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The IP Multimedia Subsystem (IMS) is an architectural framework for delivering Internet Protocol (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. Its original formulation (3GPP R5) represented an approach to delivering "Internet services" over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed line.

To ease the integration with the Internet, IMS uses IETF protocols wherever possible, e.g. Session Initiation Protocol (SIP). According to the 3GPP , IMS is not intended to standardize applications but rather to aid the access of multimedia and voice applications from wireless and wireline terminals, i.e. create a form of fixed-mobile convergence (FMC). This is done by having a horizontal control layer that isolates the access network from the service layer. From a logical architecture perspective, services need not have their own control functions, as the control layer is a common horizontal layer. However in implementation this does not necessarily map into greater reduced cost and complexity.

Alternative and overlapping technologies for access and provisioning of services across wired and wireless networks include combinations of Generic Access Network, soft switches and "naked" SIP. It is easier to sell services than to sell the virtues of "integrated services", but additionally the task to sell an IMS based on a single service is also difficult as there are often (cheaper) alternatives to creating and deploying that particular service.

Since it is becoming increasingly easier to access content and contacts using mechanisms outside the control of traditional wireless/fixed operators, the interest of IMS is being challenged.

History

 * IMS was originally defined by an industry forum called 3G.IP, formed in 1999. 3G.IP developed the initial IMS architecture, which was brought to the 3rd Generation Partnership Project (3GPP), as part of their standardization work for 3G mobile phone systems in UMTS networks. It first appeared in Release 5 (evolution from 2G to 3G networks), when SIP-based multimedia was added. Support for the older GSM and GPRS networks was also provided.
 * 3GPP2 (a different organization than 3GPP) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for CDMA2000.


 * 3GPP release 6 added interworking with WLAN, Inter-operability between IMS using different IP-connectivity networks, routing group identities, multiple registration and forking, presence, speech recognition and speech-enabled services (Push to talk).


 * 3GPP release 7 added support for fixed networks, by working together with TISPAN release R1.1, the function of AGCF(Access Gateway control function) and PES (PSTN Emulation Service) are introduced to the wire-line network for the sake of inheritance of services which can be provided in PSTN network. Also added voice call continuity between circuit switching and packet switching domain (VCC), fixed broadband connection to the IMS, interworking with non-IMS networks, Policy and Charging Control (PCC), emergency sessions.


 * 3GPP release 8 added support for Long Term Evolution (LTE), System Architecture Evolution (SAE), Multimedia Session Continuity, Enhanced emergency sessions and IMS centralized services.

Architecture
Each of the functions in the diagram are explained below.

The IP Multimedia Core Network Subsystem is a collection of different functions, linked by standardized interfaces, which grouped form one IMS administrative network. 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 single network, for dimensioning, load balancing or organizational issues.

Access network
The user can connect to an IMS network in various ways, most of which use the standard Internet Protocol (IP). IMS terminals (such as mobile phones, personal digital assistants (PDAs) and computers) can register directly on an IMS network, even when they are roaming in another network or country (the visited network). The only requirement is that they can use IP and run Session Initiation Protocol (SIP) user agents. Fixed access (e.g., Digital Subscriber Line (DSL), cable modems, Ethernet), mobile access (e.g. W-CDMA, CDMA2000, GSM, GPRS) and wireless access (e.g. WLAN, WiMAX) are all supported. Other phone systems like plain old telephone service (POTS—the old analogue telephones), H.323 and non IMS-compatible VoIP systems, are supported through gateways.

Home subscriber server
The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information (subscriber profiles), performs authentication and authorization of the user, and can provide information about the subscriber's location and IP information. It is similar to the GSM Home Location Register (HLR) and Authentication Centre (AuC).

A Subscriber Location Function (SLF) is needed to map user addresses when multiple HSSs are used.

User identities
Various identities may be associated with IMS: IP Multimedia Private Identity (IMPI), IP Multimedia Public Identity (IMPU), Globally Routable User Agent URI (GRUU), Wildcarded Public User Identity. Both IMPI and IMPU are not phone numbers or other series of digits, but Uniform Resource Identifier (URIs), that can be digits (a Tel URI, like tel:+1-555-123-4567) or alphanumeric identifiers (a SIP URI, like sip:john.doe@example.com).

IP Multimedia Private Identity
The IP Multimedia Private Identity (IMPI) is a unique permanently allocated global identity assigned by the home network operator, and is used, for example, for Registration, Authorization, Administration, and Accounting purposes. Every IMS user shall have one or more IMPI.

IP Multimedia Public Identity
The IP Multimedia Public Identity (IMPU) is used by any user for requesting communications to other users (e.g. this might be included on a business card). There can be multiple IMPU per IMPI. The IMPU can also be shared with another phone, so that both can be reached with the same identity (for example, a single phone-number for an entire family).

Globally Routable User Agent URI
Globally Routable User Agent URI (GRUU) is an identity that identifies a unique combination of IMPU and UE instance. There are two types of GRUU: Public-GRUU (P-GRUU) and Temporary GRUU (T-GRUU).
 * P-GRUU reveal the IMPU and are very long lived.
 * T-GRUU do not reveal the IMPU and are valid until the contact is explicitly de-registered or the current registration expires (only used in REGISTER requests, i.e. initial registration, re-registration, UE-initiated deregistration).

Wildcarded Public User Identity
A wildcarded Public User Identity expresses a set of IMPU grouped together. The HSS subscriber database contains, the IMPU, IMPI, IMSI, and MSISDN, subscriber service profiles, service triggers and other information.

Call/session control
Several roles of Session Initiation Protocol (SIP) servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signalling packets in the IMS.


 * A Proxy-CSCF (P-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). Some networks may use a Session Border Controller for this function. The terminal discovers its P-CSCF with either DHCP, or it may be configured (e.g. during initial provisioning or via a 3GPP IMS Management Object (MO)) or in the ISIM or assigned in the PDP Context (in General Packet Radio Service (GPRS)).
 * it is assigned to an 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 an IPsec security association with the IMS terminal. This prevents spoofing attacks and replay attacks 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 using SigComp, which reduces the round-trip over slow radio links
 * it may include a Policy Decision Function (PDF), which authorizes media plane resources e.g. quality of service (QoS) over the media plane. It's used for policy control, bandwidth management, etc. The PDF can also be a separate function.
 * it also generates charging records


 * A Serving-CSCF (S-CSCF) is the central node of the signalling plane. It is a SIP server, but performs session control too. It is always located in the home network. It uses Diameter Cx and Dx interfaces to the HSS to download and upload user profiles — it has no local storage of the user. All necessary information is loaded from the HSS.
 * 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 signaling messages, and can inspect every message
 * it decides to which application server(s) the SIP message will be forwarded, in order to provide their services
 * it provides routing services, typically using Electronic Numbering (ENUM) lookups
 * it enforces the policy of the network operator
 * there can be multiple S-CSCFs in the network for load distribution and high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF.


 * An Interrogating-CSCF (I-CSCF) is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System (DNS) of the domain (using NAPTR and SRV type of DNS records), so that remote servers can find it, and use it as a forwarding point (e.g. registering) for SIP packets to this domain. The I-CSCF queries the HSS to retrieve the address of the S-CSCF and assign it to a user performing SIP registration. It also forward SIP request or response to the S-CSCF. Up to Release 6 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 Topology Hiding Inter-network Gateway (THIG). From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the Interconnection Border Control Function (IBCF). The IBCF is used as gateway to external networks, and provides NAT and Firewall functions (pinholing).

Application servers
SIP Application servers (AS) host and execute services, and interface with the S-CSCF using Session Initiation Protocol (SIP). An example of an application server that is being developed in 3GPP is the Voice call continuity Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA (user agent) mode or SIP B2BUA 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 or Si interfaces (for a SIP-AS).
 * SIP AS: Host and execute IMS specific services
 * IP Multimedia Service Switching Function (IM-SSF): Interfaces SIP to CAP to communicate with CAMEL Application Servers
 * OSA Service Capability Server (OSA SCS) : Interfaces SIP to the OSA framework

Functionnal model
The AS-ILCM and AS-OLCM store transaction state, and may optionally store session state depending on the specific service being executed. The AS-ILCM interfaces to the S-CSCF (ILCM) for an incoming leg and the AS-OLCM interfaces to the S-CSCF (OLCM) for an outgoing leg. Application Logic provides the service(s) and interacts between the AS-ILCM and AS-OLCM.

Public Service Identity
Public Service Identities (PSI) are identities that identify services, which are hosted by Application Servers. As user identities, PSI shall take the form of either a SIP or Tel URI. PSIs are stored in the HSS either as a distinct PSI or as a wildcarded PSI:
 * a distinct PSI contains the PSI that is used in routing
 * a wildcarded PSI represents a collection of PSIs.

Media Servers
The Media Resource Function (MRF) provides media related functions such as media manipulation (e.g. voice stream mixing) and playing of tones and announcements.

Each MRF is further divided into a Media Resource Function Controller (MRFC) and a Media Resource Function Processor (MRFP).


 * The MRFC is a signalling plane node that interpret information coming from an AS and S-CSCF to control the MRFP
 * The MRFP is a media plane node used to mix, source or process media streams. It can also manage access right to shared ressources.

The Media Resource Broker (MRB) is a functional entity that is responsible for both collection of appropriate published MRF information and supplying of appropriate MRF information to consuming entities such as the AS. MRB can be used in two modes:
 * Query mode: AS queries the MRB for media and sets up the call using the response of MRB
 * In-Line Mode: AS sends a SIP INVITE to the MRB. The MRB sets up the call

Breakout Gateway
A Breakout Gateway Control Function (BGCF) is a SIP proxy which processes requests for routing from an S-CSCF when the S-CSCF has determined that the session cannot be routed using DNS or ENUM/DNS. It includes routing functionality based on telephone numbers.

PSTN Gateways
A PSTN/CS gateway interfaces with PSTN circuit switched (CS) networks. For signalling, CS networks use ISDN User Part (ISUP) (or BICC) over Message Transfer Part (MTP), while IMS uses Session Initiation Protocol (SIP) over IP. For media, CS networks use Pulse-code modulation (PCM), while IMS uses Real-time Transport Protocol (RTP).
 * A Signalling Gateway (SGW) interfaces with the signalling plane of the CS. It transforms lower layer protocols as Stream Control Transmission Protocol (SCTP, an Internet Protocol (IP) protocol) into Message Transfer Part (MTP, an Signalling System 7 (SS7) protocol), to pass ISDN User Part (ISUP) from the MGCF to the CS network.
 * A Media Gateway Controller Function (MGCF) is SIP endpoint that does call control protocol conversion between SIP and ISUP/BICC and interfaces with the SGW over SCTP. It also controls the resources in a Media Gateway (MGW) across an H.248 interface.
 * A Media Gateway (MGW) interfaces with the media plane of the CS network, by converting between RTP and PCM. It can also transcode when the codecs don't match (e.g. IMS might use AMR, PSTN might use G.711).

Media Resources
Media Resources are those components that operate on the media plane and are under the control of IMS Core functions. Specifically, Media Server (MS) and Media gateway (MGW)

NGN Interconnection
There are two types of Next Generation Networking Interconnection:
 * Service oriented Interconnection (SoIx): The physical and logical linking of NGN domains that allows carriers and service providers to offer services over NGN (i.e. IMS and PES) platforms with control, signalling (i.e. session based), which provides defined levels of interoperability. For instance, this is the case of "carrier grade" voice and/or multimedia services over IP interconnection. "Defined levels of interoperability" are dependent upon the service or the QoS or the Security, etc.
 * Connectivity oriented Interconnection (CoIx): The physical and logical linking of carriers and service providers based on simple IP connectivity irrespective of the levels of interoperability. For example, an IP interconnection of this type is not aware of the specific end to end service and, as a consequence, service specific network performance, QoS and security requirements are not necessarily assured. This definition does not exclude that some services may provide a defined level of interoperability. However only SoIx fully satisfies NGN interoperability requirements.

An NGN interconnection mode can be direct or indirect. Direct interconnection refers to the interconnection between two network domains without any intermediate network domain. Indirect interconnection at one layer refers to the interconnection between two network domains with one or more intermediate network domain(s) acting as transit networks. The intermediate network domain(s) provide(s) transit functionality to the two other network domains. Different interconnection modes may be used for carrying service layer signalling and media traffic.

Charging
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, or real-time credit control of postpaid 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 Charging Data Function (CDF) located in the same domain. The CDF will collect all this information, and build a Call Detail Record (CDR), which is sent to the billing system (BS) of the domain.


 * Online charging : The S-CSCF talks to a Online Charging Function (SCF) 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 the OCF.
 * When Immediate Event Charging (IEC) is used, a number of credit units is immediately deducted from the user's account by the OCF 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 Event Charging with Unit Reservation (ECUR) is used, the OCF 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.

Each session carries an : IMS Charging Identifier (ICID) as a unique identifier generated by the first IMS entity involved in a SIP transaction and used for the correlation with CDRs. Inter Operator Identifier (IOI) is a globally unique identifier shared between sending and receiving networks. Each domain has its own charging network. BS in different domains will also exchange information, so that roaming charges can be applied.

Charging function addresses are addresses distributed to each IMS entities and provide a CDF and OCF addresses for each entity to send charging information. Charging Vector contains ICID, IOI and access network charging information.

Session handling
The S-CSCF might apply filter criteria to determine the need to forward SIP requests to AS.

Initial Filter Criteria
Initial Filter Criteria (iFC) are filter criteria that are stored in the HSS as part of the user profile and are downloaded to the S-CSCF upon user registration (for registered users). They represent a provisioned subscription of a user to an application. iFC are valid throughout the registration lifetime or until the User Profile is changed.

Security aspects of early IMS systems
It is envisaged that security defined in TS 33.203 may not be available for a while especially because of the lack of USIM/ISIM interfaces and prevalence of devices that support IPv4. For this situation, to provide some protection against the most significant threats, 3GPP defines some security mechanisms, which are informally known as "early IMS security," in TR33.978.

Books

 * "The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds" by Gonzalo Camarillo, Miguel-Angel García-Martín (John Wiley & Sons, 2006, ISBN 0-470-01818-6)
 * "The IMS: IP Multimedia Concepts and Services" by Miikka Poikselka, Aki Niemi, Hisham Khartabil, Georg Mayer (John Wiley & Sons, 2006, ISBN 0-470-01906-9)
 * "IP Multimedia Subsystem (IMS) Handbook" by Mohammad Ilyas (Editor), Syed A. Ahson (Editor) (CRC Press, 2008, ISBN 1-4200-6459-2)