Architecture
3GPP / TISPAN IMS Architectural Overview
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 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 (such as mobile phones, personal digital assistants (PDAs) and computers) can register directly on IMS, even when they are roaming in another network or country (the visited network). The only requirement is that they can use IP and run 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.Core network
HSS – 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 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
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.
CSCF – Call Session Control Function
Several roles of 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 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 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 before registration, and does not change for the duration of the registration
- it sits on the path of all signalling, and can inspect every signal; the IMS terminal must ignore any other unencrypted signalling
- it provides subscriber authentication and may establish an IPsec or TLS security association with the IMS terminal. This prevents spoofing attacks and replay attacks 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, do not obey home network's routing policy)
- 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 is 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 user profiles and upload
user-to-S-CSCF associations (the user profile is only cached locally for
processing reasons only 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.
- 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.
- 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 NAT and firewall functions (pinholing). The IBCF is practically a Session Border Controller specialized for the NNI.
Application servers
See also: Next Generation Network Services
SIP Application servers (AS) 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 (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
Functional 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 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.
- 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 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, 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.
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.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 (CDF) addresses are used for offline billing and Online Charging Function (OCF) for online billing.
- 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 CDF located in the
same domain. The CCF will collect all this information, and build a Call Detail Record (CDR), which is sent to the billing system (BS) of the domain.
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. Billing systems in different domains will also exchange information, so that roaming charges can be applied.
- Online charging : The S-CSCF talks to a Session 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 an OCF.
- When Immediate Event Charging (IEC) 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 Event Charging with Unit Reservation (ECUR) 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.
IMS-Based PES Architecture
IMS-based PES (PSTN Emulation System) 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 -- Via A-MGW (Access Media Gateway) that is linked and controlled by AGCF. AGCF is placed within the Operators network and controls multiple A-MGW. A-MGW and AGCF communicate using H.248.1 (Megaco) over the P1 reference point. POTS phone connect to A-MGW over the z interface. The signalling is converted to H.248 in the A-MGW and passed to AGCF. AGCF interprets the H.248 signal and other inputs from the A-MGW to format H.248 messages into appropriate SIP messages. AGCF presents itself as P-CSCF to the S-CSCF and passes generated SIP messages to S-CSCF or to IP border via IBCF (Interconnection Border Control Function). Service presented to S-CSCF in SIP messages trigger PES AS. AGCF has also certain service independent logic, for example on receipt of off-hook event from A-MGW, the AGCF requests the A-MGW to play dial tone.
- Via VGW (VoIP-Gateway) or SIP Gateway/Adapter on customer premises. POTS phones via VOIP Gateway connect to P-CSCF directly. Operators mostly uses Session Border Controller between VoIP Gateway and P-CSCF for security and to hide network topology. VoIP Gateway link to IMS using SIP over Gm reference point. The conversion from POTS service over the z interface to SIP occurs in the customer premises VoIP Gateway. POTS signaling is converted to SIP and passed on to P-CSCF. VGW acts as SIP user agent ans appears to P-CSCF as SIP terminal.
Interfaces description
| Interface Name | IMS entities | Description | Protocol | Technical Specification |
|---|---|---|---|---|
| Cr | MRFC, AS | Used by MRFC to fetch documents (e.g. scripts, announcement files, and other resources) from an AS. Also used for media control related commands. | TCP/SCTP channels | |
| Cx | (I-CSCF, S-CSCF), HSS | Used to send subscriber data to the S-CSCF; including Filter criteria and their priority. Also used to furnish CDF and/or OCF addresses. | Diameter | |
| Dh | AS (SIP AS, OSA, IM-SSF) <-> SLF | Used by AS to find the HSS holding the User Profile information in a multi-HSS environment. DH_SLF_QUERY indicates a IMPU and DX_SLF_RESP return the HSS name. | Diameter | |
| Dx | (I-CSCF or S-CSCF) <-> SLF | Used by I-CSCF or S-CSCF to find a correct HSS in a multi-HSS environment. DX_SLF_QUERY indicates a IMPU and DX_SLF_RESP return the HSS name. | Diameter | |
| Gm | UE, P-CSCF | Used to exchange messages between SIP user equipment (UE) or Voip Gateway and P-CSCF | SIP | |
| Go | PDF, GGSN | Allows operators to control QoS in a user plane and exchange charging correlation information between IMS and GPRS network | COPS (Rel5), Diameter (Rel6+) | |
| Gq | P-CSCF, PDF | Used to exchange policy decisions-related information between P-CSCF and PDF | Diameter | |
| Gx | PCEF,PCRF | Used to exchange policy decisions-related information between PCEF and PCRF | Diameter | TS29.211, TS29.212 |
| ISC | S-CSCF <-> AS | Reference point between S-CSCF and AS. Main functions are to :
|
SIP | |
| Ici | IBCFs | Used to exchange messages between an IBCF and another IBCF belonging to a different IMS network. | SIP | |
| Izi | TrGWs | Used to forward media streams from a TrGW to another TrGW belonging to a different IMS network. | RTP | |
| Ma | I-CSCF <-> AS | Main functions are to:
|
SIP | |
| Mg | MGCF -> I,S-CSCF | ISUP signalling to SIP signalling and forwards SIP signalling to I-CSCF | SIP | |
| Mi | S-CSCF -> BGCF | Used to exchange messages between S-CSCF and BGCF | SIP | |
| Mj | BGCF -> MGCF | Used 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 MGCF | SIP | |
| Mk | BGCF -> BGCF | Used 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 network | SIP | |
| Mm | I-CSCF, S-CSCF, external IP network | Used for exchanging messages between IMS and external IP networks | SIP | |
| Mn | MGCF, IM-MGW | Allows control of user-plane resources | H.248 | |
| Mp | MRFC, MRFP | Allows an MRFC to control media stream resources provided by an MRFP. | H.248 | |
| Mr Mr' |
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 |
| Mx | BGCF/CSCF, IBCF | Used 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 network | SIP | |
| Mw | P-CSCF, I-CSCF, S-CSCF, AGCF | Used to exchange messages between CSCFs. AGCF appears as a P-CSCF to the other CSCFs | SIP | |
| P1 | AGCF, A-MGW | Used for call control services by AFCG to control H.248 A-MGW and Residential Gateways | H.248 | |
| P2 | AGCF, CSCF | Reference point between AGCF and CSCF. | SIP | |
| Rc | MRB, AS | Used by the AS to request that media resources be assigned to a call when utilizing MRB In-Line mode or In Query mode | SIP, In Query mode (Not specified) | |
| Rf | P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS | Used to exchange offline charging information with CDF | Diameter | |
| Ro | AS, MRFC, S-CSCF | Used to exchange online charging information with OCF | Diameter | TS32.299 |
| Rx | P-CSCF, PCRF | Used to exchange policy and charging related information between P-CSCF and PCRF
Replacement for the Gq reference point. |
Diameter | |
| Sh | AS (SIP AS, OSA SCS), HSS | Used to exchange User Profile information (e.g. user related data, group lists, user service related information or user location information or charging function addresses (used when the AS has not received the third party REGISTER for a user)) between an AS (SIP AS or OSA SCS) and HSS. Also allow AS to activate/deactivate filter criteria stored in the HSS on a per subscriber basis | Diameter | |
| Si | IM-SSF, HSS | Transports CAMEL subscription information including triggers for use by CAMEL based application services information. | MAP | |
| Sr | MRFC, AS | Used by MRFC to fetch documents (scripts and other resources) from an AS | HTTP | |
| Ut | UE and SIP AS (SIP AS, OSA SCS, IM-SSF) PES AS and AGCF | Facilitates the management of subscriber information related to services and settings | HTTP(s), XCAP | |
| z | POTS, Analog phones and VoIP Gateways | Conversion 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 (based on the user's profile) 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
Initial Filter Criteria (iFC) are filter criteria that are stored in the HSS as part of the IMS Subscription Profile and are downloaded to the S-CSCF upon user registration (for registered users) or on processing demand (for services, acting as unregistered 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. The term Shared iFC denotes an iFC which, due to its common use for a large number of subscribers, is only referenced in the Subscription Profile and provisioned on a different path between the HSS and the S-CSCF.The iFC may be composed of:
- An Application Server URI where the request is to be forwarded in case of a match.
- A Trigger Point in the form of a logical condition which is verified against initial dialog creating SIP requests or stand-alone SIP requests.
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