Editing IP Multimedia Subsystem (section)

=== Core network ===
'''HSS – Home subscriber server:'''<br />
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 [[Telecommunication|calls]]. It contains the subscription-related information (subscriber [[User profile|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 [[Network Switching Subsystem#Home location register (HLR)|home location register]] (HLR) and [[Network Switching Subsystem#Authentication centre (AUC)|Authentication centre]] (AuC).

A ''[[subscriber location function]]'' (SLF) is needed to map user addresses when multiple HSSs are used.

'''User identities:'''<br />
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, such as ''tel:+1-555-123-4567'') or alphanumeric identifiers (a SIP URI, such as ''sip:john.doe@example.com"  ).

'''IP Multimedia Private Identity:'''<br />
The ''IP Multimedia Private Identity'' (IMPI) is a unique permanently allocated global identity assigned by the home network operator. It has the form of a Network Access Identifier(NAI) 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:'''<br />
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]]). Also known as Address of Record (AOR). 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:'''<br />
''Globally Routable User Agent URI'' (GRUU) is an identity that identifies a unique combination of IMPU and [[User equipment|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

'''Wildcarded Public User Identity:'''<br />
A ''wildcarded Public User Identity'' expresses a set of IMPU grouped together.
		 
The HSS subscriber database contains the IMPU, IMPI, [[International Mobile Subscriber Identity|IMSI]], [[MSISDN]], subscriber service profiles, service triggers, and other information.

====Call Session Control Function (CSCF) ====
Several roles of SIP servers or proxies, collectively called Call Session Control Function (CSCF), are used to process SIP signaling 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 is not IMS compliant yet). Some networks may use a [[Session Border Controller]] (SBC) for this function. The P-CSCF is at its core a specialized SBC for the [[User–network interface]] which not only protects the network, but also the IMS terminal. The use of an additional SBC between the IMS terminal and the P-CSCF is unnecessary and infeasible due to the signaling being encrypted on this leg. The terminal discovers its P-CSCF with either [[Dynamic Host Configuration Protocol|DHCP]], or it may be configured (e.g. during initial provisioning or via a 3GPP IMS Management Object (MO)) or in the [[IP Multimedia Services Identity Module|ISIM]] or assigned in the [[PDP Context]] (in [[General Packet Radio Service]] (GPRS)).
** It is assigned to an IMS terminal before registration, and does not change for the duration of the registration.
** It sits on the path of all signaling, and can inspect every signal; the IMS terminal must ignore any other unencrypted signaling.
** It provides subscriber authentication and may establish an [[IPsec]] or [[Transport Layer Security|TLS]] security association with the IMS terminal. This prevents [[spoofing attack]]s and [[replay attack]]s and protects the privacy of the subscriber.
** It inspects the signaling and ensures that the IMS terminals do not misbehave (e.g. change normal signaling routes, disobey home network's routing policy).
** It can 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 is used for policy control, bandwidth management, etc. The PDF can also be a separate function.
** It also generates charging records.
* 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 record|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.
** it queries the HSS to retrieve the address of the S-CSCF and assign it to a user performing SIP registration
** it also forwards 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 parts 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 [[Network address translation|NAT]] and [[Firewall (networking)|firewall]] functions ([[Firewall pinhole|pinholing]]). The IBCF is a [[session border controller]] specialized for the [[network-to-network interface]] (NNI).
* A ''Serving-CSCF'' (S-CSCF) is the central node of the signaling 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 user profiles and upload user-to-S-CSCF associations (the user profile is only cached locally for processing reasons and is not changed). All necessary subscriber profile 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 of the locally registered users, 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. There are multiple options for this purpose, including a mandatory/optional capabilities to be matched between subscribers and S-CSCFs.

==== Application servers ====
{{see also|Next generation network services}}
SIP [[Application server]]s (AS) host and execute [[Service (systems architecture)|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 (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 [[Camel Application Part|CAP]] to communicate with [[Customized Applications for Mobile networks Enhanced Logic|CAMEL]] Application Servers
* [[Open Services Access|OSA]] service capability server (OSA SCS): Interfaces SIP to the OSA framework;

===== Functional model =====
The AS-ILCM (Application Server - Incoming Leg Control Model) and AS-OLCM (Application Server - Outgoing Leg Control Model) 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 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:
* 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 interprets 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 resources.

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 switching|circuit switched]] (CS) networks. For signalling, CS networks use [[ISDN User Part]] (ISUP) (or [[Bearer Independent Call Control|BICC]]) over [[Message Transfer Part]] (MTP), while IMS uses 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 IP protocol) into [[Message Transfer Part]] (MTP, a [[Signalling System 7]] (SS7) protocol), to pass [[ISDN User Part]] (ISUP) from the MGCF to the CS network. The SGW does call control protocol conversion between SIP and ISUP/BICC under the control of the MGCF.
* A ''media gateway controller function'' (MGCF) is a SIP endpoint that interfaces with the SGW over SCTP. It also controls the resources in a ''[[Media Gateway]]'' (MGW) across an [[Gateway Control Protocol|H.248]] interface.
* A ''[[media gateway]]'' (MGW) interfaces with the media plane of the CS network, by converting between [[Real-time Transport Protocol|RTP]] and [[PCM]]. It can also transcode when the [[codec]]s don't match (e.g., IMS might use [[Adaptive Multi-Rate|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)