IP Multimedia Subsystem


The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is an architectural framework for delivering IP multimedia services. Historically, mobile phones have provided voice call services over a circuit-switched-style network, rather than strictly over an IP packet-switched network. Alternative methods of delivering voice or other multimedia services have become available on smartphones, but they have not become standardized across the industry. IMS is an architectural framework to provide such standardization.
IMS was originally designed by the wireless standards body 3rd Generation Partnership Project, as a part of the vision for evolving mobile networks beyond GSM. Its original formulation represented an approach for delivering Internet services over GPRS. This vision was later updated by 3GPP, 3GPP2 and ETSI TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed lines.
IMS uses IETF protocols wherever possible, e.g., the Session Initiation Protocol. 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., to create a form of fixed-mobile convergence. 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, softswitches and "naked" SIP.
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.
Examples of global standards based on IMS are MMTel which is the basis for Voice over LTE, Wi-Fi Calling and Rich Communication Services which is also known as joyn or Advanced Messaging.

History

Some operators opposed IMS because it was seen as complex and expensive.
In response, a cut-down version of IMS -- enough of IMS to support voice and SMS over the LTE network -- was defined and standardized in 2010 as Voice over LTE.

Architecture

Each of the functions in the diagram is 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 : An implementer is free to combine two functions in one node, or to split a single function into two 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 IMS in various ways, most of which use the standard IP. IMS terminals can register directly on IMS, even when they are roaming in another network or country. The only requirement is that they can use IP and run SIP user agents. Fixed access, mobile access and wireless access are all supported. Other phone systems like plain old telephone service, H.323 and non IMS-compatible systems, are supported through gateways.

Core network

HSS – Home subscriber server:

The home subscriber server, or user profile server function, is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information, 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 and Authentication centre.
A subscriber location function is needed to map user addresses when multiple HSSs are used.
User identities:

Various identities may be associated with IMS: IP multimedia private identity, IP multimedia public identity, globally routable user agent URI, wildcarded public user identity. Both IMPI and IMPU are not phone numbers or other series of digits, but uniform resource identifier, that can be digits or alphanumeric identifiers.
IP Multimedia Private Identity:

The IP Multimedia Private Identity is a unique permanently allocated global identity assigned by the home network operator, it has the form of an Network Access Identifier i.e. user.name@domain, and is used, for example, for Registration, Authorization, Administration, and Accounting purposes. Every IMS user shall have one IMPI.
IP Multimedia Public Identity:

The IP Multimedia Public Identity is used by any user for requesting communications to other users. Also known as Address of Record. 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.
Globally Routable User Agent URI:

Globally Routable User Agent URI is an identity that identifies a unique combination of IMPU and UE instance.
There are two types of GRUU: Public-GRUU and Temporary GRUU.
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, MSISDN, subscriber service profiles, service triggers, and other information.

Call Session Control Function (CSCF)

Several role of SIP servers or proxies, collectively called Call Session Control Function, are used to process SIP signaling packets in the IMS.
SIP Application servers host and execute services, and interface with the S-CSCF using SIP. An example of an application server that is being developed in 3GPP is the Voice call continuity Function. Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA 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.
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 for an incoming leg and the AS-OLCM interfaces to the S-CSCF for an outgoing leg.
Application Logic provides the service and interacts between the AS-ILCM and AS-OLCM.
Public Service Identity
Public Service Identities are identities that identify services, which are hosted by application servers. As user identities, PSI takes 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:
The Media Resource Function provides media related functions such as media manipulation and playing of tones and announcements.
Each MRF is further divided into a media resource function controller and a media resource function processor.
The Media Resource Broker 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:
A Breakout Gateway Control Function 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 networks. For signalling, CS networks use ISDN User Part over Message Transfer Part, while IMS uses SIP over IP. For media, CS networks use Pulse-code modulation, while IMS uses Real-time Transport Protocol.
Media Resources are those components that operate on the media plane and are under the control of IMS core functions. Specifically, Media Server and Media gateway

NGN interconnection

There are two types of next-generation networking interconnection:
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 acting as transit networks. The intermediate network domain provide 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. 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.
Charging function addresses are addresses distributed to each IMS entities and provide a common location for each entity to send charging information. charging data function addresses are used for offline billing and Online Charging Function for online billing.
IMS-based PES provides IP networks services to analog devices. IMS-based PES allows non-IMS devices to appear to IMS as normal SIP users. Analog terminal using standard analog interfaces can connect to IMS-based PES in two ways:
Both A-MGW and VGW are unaware of the services. They only relay call control signalling to and from the PSTN terminal. Session control and handling is done by IMS components.

Interfaces description

Interface nameIMS entitiesDescriptionProtocolTechnical specification
CrMRFC, ASUsed by MRFC to fetch documents from an AS. Also used for media control related commands.TCP/SCTP channels-
Cx, HSSUsed to send subscriber data to the S-CSCF; including filter criteria and their priority. Also used to furnish CDF and/or OCF addresses.DiameterTS29.229, TS29.212
DhAS <-> SLFUsed by AS to find the HSS holding the user profile information in a multi-HSS environment. DH_SLF_QUERY indicates an IMPU and DX_SLF_RESP return the HSS name.Diameter-
Dx <-> SLFUsed by I-CSCF or S-CSCF to find a correct HSS in a multi-HSS environment. DX_SLF_QUERY indicates an IMPU and DX_SLF_RESP return the HSS name.DiameterTS29.229, TS29.212
GmUE, P-CSCFUsed to exchange messages between SIP user equipment or Voip gateway and P-CSCFSIP-
GoPDF, GGSNAllows operators to control QoS in a user plane and exchange charging correlation information between IMS and GPRS networkCOPS, diameter -
GqP-CSCF, PDFUsed to exchange policy decisions-related information between P-CSCF and PDFDiameter-
GxPCEF, PCRFUsed to exchange policy decisions-related information between PCEF and PCRFDiameterTS29.211, TS29.212
GyPCEF, OCSUsed for online flow-based bearer charging. Functionally equivalent to Ro interfaceDiameterTS23.203, TS32.299
ISCS-CSCF <-> ASReference point between S-CSCF and AS. Main functions are to :
  • Notify the AS of the registered IMPU, registration state and UE capabilities
  • Supply the AS with information to allow it to execute multiple services
  • Convey charging function addresses
SIP-
IciIBCFsUsed to exchange messages between an IBCF and another IBCF belonging to a different IMS network.SIP-
IziTrGWsUsed to forward media streams from a TrGW to another TrGW belonging to a different IMS network.RTP-
MaI-CSCF <-> ASMain functions are to:
  • Forward SIP requests which are destined to a public service identity hosted by the AS
  • Originate a session on behalf of a user or public service identity, if the AS has no knowledge of a S-CSCF assigned to that user or public service identity
  • Convey charging function addresses
    		• SIP-
    MgMGCF -> I,S-CSCFISUP signalling to SIP signalling and forwards SIP signalling to I-CSCFSIP-
    MiS-CSCF -> BGCFUsed to exchange messages between S-CSCF and BGCFSIP-
    MjBGCF -> MGCFUsed for the interworking with the PSTN/CS domain, when the BGCF has determined that a breakout should occur in the same IMS network to send SIP message from BGCF to MGCFSIP-
    MkBGCF -> BGCFUsed for the interworking with the PSTN/CS domain, when the BGCF has determined that a breakout should occur in another IMS network to send SIP message from BGCF to the BGCF in the other networkSIP-
    MmI-CSCF, S-CSCF, external IP networkUsed for exchanging messages between IMS and external IP networksSIP-
    MnMGCF, IM-MGWAllows control of user-plane resourcesH.248-
    MpMRFC, MRFPAllows an MRFC to control media stream resources provided by an MRFP.H.248-
    MrMr'S-CSCF, MRFC
    AS, MRFC    		Used to exchange information between S-CSCF and MRFC
    Used to exchange session controls between AS and MRFC    		Application server sends SIP message to MRFC to play tone and announcement. This SIP message contains sufficient information to play tone and announcement or provide information to MRFC, so that it can ask more information from application server through Cr Interface.SIP
    MwP-CSCF, I-CSCF, S-CSCF, AGCFUsed to exchange messages between CSCFs. AGCF appears as a P-CSCF to the other CSCFsSIP-
    MxBGCF/CSCF, IBCFUsed for the interworking with another IMS network, when the BGCF has determined that a breakout should occur in the other IMS network to send SIP message from BGCF to the IBCF in the other networkSIP-
    P1AGCF, A-MGWUsed for call control services by AGCF to control H.248 A-MGW and residential gatewaysH.248-
    P2AGCF, CSCFReference point between AGCF and CSCF.SIP-
    RcMRB, ASUsed by the AS to request that media resources be assigned to a call when using MRB in-line mode or in query modeSIP, in query mode -
    RfP-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, ASUsed to exchange offline charging information with CDFDiameterTS32.299
    RoAS, MRFC, S-CSCFUsed to exchange online charging information with OCFDiameterTS32.299
    RxP-CSCF, PCRFUsed to exchange policy and charging related information between P-CSCF and PCRF
    Replacement for the Gq reference point.    		DiameterTS29.214
    ShAS, HSSUsed to exchange User Profile information between an AS and HSS. Also allow AS to activate/deactivate filter criteria stored in the HSS on a per-subscriber basisDiameter-
    SiIM-SSF, HSSTransports CAMEL subscription information, including triggers for use by CAMEL-based application services information.MAP-
    SrMRFC, ASUsed by MRFC to fetch documents from an ASHTTP-
    UtUE and SIP AS PES AS and AGCFFacilitates the management of subscriber information related to services and settingsHTTP, XCAP-
    zPOTS, Analog phones and VoIP gatewaysConversion of POTS services to SIP messages-

    Session handling

    One of the most important features of IMS, that of allowing for a SIP application to be dynamically and differentially triggered, is implemented as a filter-and-redirect signalling mechanism in the S-CSCF.
    The S-CSCF might apply filter criteria to determine the need to forward SIP requests to AS. It is important to note that services for the originating party will be applied in the originating network, while the services for the terminating party will be applied in the terminating network, all in the respective S-CSCFs.

    Initial filter criteria

    An initial filter criteria is an XML-based format used for describing control logic. iFCs represent a provisioned subscription of a user to an application. They are stored in the HSS as part of the IMS Subscription Profile and are downloaded to the S-CSCF upon user registration or on processing demand. iFCs are valid throughout the registration lifetime or until the User Profile is changed.
    The iFC is composed of:
    There are two types of iFCs:
    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. This mechanism relies on the authentication performed during the network attachment procedures, which binds between the user's profile and its IP address. This mechanism is also weak because the signaling is not protected on the user–network interface.
    CableLabs in PacketCable 2.0, which adopted also the IMS architecture but has no USIM/ISIM capabilities in their terminals, published deltas to the 3GPP specifications where the Digest-MD5 is a valid authentication option. Later on, TISPAN also did a similar effort given their fixed networks scopes, although the procedures are different. To compensate for the lack of IPsec capabilities, TLS has been added as an option for securing the Gm interface. Later 3GPP Releases have included the Digest-MD5 method, towards a Common-IMS platform, yet in its own and again different approach. Although all 3 variants of Digest-MD5 authentication have the same functionality and are the same from the IMS terminal's perspective, the implementations on the Cx interface between the S-CSCF and the HSS are different.