IP multimedia subsystem

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The IP Multimedia Subsystem ( IMS ) is a telecommunications system that aims at standardized access to services from different networks. It was specified by the 3rd Generation Partnership Project Committee (3GPP). IMS uses an All- IP network in which all communication is IP-based. The basic protocol of IMS is the SIP . IMS systems are mainly used in cellular networks. The main tasks of IMS systems are:

  • Connection of traditional networks (cellular networks according to the GSM or UMTS standard, conventional analog or digital (ISDN) telephone networks ) with IP-based networks that use Voice over IP .
  • Provision of additional services for IP-based networks, e.g. B. Provision of voice services for LTE networks ( Voice over LTE ).
  • Connection of additional service platforms, e.g. B. Presence Services , RCS

aims

The basics of the IMS are described in the "Technical Specifications" of the 3GPP standardization bodies . The specifications can be obtained free of charge from the 3GPP website. The main features of IMS can be found in the specifications 3GPP TS 23.228 and 3GPP TS 23.002 (overall architecture). (see also chapter IMS specifications ). IMS also forms the basis for the Next Generation Network Release 1 described at ETSI (European Telecommunications Standards Institute) in the TISPAN group .

IMS supports the following services:

  • packet-switched connections between two or more participants
  • Cooperation between the circuit-switching and the packet-oriented domains
  • an end-to-end negotiation of the QoS ( Quality of Service )
  • service-dependent billing
  • Provision of the home network environment in foreign networks
  • Various media types support
  • fast and flexible creation of services through predefined service modules ("service enabler")
  • Services should be independent of the access network

history

IMS was created at 3GPP as part of the UMTS standardization. In "Release 4" published in 2001, initial ideas for these properties were explained. The first specifications for the IMS came as early as 2003 with "Release 5". Even then, the SIP was used for signaling . Interoperability with the GSM and GPRS networks should also be ensured. With "Releases" 6 and 7, a number of other properties were added - WLAN and fixed network as additional access networks as well as DRM , services for location determination, generic user profiles and others.

architecture

IMS architecture overview
3GPP / TISPAN IMS Architectural Overview - HSS in IMS layer (as by standard)

IMS function blocks

An IMS system consists of up to 20 function blocks. These function blocks can be assigned to four levels:

  • Level 1: User Plane / Gateways : Functions for managing and processing the IP data stream
    The IP data stream is connected to the IMS system via so-called gateways. There are gateway functions for
    • Data interfaces to the own IP-based telecommunication network (IMS function "Access Transfer Gateway" ATGW )
    • Data interfaces to external IP-based telecommunication networks (IMS function "Interconnect Border Gateway" I-BGF )
    • Data interfaces to non-IP-based telecommunication networks (classic GSM or UMTS networks, classic fixed network ) (IMS function "Multi Media Gateway" IM-MGW )
    • Data interfaces to internal IMS data sources such as announcements, sources for data to be downloaded (IMS function "Multimedia Resource Function Processor" MRFP )
  • Level 2: Control Plane / Gateway control : Functions for direct control and signaling of the data streams
    Each gateway that directs the data streams to and from the IMS system has a control unit that controls these data streams and connects to its own network or other networks Exchanges signaling information. Information about the data packets is transmitted via these signaling interfaces, for example information about the participants (phone number, IP address, APN), signals to control the calls or sessions ("sessions") are exchanged (ringing, answering the call, End of call), and ongoing calls or "sessions" are monitored and controlled (exchange charge information, check subscriber credit, guarantee connection quality or data rate).
    There are functions for
    • Signaling interfaces to your own IP-based telecommunication network (IMS function "Access Transfer Control Function" ATCF )
    • Signaling interfaces to external IP-based telecommunication networks (IMS function "Interconnect Border Control Function" I-BCF )
    • Signaling interfaces to non-IP-based telecommunications networks (classic GSM or UMTS networks, classic landline network ) (IMS function "Media Gateway Control Function" MGCF )
    • Signaling interfaces to IMS-internal data sources such as announcements, sources for data to be downloaded (IMS function "Multimedia Resource Function Controller" MRFC )
  • Level 3: Call Control / Session Control : Functions for controlling the course of calls or the "session" sequence.
    The entire course of a call or session is controlled in these signaling functions. This is about functions to locate a participant in your own or in foreign networks, to set up a conversation or session with this participant, to monitor and control this conversation or this session and finally this conversation or this session again dismantle.
    There are functions for
    • Authentication of a participant, definition of participant profiles (IMS function "Proxy Call Session Control Function" P-CSCF )
    • Searching for a participant in IMS networks (IMS function "Interrogating Call Session Control Function" I-CSCF )
    • Log in a participant and control his call or his session (IMS function "Service Call Session Control Function" S-CSCF )
    • Control of emergency calls (IMS function "Emergency Call Session Control Function" E-CSCF )
    • Forwarding the call or session from the IMS system to other network components. (IMS function "Breakout Gateway Control Function" BGCF )
  • Level 4: Service functions : Functions for integrating additional services into the conversation or session.
    Above all, this includes functions to support an existing call or session and functions to connect additional data sources.
    There are functions for
    • Monitoring of the connection quality of an existing conversation or an existing session in order to e.g. B. Avoid interruptions in voice communication. (IMS function "Service Centralization and Continuity Application Server" SCC-AS )
    • Monitoring of the connection quality of emergency calls (IMS function "Emergency Access Transfer Function" EATF )
    • Connection of classic telephone services (e.g. to manage prepaid subscribers) (IMS function "IP Multimedia Service Switching Function" IM-SSF )
    • Connection of message services such as SMS (IMS function "IP Multimedia Short Message Gateway" IP-SM-GW )
    • Integration of new services (IMS function "Multimedia Telephony Service" MMTEL )

Access networks

If services of the IMS system are required for a specific conversation or for a specific data connection (“data session”), the telephone network requests the corresponding IMS services. This can be done both from your own network (e.g. by directly forwarding a service request to the "IMS Call Session Control Function") and from external networks, in that the external network signals this to the gateway controller is responsible for the network transition. The telephone network decides on the basis of the services requested by a user and on the basis of the user properties which system is to process this call or this session or whether another system is used for this purpose. Services based on the use of an IMS system can be provided for almost all users of a telephone network, because different IMS gateways allow access to the IMS system for a large number of different end devices and different connections of the users to the telephone network.

If a terminal device ( mobile phones , PDAs and computers) supports IMS-relevant properties (such as Session Initiation Protocol (SIP) user agents), it can be connected relatively directly to the IMS system via the telephone network. For terminals without direct IMS support, the telephone network provides appropriate gateways and auxiliary systems to connect these systems. B. Gateways for fixed network access (for example: Digital Subscriber Line (DSL), cable modems , Ethernet ), gateways for mobile access (for example: W-CDMA , CDMA2000 , GSM , GPRS ) and gateways for wireless access (for example: WLAN , WiMAX ), as well as gateways for other telephony systems such as plain old telephone service (POTS — the old analog telephones), H.323 and for non-compatible VoIP systems.

The IMS system can either be accessed via one of the gateways and the associated gateway controller, but it can also go directly to the central call session control function .

Gateways and Gateway Controllers

The gateways connect the data streams from other systems and networks to the IMS system and route them through. Corresponding gateway controllers belong to each gateway , which exchange signaling information belonging to a data stream with other network components or other networks and specify to the gateways how the data streams are to be directed and manipulated.

The Access Transfer Gateway ( ATGW ) and the Access Transfer Control Function ( ATCF ) are available for access from your own packet-switching network :

  • Access Transfer Gateway ( ATGW ) / Application Gateway ( AGW ) can be implemented as an independent function or as part of the I-BGF. The ATGW carries out the adaptations to the conditions of the remote station in detail (e.g. protocol adaptations, converting from packet to circuit-switched data). This function is also known as a trunking gateway ( TGW , TrGW ).
  • Access Transfer Control Function ( ATCF ) / Application Layer Gateway ( ALG ) can be executed as a standalone function or as part of the I-BCF and serves as an anchor point for incoming and outgoing calls. Together with the SCC-AS, freedom from interruptions is implemented as far as necessary for the call (e.g. for voice calls). Also supports handover to circuit-switched networks

The "Interconnect Border Gateway" (I-BGF) and the "Interconnect Border Control Function" (I-BCF) are available for access from other IMS networks.

  • Interconnect Border Gateway Function ( I-BGF ) controls the lower transport layers, includes firewall functionalities and implements IP addresses for media connections. Protocol adjustments that are required depending on the remote station and call type are also carried out here (e.g. interpreting or importing fax signals and ring tones).
  • Interconnect Border Control Function ( I-BCF ): Serves as an interface to other networks or network segments, in particular to circuit-switching systems. This function is used to monitor and control the transport layers and to determine and assign the resources necessary for a call. Special functions such as emergency calls or telecommunication monitoring are supported by this function.

The "IMS Multi Media Gateway" (IM-MGW) and the "Media Gateway Control Function" (MGCF) are available for access from circuit-switching networks.
The circuit switched (CS) network and the IP-based IMS network differ in some points. While IMS uses SIP for signaling, the Signaling System 7 (SS7) protocol stack is often used in CS networks . ISDN , for example, uses the ISDN User Part (ISUP), which is routed via the Message Transfer Part ( MTP ).
IMS uses the Real-Time Transport Protocol (RTP) to transmit media streams , while CS networks use pulse code modulation .
In order to be able to exchange data between the different networks in spite of these differences, you need appropriate interfaces (gateways).

  • An IP-Multi-Media Gateway ( IM-MGW ) is the connection between IMS and the circuit-switching systems at the data level. It converts the data streams in RTP and PCM into the other format. The gateway can also adapt the data stream, for example if the audio / video codec used is not understood in the other network (for example IMS uses AMR and PSTN G.711 ).
  • The Media Gateway Controller Function ( MGCF ) serves as an interface between SIP and ISUP as well as to the SGW via SCTP. The MGCF monitors the requested resources in the media gateway (MGW) via the H.248 interface.

Call control / session control

The central functions of the IMS system are the call session control functions , in which the course of a call or session is determined. The Call Session Control Functions exchange this information with the gateway controllers.

In addition, the call session control functions communicate with the service applications in order to provide additional services and to integrate them into the conversation or session.

Call session control functions

In an IMS system, all conversations, data transfers, file downloads, etc. are viewed as "multimedia sessions". The central element of IMS is the Call Session Control Function ( CSCF ), which sets up these "multimedia sessions", monitors and supports their operation and also dismantles them at the end. Controlled by the CSCF, the gateways and their controllers regulate the data transfer for these “multimedia sessions”. The "Service Applications" are integrated by the CSCF in order to provide additional services that are desired by the participants or that are required for the type of conversation or session that the participant is conducting.

The CSCF consists of four different components: S-CSCF (service), P-CSCF (proxy), I-CSCF (interrogating) and E-CSCF (emergency). The SIP protocol is used as a call control protocol for communication between the IMS components and with the "user clients". The components of the CSCF essentially fulfill the following tasks:

  • Proxy call session control function P-CSCF : If a subscriber uses a service that requires the integration of the IMS system, the P-CSCF represents the first contact point for this service request. If the subscriber is in a foreign network, so the P-CSCF of the visited network can be used, or the P-CSCF of the home network can be used (e.g. if the visited network is not IMS-compatible). The P-CSCF can find out either with DHCP or via a PDP context (in GPRS networks). The P-CSCF has the following tasks:
    • works as a SIP proxy and forwards requests to the I-CSCF and S-CSCF as well as responses and requests to the “user client”
    • Authentication of the user when entering the network
    • Compress and decompress SIP messages
    • contains the Policy Decision Function (PDF) which negotiates the Quality of Service for the underlying networks
    • generates necessary data for billing
  • Interrogating Call Session Control Function I-CSCF : The task of the I-CSCF is to inquire about the S-CSCF that is to manage the desired “multimedia session”. The IP address of the I-CSCF can be determined using DNS. Remote servers can use this IP to forward SIP requests to the domain of the I-CSCF. The I-CSCF queries the S-CSCF in which the subscriber is registered on the HSS ( Home Subscriber Server ) using the diameter protocol . If the subscriber is in a foreign network, the I-CSCF is used to establish contact with the home network. The I-CSCF then forwards the incoming SIP requests to the corresponding S-CSCF.
  • Service Call Session Control Function S-CSCF : The S-CSCF is the central unit of the IMS that controls the multimedia sessions of the IMS. All actions for session management (setup, dismantling, monitoring) are coordinated by the S-CSCF. S-CSCF downloads and uploads user profiles from the HSS. It does not temporarily store this information, but leaves the consistent storage to the HSS. Further tasks are:
    • Handling of a user's SIP registration
    • Forwarding of all SIP messages
    • Decision based on the required services and the selected filter criteria, to which "application server" the signaling messages are forwarded
    • Enforcement of the network operator's policy
    • Collection of session data for billing
  • Emergency Call Session Control Function E-CSCF : Serves as a contact point for emergency services (police, fire brigade). Emergency calls are forwarded from the P-CSCF or the S-CSCF to the E-CSCF. The E-CSCF uses the Emergency Access Transfer Function (EATF) to establish the connection to the emergency services.

Media server

If the data streams of a multimedia session are to be processed or if additional data is to be imported, the Multimedia Resource Function ( MRF ) is triggered by the S-CSCF for this purpose. With the help of the MRF, data streams can be manipulated (for example: mixing several voice channels in conference calls), or data from other sources can be played back (voice announcements, voicemail box). The MRF is divided into 2 main functionalities, the Multimedia Resource Function Controller (MRFC) and the Multimedia Resource Function Processor (MRFP).

  • The Multimedia Resource Function Controller ( MRFC ) controls the resources of the MRFP available for the media streams (via the H.248 interface). Application Server (AS) and S-CSCF provide the necessary information for this.
  • The Multimedia Resource Function Processor ( MRFP ) handles the corresponding data streams. Controlled by the MRFC, the MRFP can mix media streams (e.g. conference call), serve as a provider of media streams ( e.g. multimedia announcements) or process media streams (e.g. voice and image transcoding, analysis of media).

Breakout gateway

Once the details of a session have been determined and a session is to be established, the data streams must be routed from the source to the destination. This means that the gateway controllers must be informed how the data streams are to be directed. For this purpose, the Breakout Gateway Control Function ( BGCF ) is used to determine via which gateway and which gateway controller the session is to be conducted.

The BGCF is a SIP server that makes routing decisions based on a telephone number . The BGCF always used when out of the IMS in a circuit-switched (engl. Circuit switched network) network, a connection should be established. That would be the case with calls to the fixed network or to the PLMN (Public Land Mobile Network), the public land-based mobile network (in Germany GSM and UMTS ).

If the caller and the called party are in the same network, the BGCF selects a suitable MGCF (Media Gateway Control Function) that takes over the signaling to the circuit-switching network. If it is necessary to switch to another network, the BGCF forwards the signaling via the I-BCF to the BGCF of the corresponding network.

Service functions / application server

The S-CSCF is supported by a number of service functions that are housed in different service applications. Application servers ( AS ) manage services and execute them when required. The S-CSCF interacts with an AS via SIP. The AS can act as a SIP UA (user agent), SIP proxy or SIP B2BUA (back-to-back user agent) to the S-CSCF. As a SIP UA, it can both receive calls (e.g. mailbox) and act as a caller (e.g. automatic wake-up service). As a SIP proxy it forwards calls, and as a SIP B2BUA it can initiate calls itself (for example Click2Dial). If the AS is in the home network, it can address the HSS via the Diameter protocol, for example.

Among other things there are:

  • SIP AS: Standard IMS Application Server
  • IP Multimedia Service Switching Function ( IM-SSF ): integrates the CAMEL services into the IMS; uses Camel Application Part (CAP)
  • Open Service Access Service Capability Server ( OSA-SCS ): Interface to the OSA framework application server
  • Service Centralization and Continuity Application Server ( SCC-AS ), ensures that calls from "Circuit Switched" to "Packet Switched" are made without disconnection. Communicates with the circuit switched systems such as MSS. Works together with IM-SSF, MMTel and IP-SW-GW
  • IP Multimedia Short Message Gateway ( IP-SM-GW ), interface for sending SMS using the IMS system

Via the application layer it is also possible to integrate multimedia standards such as

  • Multimedia Telephony Service ( MMTel ), 3GPP standard for the integration of voice, real-time video, file transfer, distribution of images, audio and video clips, etc. Offers a standardized interface to other networks.
  • Rich Communication Suite ( RCS ) for the transmission of short messages, files, voice, video telephony

Access to other systems

Home Subscriber Server

The IMS system requires access to a participant database in which all participant data relevant to establishing a session are stored. With the help of the subscriber database, the subscriber is identified as a member of his own network, the permitted services of the IMS are communicated so that the IMS can then set up the session according to the subscriber properties.

The Home Subscriber Server ( HSS ) or User Profile Server Function ( UPSF ) is a database in which user profiles are stored. The HSS is responsible for authenticating and authorizing users. It also contains information about which services and media a user has authorizations for. The filter criteria stored in the HSS are used when selecting the application server to be used.

The HSS has the same role in the IMS as the Home Location Register (HLR) and the authentication center in GSM. If a user is entered in several HSSs, the Subscription Locator Function (SLF) decides which HSS is used. HSS and SLR both use the Diameter protocol . Everything together is also known as the Triple A System (AAA).

Signaling gateway

The Signaling Gateway ( SGW ) is responsible for translating the signaling at the transport level - for example Stream Control Transmission Protocol (SCTP, an IP protocol) to Message Transfer Part (MTP, a Signaling System 7 protocol).

IMS connection establishment

This section describes the establishment of a connection with IMS systems using the example of an IMS system that is integrated into a cellular network.

Structure of an IMS system with interfaces to a cellular network

Determine the system that handles an IMS service request

Proxy Call Session Control Function (P-CSCF)

If a terminal device within a telecommunications network requires a service with IMS support (e.g. voice via LTE for cell phones ), a corresponding request is made to the responsible P-CSCF (Proxy Call Session Control Function). The P-CSCF can be specified by the terminal (e.g. via a corresponding entry on the SIM card of the mobile phone), but it can also be determined by the network (e.g. during the dial-in procedure into the data network via a corresponding PDP -Context ) or via DHCP .

The P-CSCF authenticates the user. The P-CSCF can exchange information about the subscriber services and the data rates with the PCRF and also create charging records. The PCRF can also be used to influence the properties of the connection (e.g. throttling data rates, setting the priority of the subscriber, etc.)

The P-CSCF forwards the received IMS service request:

  • If the subscriber is in the home network, the request requested by the system is passed on to the I-CSCF in which the subscriber is registered.
  • If the P-CSCF detects that the subscriber is not in its own network, the P-CSCF can also forward the request to the I-BCF and thus forward the service request to another network (e.g. to the subscriber's home network) .
  • If the P-CSCF detects that it is an emergency call, the request is immediately forwarded to the E-CSCF, which handles emergency calls.

Their responses and inquiries are returned to the terminal accordingly via the P-CSCF.

Interrogating Call Session Control Function (I-CSCF)

The I-CSCF is the contact point for all connection requests to participants in your own network. The main task is to determine the S-CSCF system in which the participant is registered.

The I-CSCF (Interrogating Call Session Control Function) requests the subscriber data required to establish the connection from the Home Subscriber Server ( HSS ). If there are several HSS systems in your own network, the HSS in which the subscriber data is stored can be determined via a request to the Subscriber Locator Function ( SLF ). The subscriber data contain, among other things, the address of the S-CSCF system in which the subscriber is currently registered. This address shows whether the subscriber is logged into his own IMS system or another IMS system. If the participant is logged into an external IMS system, the external IMS system can be informed of the connection request via the I-BCF .

The I-CSCF forwards the connection request to the responsible S-CSCF . The I-CSCF also generates charging information (charging records).

Run the IMS service

Service Call Session Control Function (S-CSCF)

The S-CSCF (Service Call Session Control Function) is the central instance that controls the service request or the connection establishment of the subscriber. The participant is booked into the IMS system via the S-CSCF. To do this, the S-CSCF queries the subscriber profiles from the HSS and thus receives information about the services and functions that are relevant for the subscriber. The S-CSCF then controls the establishment of the connection, monitors the connection and, at the end, clears the connection again. In addition, she monitors the billing for the participant during the entire time.

Guaranteeing freedom from interruption

Service Centralization and Continuity Application Server (SCC-AS)

For connections that require uninterrupted communication (e.g. voice, video), the connection must be continuously monitored and network resources must be allocated in such a way that interruptions are avoided. The organization of such connections is carried out by the SCC-AS (Service Centralization and Continuity Application Server). The participant session is anchored in the SCC-AS when the session is set up, and the SCC-AS then takes care of the control and prioritization of the service so that interruptions are avoided as far as possible.

Inquiries to other service systems

IP Multimedia Service Switching Function (IM-SSF)

Since services that are outside the IMS network are also required for an IMS service, the S-CSCF must also be able to access them. The IM-SSF (IP Multimedia Service Switching Function), which provides interfaces between IMS and "classic" systems and protocols, serves as an interface to other systems . This means that other network elements such as SCP (IN) , MSS , HLR can be accessed to support the IMS service .

The IM-SSF also carries out the necessary protocol conversions between the SIP protocol used in the IMS and the protocols used in the other network elements such as CAMEL , MAP , INAP . With the help of the IM-SSF z. B. Supports functions such as online billing for prepaid subscribers, for which access to the IN (SCP) systems is required, since the subscribers' prepaid credit is managed there.

Multimedia Telephony Service (MMTel)

The MMTel (Multimedia Telephony Service) establishes the connection to other multimedia systems and supports multimedia communication (e.g. voice, speech, real-time video, file transfer, distribution of pictures, audio and video clips etc.) between two (or more) participants. MMTel provides interfaces to other multimedia systems and also carries out the protocol conversions required for this.

Sending SMS

IP Short Message Gateway (IP-SM-GW)

In order to be able to send SMS short messages from the IMS system (e.g. to inform the subscriber about failed connection attempts, expired prepaid credit, etc.), an interface from the IMS system to the SMS or instant messaging systems is required ( e.g. SMSC ). The IP-SM-GW (IP Short Message Gateway), which is controlled by the S-CSCF when required, is used for this.

Ring tones and announcements

Multimedia Resource Function (MRF)

The MRF (Multimedia Resource Function) plays ringing tones and announcements when they are required in a session (e.g. signal tones from the telephone or "The participant is currently unavailable"). In addition, the MRF also supports advanced media functions, such as B. Conference calls.

Multimedia Resource Function Controller (MRFC)

If multimedia functions are requested that are supported by the MRF, such as B. the import of ring tones and announcements, the S-CSCF accesses the MRFC (Multimedia Resource Function Controller). The MRFC then coordinates e.g. B. importing the announcements and tones.

Multimedia Resource Function Processor (MRFP)

The MRFP (Multimedia Resource Function Processor) is triggered by the MRFC and then feeds e.g. B. the appropriate announcements.

Organize the data stream

The S-CSCF also organizes the data stream that begins after the connection has been established in its own network or to external networks. If “classic”, circuit-switched telephone networks are required to establish a connection, the S-CSCF organizes access to circuit-switched landline and mobile networks with the help of the BGCF . If the connection remains in the packet-switched network, the S-CSCF uses the I-BCF to organize the forwarding to other IMS networks or within its own packet-switched network.

Emergency calls

Emergency Call Session Control Function (E-CSCF)

If the connection request is an emergency call, the P-CSCF or the S-CSCF triggers the E-CSCF (Emergency Call Session Control Function), which organizes the emergency call. Instead of the S-CSCF , the connection establishment is then controlled by the E-CSCF.

The E-CSCF connects the participant with the emergency call center. If the emergency call center is connected via the classic circuit-switched telephone network, the transition point to the classic telephone network is determined with the help of the BGCF . If the emergency call center is connected via Voice-over-IP or is in a different IMS network, the connection is organized via the I-BCF .

Emergency Access Transfer Function (EATF)

The connection quality for emergency calls is not monitored via SCC-AS , as is the case with normal connections , but via the Emergency Access Transfer Function ( EATF ). The emergency call connection is anchored and monitored in the EATF to ensure that there is no interruption and that the connection quality is good.

Transition to classic telephone networks

Breakout Gateway Control Function (BGCF)

If it is necessary to establish a connection to destinations outside the IMS networks (e.g. in the fixed network or in areas of cellular networks in which only GSM or UMTS are available), the BGCF (Breakout Gateway Control Function) is used by the S- CSCF triggered (English: trigger) to organize the handover to the circuit-switched network. For this purpose, the BGCF determines the route into the circuit-switched network.

If the transfer point is in its own network, the BGCF forwards the process to the appropriate MGCF (Media Gateway Control Function), which organizes the transfer. If the transfer point is in another IMS network, the BGCF forwards the trigger to the I-BCF , which coordinates the transition to the foreign IMS network.

Media Gateway Control Function (MGCF)

The MGCF (Media Gateway Control Function) organizes the transition to the circuit-switched network with the help of the IM-MGW. In the MGCF, the protocol is converted into the ISUP / BICC protocols, which control the traffic in the circuit-switched network. The MGCF directly controls the resources in the IM-MGW , i. H. how the connection should be established in detail.

IMS Multi Media Gateway (IM-MGW)

While all the IMS functions mentioned so far only concern the signaling and the organization of the connection setup, the data packets or data lines are managed in the IM-MGW (IMS Multi Media Gateway). The IM-MGW represents the interface between packet and circuit-switched systems. Here, data that arrive from the circuit-switched system on the corresponding data lines are converted into data packets for the packet-switched network and vice versa. This is controlled by the MGCF , which specifies exactly how the data arriving on a certain line should be converted into data packets and vice versa.

Transition to other IMS networks

Interconnection Border Control Function (I-BCF)

The I-BCF (Interconnection Border Control Function) organizes the transition to other IMS networks. If a participant is in another IMS network, the data and signaling connections to the other IMS network are organized via the I-BCF. The I-BCF directly controls the resources in the I-BGF . As an interface to external networks, I-BCF and I-BGF monitor the interface from a security point of view and encrypt the traffic.

If a connection to a subscriber in your own network is required from an external network, the I-BCF accepts this connection request. If necessary, the messages and the data traffic are adapted to the protocols used in the own network. The I-BCF reserves the resources required for the connection and monitors the traffic. The actual data stream is routed via the I-BGF , with the I-BCF controlling this data stream.

In order to find your own participant, the connection request is then passed on to the P-CSCF .

The I-BCF can also organize the transition to its own packet-switched network. The sub-function of the I-BCF for the transition to its own packet-switched network is the Access Transfer Control Function ( ATCF ). Instead of being part of the I-BCF, this function can also be implemented as an independent function.

Interconnection Border Gateway Function (I-BGF)

The I-BGF (Interconnection Border Gateway Function) is analogous to the IM-MGW in the IMS data stream. The data traffic to external IMS networks is forwarded via the I-BGF. The I-BGF represents the data-side interface between external IMS networks and your own network. The traffic from and to external IMS networks is forwarded to the packet-switching network elements of your own network (e.g. GGSN , SAE-GW ) or to IM-MGW, where, if necessary, the transition to the own circuit-switching network takes place.

The I-BGF can also serve as an interface for data traffic into your own packet-switched network. The sub-function of the I-BGF for the transition to its own packet-switched network is the Access Transfer Gateway ( ATGW ). Instead of being part of the I-BGF, this function can also be implemented as an independent function.

Transition to your own network

Access Transfer Control Function (ATCF)

The ATCF (Access Transfer Control Function) organizes the transition to your own packet-switched network. For subscribers in their own network who can be reached via packet-switched services, the data and signaling connections are organized via ATCF. The ATCF directly controls the resources in the ATGW . This interface is used when IMS data services are used by the subscriber (such as MMTel , RCS etc.) or when IP voice services are used (such as Voice over IP , Voice over LTE ).

For connection requests from and to one's own network, the ATCF provides the interface via which signaling information is exchanged between one's own network and the IMS system. The ATCF reserves the resources required for the connection and monitors the traffic. The actual data stream is routed via the ATGW , whereby the ATCF controls this data stream.

The ATCF can be implemented as part of the I-BCF , then the I-BCF takes over the organization of the traffic to its own as well as to external networks. The ATCF can also be set up as an independent IMS function. In this case, the ATCF is sometimes referred to as the Access Border Control Function A-BCF .

Access Transfer Gateway (ATGW)

The ATGW (Access Transfer Gateway) is analogous to the IM-MGW and the I-BGF in the IMS data stream. If the IMS service used requires a transition to your own packet-switched network, the IMS data traffic is forwarded to the packet-switching network elements in your own network (e.g. GGSN , SAE-GW ) via the ATGW .

The ATGW can be implemented as part of the I-BGF , then both the data streams to the own network and to external networks are routed via the I-BGF. The ATGW can also be set up as an independent IMS function. In this case, the ATGW is sometimes referred to as the Access Border Gateway Function (A-BGF).

IMS specifications

The components of the IMS are described in the following 3GPP specifications:

IMS functions 3GPP specifications
IMS overall architecture 3GPP TS 23.002
P-CSCF , I-CSCF , S-CSCF , E-CSCF , BGCF 3GPP TS 23.167, 3GPP TS 23.228
SCC-AS 3GPP TS 23.237
IM-SSF 3GPP TS 23.278
IP-SM-GW 3GPP TS 23.204
MMTel , RCS , Presence Server 3GPP TS 22.173, 3GPP TS 24.173
MRFC , MRFP 3GPP TS 23.228, 3GPP TS 23.333
MGCF , IM-MGW 3GPP TS 23.002, 3GPP TS 24.229, 3GPP TS 29.163
I-BCF , I-BGF 3GPP TS 29.162
ATCF , ATGW 3GPP TS 23.237, 3GPP TS 24.237

See also

literature

  • Gonzalo Camarillo et al. a .: The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds . John Wiley & Sons, 2006, ISBN 0-470-01818-6 .
  • Miikka Poikselka u. a .: The IMS: IP Multimedia Concepts and Services . John Wiley & Sons, 2006, ISBN 0-470-01906-9 .

Web links