Post on 26-Jan-2016
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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