3-general principles of the ims architecture

8/8/2019 3-General Principles of the IMS Architecture

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Chapter 3

General Principles of the IMSArchitecture


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From Circuit-switched to Packet-switched

The 3G CS and PS domains are based on GSM modes of operation.CS networks have two different planes: the signaling and the media plane. Signaling plane: the establishment of CS path between terminals, service

invocation Media plane: the data transmitted over CS pathThe GSM version of IN service is known as CAMEL (CustomizedApplications for Mobile Networks using Enhanced Logic) services3GPP has gone a step further in the separation of signaling and media

planes with the introduction of the split architecture for the MSC (MobileSwitching Center). The MSC is split into an MSC server and a mediagateway. The MSC server handles signaling plane and the media gatewayhandles the media plane (Release 4 of the 3GPP specifications)

GSM PS network: GPRS (General Packet Radio Service)Three applications designed to boost the usage of the PS domain The Wireless Application Protocol (WAP) Access to corporate networks Access to the public Internet


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IMS RequirementsThe IMS aims to: Combine the latest trends in technology Make the mobile Internet paradigm come true Create a common platform to develop diverse multimedia services Create a mechanism to boost margins due to extra usage of mobile PS networksIMS framework requirements Support for establishing IP Multimedia Sessions

The need to support the main service (audio, video communication) to be delivered by the IMS Support for a mechanism to negotiate QoS

The QoS for a particular session is determined by a number of factors, such as the maximum bandwidth that can be allocated to the user based on the users subscription or the current stateof the network

Support for interworking with the Internet and circuit-switched networksInternet interworks with CS networks, such as the PSTN, or existing cellular networks.

Support for roaming Support for strong control imposed by the operator with respect to the services delivered

to the end-user General policies applicable to all the users in the network Individual policies that apply to a particular user

Support for rapid service creation without requiring standardizationThe IMS reduces development delay by standardizing service capabilities instead of services

Access independence of the IMS: The release 6 version of 3GPP TS 22.228 added a newrequirement to support access from networks other than GPRS


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Overview of Protocols used in the IMSSession Control Protocol Bear Independent Call Control (BICC) H.323 SIP (Session Initiation Protocol, RFC 3216):

Not differentiate the UNI from a NNIText-based protocol: easier to extend, debug, and use to build services

AAA Protocol Diameter (whose base protocol is specified in RFC 3588) An evolution of RADIUSOther Protocols COPS (Common Open Policy Service, RFC 2748)

H.248 and it packages: used by signaling nodes to control nodes in themedia plane (e.g., a media gateway controller controlling a mediagateway), MEGACO (Media Gateway Control)

RTP: RFC 3550 RTCP: RFC 3550


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Overview of IMS ArchitectureIMS architecture is a collection of functions linked bystandardized interfaces.IP Multimedia Core Network Subsystem One or more user database, called HSSs (Home Subscriber Servers)

and SLFs (Subscriber Location Functions) One or more SIP servers, collectively known as CSCFs (Call/SessionControl Functions)

One or more ASs (Application Server) One or more MRFs (Media Resource Functions), each one further

divided into MRFC (Media Resource Function Controllers) and MRFP

(Media Resource Function Processors) One or more BGCFs (Break Gateway Control Functions) One or more PSTN gateways, each one decomposed into an SGW

(Signaling Gateway ), an MGCF (Media Gateway Control Function),and an MGW (Media Gateway)


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3GPP IMS architecture overview


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The Databases: the HSS and the SLF The HSS contains all the user-related subscription data

required to handle multimedia sessions. These data include, among other items, location

information, security information (including bothauthentication and authorization information), user profile

information (including the services that the user issubscribed to), and the S-CSCF (Serving-CSCF) allocatedto the user.

Networks with more than one HSS do require an

Subscription Locator Function (SLF). Both the HSS and the SLF implement the Diameter (RFC3588) with an IMS-specific Diameter application.


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The CSCF (Call/Session Control Function) SIP server Three categories P-CSCF (Proxy-CSCF)

Forward SIP requests and responses in the appropriate direction(i.e., toward the IMS terminal or toward the IMS network)

Functions IPSec security association Verification of SIP request A compressor and a decompressor of SIP message PDF (Policy Decision Function): authorizes media plane resource and

manages QoS over the media plane. Generates charging information toward a charging collection node

Location Located either in the visited network or in the home network


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I-CSCF (Interrogating-CSCF)Located at the edge of an administrative domain

Functions SIP proxy server functionality Interface (Diameter protocol) to the SLF and the HSS Retrieves user location information and routes the SIP request

to the appropriate destination (typically an S-CSCF)

THIG (Topology Hiding Inter-network Gateway) functionality, being optional Encrypt the SIP messages that contain sensitive

information about the domain, such as the number of servers in the domain, their DNS names, or their capacity

Location Usually located in the home network, although in some special

cases, such as an I-CSCF(THIG), it may be located in a visitednetwork as well


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S-CSCF (Serving-CSCF) The central node of the signaling plane SIP server SIP registrar

A binding between the user location and the users SIP address of record(Public User Identity)

Interface to the HSSTo download the authentication vectors of the user who is trying to access

the IMS from the HSS. The S-CSCF uses these vectors to authenticate theuser To download the user profile from the HSS. The user profile includes theservice profile, which is a set of triggers that may cause a SIP message to

be routed through on or more application servers.To inform the HSS that this is the S-CSCF allocated to the user for theduration of the registration.

SIP routing service: DNS E.164 Number Translation (RFC 2916) Policy for authorization Location

Home network


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Three types of Application Servers


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Modes: SIP proxy mode, SIP UA (User Agent) mode, SIPB2BUA (Back-to-Back User Agent) mode (i.e., aconcatenation of two SIP User Agents)

Interfaces the S-CSCF using SIPTypes SIP AS (Application Server) OSA-SCS (Open Service Access-Service Capability Server)

inherits all of OSA capabilities, especially the capability to access the IMSsecurely from external network, using OSA API interfaced to OSA AS

IM-SSF (IP Multimedia Service Switching Function)Reuse CAMLE (Customized Applications for Mobile network EnhancedLogic)CAP (CAMEL Application Part, defined in 3GPP TS 29.278)

Interface to the HSS (Diameter)The IM-SSF interface toward the HSS is based on MAP(Mobile Application Part, defined in 3GPP TS 29.002)Location Either in the home network or in an external third-party network In any case, if the AS is located outside the home network, it does not

interface the HSS.


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The MRF (Media Resource Function)A source of media in the home network Play announcements, mix media stream (e.g., in centralizedconference bridge), transcode between different codecs, obtainstatistics, and do any sort of media analysisFurther divided into MRFC (Media Resource Function Controller): signaling plane MRFP (Media Resource Function Processor): media planeActs as a SIP UA and contains a SIP interface towards the S-CSCFThe MRFC controls the resources in the MRFP via an H.248interface.MRFP: all the media-related functions, such as playing andmixing mediaLocation Home network


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The BGCF

Routing functionality based on telephone numbersIMS terminal, addressed to a user in a circuit-switched network, such as PSTN or the PLMNThe main functionality To select an appropriate network where interworking with

the circuit-switched domain is to occur Or, to select an appropriate PSTN/CS gateway, if

interworking is to occur in the same network where theBGCF is located.


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The IMS-ALG and TrGW


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IMS supports two IP versions, namely IPv4 and IPv6 IMS Application Layer Gateway (IMS-ALG): control

plane signaling (e.g., SIP and SDP message)Act as a SIP B2BUA by maintaining two independent signalinglegs (i.e., the internal IMS network, the other network)

Transition Gateway (TrGW): user plane traffic (e.g., RTP,RTCP)

The IMS-ALG interfaces the I-CSCF for incomingtraffic and the S-CSCF for outgoing traffic throughthe Mx interface (not standardized by 3GPP R6, SIP).The TrGW is effectively a NAT-PT/NAPT-PT(Network Address Port Translator-ProtocolTranslator).3GPP TS 29.162: IPv4/IPv6 interworking of theIMS-ALG and TrGWIx interface: not standardized in 3GPP R6


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The PSTN/CS gateway interfacing a

CS network


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The PSTN Gateway SGW (Signaling Gateway)

Lower layer protocol conversionExample) replace the lower MTP transport with SCTPTransforms ISUP or BICC over MTP into ISUP or BICC over SCTP/IP

MGCF (Media Gateway Control Function)Call control protocol conversion: maps SIP to either ISUP over IP or BICC over IPControl the resources in an MGW using H.248

MGW (Media Gateway)RTP to PCM (Pulse Code Modulation)Transcoding


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The P-CSCF located in the visitednetwork


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The P-CSCF located in the homenetwork


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Roaming agreement between home network operator and visited network operator P-CSCF : the home or the visited network Go Interface Near term: P-CSCF is located in the home network This creates an undesired trombone effect that

causes delays in the media plane Long term: P-CSCF is located in the visited

network


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Identification in the IMSIdentification: users and servicesPublic User Identities SIP URI or TEL URI Route SIP signaling MSISDN (Mobile Subscriber ISDN Number) to GSM Form: sip:[email protected] (Can change this scheme) Sip:[email protected];user=phone TEL URI: tel:+1-212-555-0293 An interesting concept: a set of implicitly registered public user identities Register PUIs in one message to save time and bandwidthPrivate User Identities The format of a NAI (network Access Identifier, RFC 2486)

[email protected] Not used of routing SIP requests, but explicitly used for subscription identification andauthentication purpose

IMSI (International Mobile Subscriber Identifier) in GSM A PUI need not be known by the user, because it might be stored in a smart card, in the

same way that an IMSI is stored in a SIM (Subscriber Identity Module)


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Relation of Private and Public User Identities in 3GPP R5


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Relation of Private and Public User Identities in 3GPP R6


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The relation between Public and Private User Identities The HSS, as a general database for the data related

to a subscriber, stores the PUI (Private) and thecollection of PUIs (Public) allocated the user.

3GPP R5 3GPP R6 has extended relationship of Private and

Public User Identities.

Public Service Identities (PSI)

A service hosted in an Application Server The format of SIP URI or TEL URI No Private


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SIM, USIM, and ISIM in 3GPPUICC (Universal Integrated Circuit Card) may several logical applications SIM (Subscriber Identity Module)

Storage for a collection of parameters (e.g., user subscription information, user preference,

authentication keys, and storage of messages) USIM (Universal Subscriber Identity Module)Resides in 3G UICCAnother set of parameters: user subscription information, authentication information, paymentmethods, and storage of messagesTo access UMTS network The parameters

IMSI (International Mobile Subscriber Identity) MSISDN (Mobile Subscriber ISDN Number) CK (Ciphering Key) and IK (Integrity Key) Long-term secrete SMS (Short Message Service) SMS parameters MMS (Multimedia Messaging Service) user connectivity parameters MMS user preferences

ISIM (IP multimedia Services Identity Module)A third application that may be present in the UICC

The collection of parameters that are used for user identification, user authentication, andterminal configurationThe relevant parameters

Private User Identity Public User Identity Home Network Domain URI Long-term secrete

Additionally, other applications, such as a telephone book


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SIM, USIM, and ISIM in the UICC of 3GPP IMS terminals


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Simplified representation of thestructure of the USIM application


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Structure of an ISIM application

3-general principles of the ims architecture

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