Chapter 3 General Principles of the IMS Architecture

1. Chapter 3General Principles of the IMS Architecture

2. 2 Contents From Circuit-switched to Packet-switched IMS Requirements Overview of Protocols used in the IMS Overview of IMS Architecture Identification in the IMS SIM, USIM, and ISIM in 3GPP

3. 3 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 path The GSM version of IN service is known as CAMEL (Customized Applications for Mobile Networks using Enhanced Logic) services 3GPP has gone a step further in the separation of signaling and media planes with the introduction of the split architecture for the MSC (Mobile Switching Center). The MSC is split into an MSC server and a media gateway. The MSC server handles signaling plane and the media gateway handles 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

4. 4 IMS Requirements The 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 networks IMS 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 user’s subscription or the current state of the network Support for interworking with the Internet and circuit-switched networks Internet 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 standardization The 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 new requirement to support access from networks other than GPRS

5. 5 Overview of Protocols used in the IMS Session Control Protocol Bear Independent Call Control (BICC) H.323 SIP (Session Initiation Protocol, RFC 3216): Not differentiate the UNI from a NNI Text-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 RADIUS Other Protocols COPS (Common Open Policy Service, RFC 2748) H.248 and it packages: used by signaling nodes to control nodes in the media plane (e.g., a media gateway controller controlling a media gateway), MEGACO (Media Gateway Control) RTP: RFC 3550 RTCP: RFC 3550

6. 6 Overview of IMS Architecture IMS architecture is a collection of functions linked by standardized 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/Session Control 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)

7. 7 3GPP IMS architecture overview

8. 8 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 both authentication and authorization information), user profile information (including the services that the user is subscribed to), and the S-CSCF (Serving-CSCF) allocated to the user. Networks with more than one HSS do require an Subscription Locator Function (SLF). Both the HSS and the SLF implement the Diameter (RFC 3588) with an IMS-specific Diameter application.

9. 9 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

10. 10 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 visited network as well

11. 11 S-CSCF (Serving-CSCF) The central node of the signaling plane SIP server SIP registrar A binding between the user location and the user’s SIP address of record (Public User Identity) Interface to the HSS To 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 the user To download the user profile from the HSS. The user profile includes the service 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 the duration of the registration. SIP routing service: DNS E.164 Number Translation (RFC 2916) Policy for authorization Location Home network

12. 12 Three types of Application Servers

13. 13 Modes: SIP proxy mode, SIP UA (User Agent) mode, SIP B2BUA (Back-to-Back User Agent) mode (i.e., a concatenation of two SIP User Agents) Interfaces the S-CSCF using SIP Types SIP AS (Application Server) OSA-SCS (Open Service Access-Service Capability Server) inherits all of OSA capabilities, especially the capability to access the IMS securely from external network, using OSA API interfaced to OSA AS IM-SSF (IP Multimedia Service Switching Function) Reuse CAMLE (Customized Applications for Mobile network Enhanced Logic) 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.

14. 14 The MRF (Media Resource Function) A source of media in the home network Play announcements, mix media stream (e.g., in centralized conference bridge), transcode between different codecs, obtain statistics, and do any sort of media analysis Further divided into MRFC (Media Resource Function Controller): signaling plane MRFP (Media Resource Function Processor): media plane Acts as a SIP UA and contains a SIP interface towards the S-CSCF The MRFC controls the resources in the MRFP via an H.248 interface. MRFP: all the media-related functions, such as playing and mixing media Location Home network

15. 15 The BGCF Routing functionality based on telephone numbers IMS terminal, addressed to a user in a circuit-switched network, such as PSTN or the PLMN The 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 the BGCF is located.

16. 16 The IMS-ALG and TrGW

17. 17 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 signaling legs (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 incoming traffic and the S-CSCF for outgoing traffic through the Mx interface (not standardized by 3GPP R6, SIP). The TrGW is effectively a NAT-PT/NAPT-PT (Network Address Port Translator-Protocol Translator). 3GPP TS 29.162: IPv4/IPv6 interworking of the IMS-ALG and TrGW Ix interface: not standardized in 3GPP R6

18. 18 The PSTN/CS gateway interfacing a CS network

19. 19 The PSTN Gateway SGW (Signaling Gateway) Lower layer protocol conversion Example) replace the lower MTP transport with SCTP Transforms 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 IP Control the resources in an MGW using H.248 MGW (Media Gateway) RTP to PCM (Pulse Code Modulation) Transcoding

20. 20 The P-CSCF located in the visited network

21. 21 The P-CSCF located in the home network

22. 22 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

23. 23 Identification in the IMS Identification: users and services Public User Identities SIP URI or TEL URI Route SIP signaling MSISDN (Mobile Subscriber ISDN Number) to GSM Form: sip:first.last@operator.com (Can change this scheme) Sip:+1-212-555-0293@operator.com;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 bandwidth Private User Identities The format of a NAI (network Access Identifier, RFC 2486) username@operator.com Not used of routing SIP requests, but explicitly used for subscription identification and authentication 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)

24. 24 Relation of Private and Public User Identities in 3GPP R5

25. 25 Relation of Private and Public User Identities in 3GPP R6

26. 26 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 the collection 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

27. 27 SIM, USIM, and ISIM in 3GPP UICC (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 UICC Another set of parameters: user subscription information, authentication information, payment methods, and storage of messages To 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, and terminal configuration The relevant parameters Private User Identity Public User Identity Home Network Domain URI Long-term secrete Additionally, other applications, such as a telephone book

28. 28 SIM, USIM, and ISIM in the UICC of 3GPP IMS terminals

29. 29 Simplified representation of the structure of the USIM application

30. 30 Structure of an ISIM application