The IP Multimedia Subsystem (IMS) is an open, standardised, operator friendly, Next Generation Networking (NGN) multi-media architecture for mobile and fixed IP services. It s a Voice_over_IP implementation based on a 3GPP variant of Session_Initiation_Protocol, and runs over the standard Internet protocol (Internet_Protocol). It s used by Telco in NGN networks (which combine voice and data in a single packet switched network), to offer network controlled Multimedia services.
The aim of IMS is not only to provide new services but to provide all the services, current and future, that the Internet provides. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals the IMS uses open standard IP protocols, defined by the IETF. So, a multi-media session between 2 IMS users, between an IMS user and a user on the Internet, and between 2 users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based in IP protocols. This is why the IMS truly merges the Internet with the cellular world; it uses cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services.
=History=
IMS was originally defined by the 3rd Generation Partnership Project (3GPP), as part of their standardisation work for third generation (3G) cell phone systems for W-CDMA networks. But the service model has been used by 3GPP2 (a different organisation for CDMA2000 networks) and TISPAN (fixed networks) too. It is a key technology for Fixed/Mobile Convergence (FMC).
IMS first appeared in release 5 of the Evolution_to_3G for W-CDMA networks (UMTS), when SIP-based multimedia domain was added to NGN networks. Support for older GSM and GPRS networks is also provided.
In 3GPP release 6, interworking with WLAN was added.
3GPP2 (a different organisation) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for CDMA2000.
3GPP release 7 adds support for fixed networks, together with TISPAN R1.
Early IMS is defined for IPv4 networks, and provides a migration path to IPv6
=Basic Principles=
Access independence: IMS will eventually work with any network (fixed, mobile or wireless) with packet-switching functions, such as GPRS, UMTS, CDMA2000, WLAN, WiMAX, Digital Subscriber Line, Cable_modem, ... Older phone systems are supported through gateway (telecommunications). Open interfaces between control and service layers allow elements and calls/sessions from different access networks to be mixed.
Different network architectures: IMS allows operators and service providers to use different underlying network architectures.
Terminal and user mobility: The mobile network provides terminal mobility (roaming), while user mobility is provided by IMS and SIP.
Extensive IP-based services: IMS should make it easier to offer just about any Internet_Protocol-based service. Examples include voice over IP (VOIP), push-to-talk over cellular (POC), multiparty gaming, videoconferencing, and content sharing.
=Architecture=
The IP Multimedia Core Network Subsystem is a collection of different functions, linked by standardized interfaces. A function is not a node (hardware box) : an implementer is free to combine 2 functions in 1 node, or to split a single function into 2 or more nodes. Each node can also be present multiple times in a network, for load balancing or organizational issues.
==Access Network==
The user can connect to an IMS network using various methods, all of which are using the standard Internet_Protocol (IP). Direct IMS terminals (Mobile_phone, Personal_digital_assistant, computers, ...), can register directly into an IMS network, even when they re roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in Early IMS ) and are running Session_Initiation_Protocol User Agents. Fixed access (e.g., Digital Subscriber Line, cable_modem, Ethernet, ...), mobile access (W-CDMA, CDMA2000, GSM, GPRS, ...) and wireless access (WLAN, WiMAX, ...) are all supported. Other phone systems like the Plain old telephone service (the old analogue telephones) and H.323, or non IMS-compatible VoIP systems are supported through gateway (telecommunications).
==Core Network==
===User Database===
The HSS ( Home Subscriber Server ) is the master user Database that supports the IMS network entities that are actually handling the Call/Session (computer science). It contains the subscription-related information (user profile), perfoms authentication and Authorization of the user, and can provide information about the physical location of user. It s similar to the GSM GSM_core_network#Home_Location_Register_.28HLR.29 and GSM_core_network#Authentication_Centre_.28AUC.29.
A SLF ( Subscriber Location Function ) is needed when multiple HSSs are used. Both the HSS and the SLF implement the DIAMETER protocol (Cx, Dx and Sh interfaces).
===Call/Session Control===
Several types of Session_Initiation_Protocol servers, collectively known as CSCF ( Call/Session Control Function ), are used to process SIP signalling packets in the IMS.
A P-CSCF ( Proxy-CSCF ) is a SIP proxy that is the first point of contact for the IMS terminal. It can be located either in the visited network (in full IMS networks) or in the home network (when the visited network isn t IMS compliant yet). The terminal will discover its P-CSCF with either Dynamic_Host_Configuration_Protocol, or it s assigned in the PDP Context (in General_Packet_Radio_Service).
it s assigned to a IMS terminal during registration, and does not change for the duration of the registration
it sits on the path of all signalling messages, and can inspect every message
it authenticates the user and establishes a IPsec security association with the IMS terminal. This prevents Spoofing_attack and Replay_attack and protects the privacy of the user. Other nodes trust the P-CSCF, and do not have to authenticate the user again.
it can also compress and decompress SIP messages, which reduces the round-trip over slow radio links
it may include a PDF ( Policy Decision Function ), which authorizes media plane resources and manages quality of service (QoS) over the media plane. It s used for policy decisions, lawful interception, bandwidth management, etc ... The PDF can also be a separate function, for example in a Session Border Controller
it also generates charging towards a charging collection node
An I-CSCF ( Interrogating-CSCF ) is a SIP proxy located at the edge of an administrative domain. Its IP_address is published in the Domain_Name_System records of the domain (using NAPTR and SRV_record), so that remote servers (e.g., a P-CSCF in a visited domain, or a S-CSCF in a foreign domain) can find it, and use it as an entry point for all SIP packets to this domain. The I-CSCF queries the HSS using the DIAMETER Cx and Dx interfaces to retrieve the user location, and then route the SIP request to its assigned S-CSCF. It can also be used to hide the internal network from the outside world (encrypting part of the SIP message), in which case it s called a THIG ( Topology Hiding Interface Gateway ).
A S-CSCF ( Serving-CSCF ) is the central node of the signalling plane. It s a SIP server, but performs session control as well. It s always located in the home network. The S-CSCF uses DIAMETER Cx and Dx interfaces to the HSS to download and upload user profiles - it has no local storage of the user.
it handles SIP registrations, which allows it to bind the user location (e.g. the IP_address of the terminal) and the SIP address
it sits on the path of all signalling messages, and can inspect every message
it decides to which application server(s) the SIP message will be forwarded to, in order to provide their services
it provides routing services, typically using Electronic_Numbering lookups
it enforces the policy of the network operator
===Application Servers===
Application_server (AS) host and execute services, and interfaces with the S-CSCF using Session_Initiation_Protocol. This allows third party providers an easy integration and deployment
of their value added services to the IMS infrastructure.
Depending on the actual service, the AS can operated in SIP proxy mode, SIP US (user agent) mode or SIP B2BUA (back-to-back user agent) mode. An AS can be located in the home network or in an external third-party network. If located in the home network, it can query the HSS with the DIAMETER Sh interface (for SIP-AS and OSA-SCS) or the Mobile_Application_Part interface (for IM-SSF).
SIP AS : native IMS application server
OSA-SCS : an Open Service Access - Service Capability Server interfaces with Open_Services_Architecture Application Servers using Parlay
IM-SSF : an IP Multimedia Service Switching Function interfaces with Customised_Applications_for_Mobile_networks_Enhanced_Logic Application Servers using Camel_Application_Part
===Media Servers===
A MRF ( Media Resource Function ) provides a source of media in the home network. It s used for :
Playing of announcements (audio/video)
Videoconferencing (e.g. mixing of audio streams)
Text-to-speech conversation (TTS) and speech recognition.
Realtime transcoding of multimedia data (i.e. conversion between different codecs)
Each MRF is further divided into :
A MRFC ( Media Resource Function Controller ) is a signalling plane node that acts as a SIP User Agent to the S-CSCF, and which controls the MFRP with a H.248 interface
A MRFP ( Media Resource Function Processor ) is a media plane node that implements all media-related functions.
===Breakout Gateway===
A BGCF ( Breakout Gateway Control Function ) is a SIP server that includes routing functionality based on telephone numbers. It s only used when calling from the IMS to a phone in a Circuit_switching network, such as the Public_Switched_Telephone_Network or the Public_land_mobile_network.
===PSTN Gateways===
A PSTN/CS gateway interfaces with PSTN Circuit_switching (CS) networks. For signalling, CS networks use ISDN_User_Part (or BICC) over Message_Transfer_Part, while IMS uses Session_Initiation_Protocol, For media, CS networks use Pulse-code_modulation, while IMS uses Real-time_Transport_Protocol.
A SGW ( Signalling Gateway ) interfaces with the signalling plane of the CS. It transforms lower layer protocols as Stream_Control_Transmission_Protocol (which is an Internet_Protocol protocol) into Message_Transfer_Part (which is a Signalling_System_7 protocol), to pass ISDN_User_Part from the MGCF to the CS network.
A MGCF ( Media Gateway Controller Function ) does call control protocol conversion between Session_Initiation_Protocol and ISDN_User_Part, and interfaces with the SGW over Stream_Control_Transmission_Protocol. It also controls the resources in a MGW with a H.248 interface.
A MGW ( Media Gateway ) interfaces with the media plane of the CS network, by converting between Real-time_Transport_Protocol and Pulse-code_modulation. It can also transcode when the codecs don t match (e.g. IMS might use Adaptive_Multi-Rate, PSTN might use G.711).
==Accounting==
Offline charging is applied to users who pay for their services periodically (e.g., at the end of the month). Online charging, also known as credit-based charging, is used for prepaid services. Both may be applied to the same session.
Offline Charging : All the SIP network entities (P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS) involved in the session use the DIAMETER Rf interface to send accounting information to a CCF ( Charging Collector Function ) located in the same domain. The CCF will collect all this information, and build a CDR ( Charging Data Record ), which is send to the billing system (BS) of the domain.Each session carries a ICID ( IMS Charging Identity ) as an unique identifier. IOI ( Inter Operator Identifier ) parameters define the originating and terminating networks.Each domain has its own charging network. Billing systems in different domains will also exchange information, so that roaming charges can be applied.
Online charging : The S-CSCF talks to a SCF ( Session Charging Function ), which looks like a regular SIP application server. The SCF can signal the S-CSCF to terminate the session when the user runs out of credits during a session. The AS and MRFC use the DIAMETER Ro interface towards a ECF ( Event Charging Function ), that also communicates with the SCF.When IEC ( Immediate Event Charging ) is used, a number of credit units is immediately deducted from the user s account by the ECF and the MRFC or AS is then authorized to provide the service. The service is not authorized when not enough credit units are available. When ECUR ( Event Charging with Unit Reservation ) is used, the ECF first reserves a number of credit units in the user s account and then authorizes the MRFC or the AS. After the service is over, the number of spent credit units is reported and deducted from the account ; the reserved credit units are then cleared.
=Advantages & Issues=
==Advantages over existing systems==
the core network is independent of a particular access technology
integrated mobility for all network applications
easier migration of applications from fixed to mobile users
faster deployment of new services based on standardized architecture
an end to unique or customized applications, leading to lower CAPEX and OPEX
new applications such as Presence_Information and Videoconferencing
evolution to combinational services, for example by combining Instant_messaging and voice
user profiles are stored in a central location
==Advantages over free VoIP==
It s possible to run free VoIP applications over the regular Internet. Then why do we need IMS, if all the power of the Internet is already available for 3G users
Quality of Service : The network offers no guarantees about the amount of bandwidth a user gets for a particular connection or about the delay the packets experience. Consequently, the quality of a VoIP conversation can vary dramatically throughout its duration.
Charging of multimedia services : Videoconferences can transfer a large amount of information, but the telecom operator can t charge separately for this data. Some business models might be more beneficial for the user (for instance: a fixed price per message, not per byte); others might charge extra for better QoS.
Integration of different services : an operator can use services developed by third parties, combine them, integrate them with services they already have, and provide the user with a completely new service. For example: if voicemail and text-to-speech is combined, a voice version of incoming text messages can be provided for blind users.
==Issues==
Benefits need to be further articulated in terms of actual savings.
IMS is operator friendly which means that it provides the operator with comprehensive control of content at the expense of the consumer.
IMS uses the 3GPP variant of SIP, which needs to interoperate with the IETF SIP.
IMS is an optimization of the network, and investments for such optimization are questionable.
=Specifications=
==3GPP Specs==
They can be downloaded from http://www.3gpp.org/specs/numbering.htm . The list below is a small selection.
TS 21.905 Vocabulary for 3GPP Specifications
TS 22.066 Support of Mobile Number Portability (MNP); Stage 1
TS 22.101 Service Aspects; Service Principles
TS 22.141 Presence Service; Stage 1
TS 22.228 Service requirements for the IP multimedia core network subsystem; Stage 1
TS 22.250 IMS Group Management; Stage 1
TS 22.340 IMS Messaging; Stage 1
TS 22.800 IMS Subscription and access scenarios
TS 23.002 Network Architecture
TS 23.003 Numbering, Addressing and Identification
TS 23.008 Organisation of Subscriber Data
TS 23.107 Quality of Service (QoS) principles
TS 23.125 Overall high level functionality and architecture impacts of flow based charging; Stage 2
TS 23.141 Presence Service; Architecture and functional description; Stage 2
TS 23.207 End-to-end QoS concept and architecture
TS 23.218 IMS session handling; IM call model; Stage 2
TS 23.221 Architectural Requirements
TS 23.228 IMS stage 2
TS 23.229 WLAN interworking
TS 23.271 Location Services (LCS); Functional description; Stage 2
TS 23.278 Customized Applications for Mobile network Enhanced Logic (CAMEL) - IMS interworking; Stage 2
TS 23.864 Commonality and interoperability between IMS core networks
TS 23.867 IMS emergency sessions
TS 23.917 Dynamic policy control enhancements for end-to-end QoS, Feasibility study
TS 23.979 3GPP enablers for Push-to-Talk over Cellular (PoC) services; Stage 2
TR 23.981 Interworking aspects and migration scenarios for IPv4-based IMS implementations (early IMS)
TS 24.141 Presence Service using the IMS Core Network subsystem; Stage 3
TS 24.147 Conferencing using the IMS Core Network subsystem
TS 24.228 Signalling flows for the IMS call control based on SIP and SDP; Stage 3
TS 24.229 IMS call control protocol based on SIP and SDP; Stage 3
TS 24.247 Messaging using the IMS Core Network subsystem; Stage 3
TS 26.235 Packet switched conversational multimedia applications; Default codecs
TS 26.236 Packet switched conversational multimedia applications; Transport protocols
TS 29.162 Interworking between the IMS and IP networks
TS 29.163 Interworking between the IMS and Circuit Switched (CS) networks
TS 29.198 Open Service Architecture (OSA)
TS 29.207 Policy control over Go interface
TS 29.208 End-to-end QoS signalling flows
TS 29.209 Policy control over Gq interface
TS 29.228 IMS Cx and Dx interfaces : signalling flows and message contents
TS 29.229 IMS Cx and Dx interfaces based on the Diameter protocol; Protocol details
TS 29.278 CAMEL Application Part (CAP) specification for IMS
TS 29.328 IMS Sh interface : signalling flows and message content
TS 29.329 IMS Sh interface based on the Diameter protocol; Protocol details
TS 29.962 Signalling interworking between the 3GPP SIP profile and non-3GPP SIP usage
TS 31.103 Characteristics of the IMS Identity Module (ISIM) application
TS 32.240 Telecommunications management; Charging management; Charging architecture and Principles
TS 32.260 Telecommunications management; Charging management; IMS charging
TS 32.421 Telecommunications management; Subscriber and equipment trace: Trace concepts and requirements
TS 33.102 3G security; Security architecture
TS 33.108 3G security; Handover interface for Lawful Interception (LI)
TS 33.141 Presence service; security
TS 33.203 3G security; Access security for IP-based services
TS 33.210 3G security; Network Domain Security (NDS); IP network layer security
==IETF Specs==
RFC 1889 Real-time Transport Protocol (Real-time_Transport_Protocol)
RFC 2327 Session Description Protocol (Session_Description_Protocol)
RFC 2748 Common Open Policy Server protocol (COPS)
RFC 2782 a DNS RR for specifying the location of services (SRV_record)
RFC 2806 URLs for telephone calls (TEL)
RFC 2915 the naming authority pointer DNS resource record (NAPTR)
RFC 2916 E.164 number and DNS
RFC 3261 Session Initiation Protocol (Session_Initiation_Protocol)
RFC 3262 reliability of provisional responses (PRACK)
RFC 3263 locating SIP servers
RFC 3264 an offer/answer model with the Session Description Protocol
RFC 3310 HTTP Digest Authentication using Authentication and Key Agreement (AKA)
RFC 3311 update method
RFC 3312 integration of resource management and SIP
RFC 3319 DHCPv6 options for SIP servers
RFC 3320 signalling compression (SIGCOMP)
RFC 3323 a privacy mechanism for SIP
RFC 3324 short term requirements for network asserted identity
RFC 3325 private extensions to SIP for asserted identity within trusted networks
RFC 3326 the reason header field
RFC 3327 extension header field for registering non-adjacent contacts (path header)
RFC 3329 security mechanism agreement
RFC 3455 private header extensions for SIP
RFC 3485 SIP and SDP static dictionary for signaling compression
RFC 3574 Transition Scenarios for 3GPP Networks
RFC 3588 DIAMETER base protocol
RFC 3589 DIAMETER command codes for 3GPP release 5 (informational)
RFC 3608 extension header field for service route discovery during registration
RFC 3680 SIP event package for registrations
RFC 3824 using E164 numbers with SIP
=See Also=
Next_Generation_Networking
Voice_over_IP
Session_Initiation_Protocol
ENUM
SIMPLE
ISIM
=External links=
http://www.3gpp.org 3GPP home page
http://www.3gpp.org/specs/numbering.htm 3GPP specifications
http://www.lightreading.com/document.aspdoc_id=70728
[http://www.alcatel.com/global/convergence/ims.html Alcatel introduces the concept of IMS and provides an overview of the architecture fundamentals]
[http://www.cisco.com/en/US/netsol/ns341/ns396/ns177/networking_solutions_packages_list.html Cisco Mobility ] Cisco solutions for Mobility & IMS
[http://www.siemens.com/index.jspsdc_p=i1306027lmo1306027pMNENcfs3t55u1436z2&sdc_bcpath=1302057.s_3%2C1307268.s_3%2C&sdc_sid=26212401203& Siemens IMS] Siemens IP-Multimedia Subsystem
[https://communication-market.siemens.de/portal/main.aspxLangID=0&MainMenuID=20&ParentID=20&pid=1&cid=0&tid=6000&xid=0 Siemens Developer Portal] Siemens IMS Developer Program
[http://www.fokus.gmd.de/bereichsseiten/testbeds/ims_playground/index.phplang=en Open IMS] IMS testbed at the FOKUS Test Lab