IP MULTIMEDIA SUBSYSTEM
LTE Evolved Packet Core Network
© Wray Castle Limited A.i LT3604/v4.0
LTE Evolved Packet Core Network
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CONTENTS CONTENTS
Appendix
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© Wray Castle Limited LT3604/v4.0
The Role of the IMS . . . A.1 Accessing the IM CN Subsystem . . . A.2 Introduction to the IMS Architecture. . . A.3 Call Session Control Functions .. . . A.4
P-CSCF ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... A.5 I-CSCF ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . A.6 S-CSCF ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... A.7 IM CN Subsystem Entities . . . A.8 SIP Signalling and User Data Flow Paths . . . A.10
Internet Multimedia Protocol Stack. . . A.11 SIP ... ... ... ... ... ... ... ... ... ... ... ... ... A.12 SIP Operation .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . A.13 RTP ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... .. A.14 RTCP ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . A.15 RTP Sessions.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . A.16
User Identities.. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . A.17 USIM, UICC and ISIM . . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. . A.18 SIP Registration and IMS Authentication . . . A.19 IMS Mobile-Originated to IMS Mobile-Terminated Session . . . A.20 SR-VCC Session Transfer . . . A.21
Emergency Calls .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. A.22 IMS Charging . . . A.23 IMS Deregistration .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . A.24
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At the end of this section you will be able to:
OBJECTIVES OBJECTIVES
Appendix
© Wray Castle Limited A.v LT3604/v4.0
■ describe the basic role of the IMS
■ describe the methods employed to provide IMS access to LTE VoLTE UEs
■ describe the basic architecture of the IMS and outline the functions of key elements such as CSCF
■ describe how IMS signalling and media flows are routed and handled
■ outline the protocols found in a generic Internet Multimedia Protocol Stack, such as SIP and RTP
■ discuss the role of SIP within the EPC and IMS and the supplementary functions performed by SDP and RTP
■ outline the set of user identities employed by the IMS, including the IMPI and the IMPU
■ discuss the role of the ISIM
■ describe the activities related to IMS registration
■ describe how IMS signalling and media flows are routed and handled
■ list the stages through which the IMS connection establishment process must proceed
■ describe the functionality of the SR-VCC service explain its role in ensuring continuity for IMS calls
■ outline the ways in which IMS Emergency Calls can be handled
■ describe the methods and connectivity employed to support the collation of IMS Charging data
■ outline the steps followed in the IMS Deregistration process
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LT3604/v4.0 © Wray Castle Limited A.1
Appendix
AS Presence
Another AS AS
PoC
User Plane Connection Packet-Switched
Network CSCF
SIP
SIP IMS
IMS finds the other terminal and connects the terminals with IP Find and connect
IP connection e.g.
CDMA, GPRS, LTE, EDGE,WLAN, ADSL
IP-based service possible between users
The Role of the IMS The Role of the IMS
The IMS is a core network framework that utilizes SIP signalling to facilitate the creation of peer-to-peer and client–server connections between IP devices. The main SIP nodes in the IMS are CSCFs (Call Session Control Functions). CSCFs deal with processes such as registration, user discovery, session establishment and termination and the interaction with IMS applications.
IMS applications are delivered using ASs (Application Servers) which run the required service logic and use SIP signalling to interact with user terminals and other IMS nodes.
IMS is referred to as being ‘access agnostic’, meaning that it can use a number of access technologies including CDMA (UMTS), GPRS, EDGE, WLAN and ADSL (Asymmetric Digital Subscriber Line).
Generally, the IMS architecture deals with control plane SIP signalling. The user plane for the connections that are established can go directly between the end user terminals.
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LTE Evolved Packet Core Network
S-GW UE
Default EPS Bearer (SIP) IM CN Subsystem PDN-GW
Dedicated EPS Bearer (media)
IP media Transport
Accessing the IM CN Subsystem Accessing the IM CN Subsystem
A UE accessing the IM CN Subsystem needs to be assigned an IP address that is logically part of the IM CN subsystem’s IP addressing domain. This is achieved by the UE establishing a PDP Context/EPS Bearer via a PDN-GW APN that provides connectivity towards the IMS. It is also possible to ensure that P-CSCF Discovery (e.g. the process of allowing the UE to discover the IP address of a useable local P-CSCF) is managed as part of the PDP Context/EPS Bearer establishment process.
The connection between an EPS-Attached UE and the IM CN subsystem is shown in the diagram. The S-GW serves the UE and handles mobility management, whilst the PDN-GW provides access to the IM CN subsystem.
In standard VoLTE using an IMS, each registered UE maintains an always on Default EPS Bearer that is available to exchange SIP signalling with the IMS. A UE will request an additional Dedicated EPS Bearer to be established to carry media traffic when a session is active. If necessary, a UE may have up to two Dedicated EPS Bearers established, one to carry real-time sessions (requiring real-time QoS handling) and another to carry any multimedia sessions that require non-real time QoS.
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Introduction to the IMS Architecture Introduction to the IMS Architecture
In a Release 99 UMTS network the core network is divided into PS and CS domains. Release 4 introduced the concept of softswitching to the CS domain, replacing MSCs with MSC servers and MGWs (Media Gateways).
In this example, user plane media connections from UE1 to CS MGW1 may be viewed as traditional CS connections. The MGW may be used to convert vocoded voice received from UE1 into packetized voice for transfer over an IP-based network to other CS MGWs, i.e. VoIP may be supported between MGWs.
Signalling from UE1 is transferred to an MSC server while control signalling between MSC servers may be transported over an IP network. A CS MGW may also be used to provide traditional TDM (Time Division Multiplexed) CS connections to legacy networks such as the PSTN or GSM using traditional SS7 control signalling.
Release 5 introduced the IMS and assumed that the access technology used would be GPRS. The diagram shows UE2 connecting to IMS through LTE (S-GW and PDN-GW).
Release 6 introduced other access technologies such as WLAN and DSL (Digital Subscriber Line). The example shows UE3 (a laptop computer) connecting to the IMS through an IP access network. The registration of IMS-enabled end-user devices is handled by the CSCF and the HSS. Call control signalling is handled by the CSCF and may involve one or more ASs.
User plane media can be transferred directly between two IP terminals. The example shows a peer-to-peer connection between a GPRS UE and a laptop connected via WLAN or ADSL, or via an IMS MGW should a connection to a legacy network be required.
In the latter case IMS MGWs are shown providing traditional TDM connections to exchanges in the PSTN and to CS MGWs in the CS core. Traditional SS7 is used for call set-up procedures between an SGW in the IMS and exchanges in the PSTN and between the SGW and MSC servers in the CS core.
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