

AT&T USA Northern America

T-Mobile USA Northern America