The
IP Multimedia Subsystem or
IP Multimedia Core Network Subsystem (
IMS) is an architectural framework for delivering
Internet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP)
multimediaMultimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
services. It was originally designed by the wireless
standardsStandardization is the process of developing and implementing technical standards.The goals of standardization can be to help with independence of single suppliers , compatibility, interoperability, safety, repeatability, or quality....
body 3rd Generation Partnership Project (3GPP), as a part of the vision for evolving mobile networks beyond GSM. Its original formulation (3GPP Rel-5) represented an approach to delivering "Internet services" over GPRS. This vision was later updated by 3GPP, 3GPP2 and
ETSIThe European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
TISPANThe Telecoms & Internet converged Services & Protocols for Advanced Networks is a standardization body of ETSI, specializing in fixed networks and Internet convergence...
by requiring support of networks other than GPRS, such as
Wireless LANA wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
,
CDMA2000CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
and fixed line.
To ease the integration with the Internet, IMS uses IETF protocols wherever possible, e.g.
Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(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 layerIn intelligent networks and cellular networks, service layer is a conceptual layer within a network service provider architecture. It aims at providing middleware that serves third-party value-added services and applications at a higher application 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 NetworkGeneric Access Network or GAN is a telecommunication system that extends mobile voice, data and IP Multimedia Subsystem/Session Initiation Protocol applications over IP networks...
,
soft switchesA softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a general-purpose computer system...
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.
An example of a global standard based on IMS is MMTel.
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
The 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
), as part of their standardization work for 3G3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
mobile phone systems in UMTSUniversal Mobile Telecommunications System is a third generation mobile cellular technology for networks based on the GSM standard. Developed by the 3GPP , UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for...
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
The 3rd Generation Partnership Project 2 is a collaboration between telecommunications associations to make a globally applicable third generation mobile phone system specification within the scope of the ITU's IMT-2000 project...
(a different organization from 3GPP) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for CDMA2000CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
.
- 3GPP release 6 added interworking with WLAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
, 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 talkPush-to-talk , also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode....
).
- 3GPP release 7 added support for fixed networks by working together with TISPAN
The Telecoms & Internet converged Services & Protocols for Advanced Networks is a standardization body of ETSI, specializing in fixed networks and Internet convergence...
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 (VCCThe 3GPP has defined the Voice Call Continuity specifications in order to describe how a voice call can be persisted, as a mobile phone moves between circuit switched and packet switched radio domains ....
), fixed broadband connection to the IMS, interworking with non-IMS networks, policy and charging control (PCC), emergency sessions.
- 3GPP release 8 added support for LTE / SAE
System Architecture Evolution is the core network architecture of 3GPP's LTE wireless communication standard.SAE is the evolution of the GPRS Core Network, with some differences:* simplified architecture* all-IP Network...
, multimedia session continuity, enhanced emergency sessions and IMS centralized services.
- 3GPP release 9 added support for IMS emergency calls over GPRS and EPS, enhancements to multimedia telephony, IMS media plane security, enhancements to services centralization and continuity.
- 3GPP release 10 added support for inter device transfer, enhancements to the single radio voice call continuity (SRVCC), enhancements to IMS emergency sessions.
- 3GPP release 11 added USSD
Unstructured Supplementary Service Data is a protocol used by GSM cellular telephones to communicate with the service provider's computers...
simulation service, network-provided location information for IMS, SMSSMS is a form of text messaging communication on phones and mobile phones. The terms SMS or sms may also refer to:- Computer hardware :...
submit and delivery without MSISDN in IMS.
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 (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
Internet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP). IMS terminals (such as
mobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s,
personal digital assistantA personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
s (PDAs) and computers) can register directly on IMS, even when they are
roamingIn wireless telecommunications, roaming is a general term referring to the extension of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device is kept connected to the network, without losing the...
in another network or country (the visited network). The only requirement is that they can use
IPThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
and run
Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP) user agents. Fixed access (e.g.,
Digital Subscriber LineDigital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
(DSL),
cable modemA cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
s,
EthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
), mobile access (e.g.
W-CDMAW-CDMA , UMTS-FDD, UTRA-FDD, or IMT-2000 CDMA Direct Spread is an air interface standard found in 3G mobile telecommunications networks. It is the basis of Japan's NTT DoCoMo's FOMA service and the most-commonly used member of the UMTS family and sometimes used as a synonym for UMTS...
,
CDMA2000CDMA2000 is a family of 3G mobile technology standards, which use CDMA channel access, to send voice, data, and signaling data between mobile phones and cell sites. The set of standards includes: CDMA2000 1X, CDMA2000 EV-DO Rev. 0, CDMA2000 EV-DO Rev. A, and CDMA2000 EV-DO Rev. B...
, GSM, GPRS) and wireless access (e.g.
WLANA wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
,
WiMAXWiMAX is a communication technology for wirelessly delivering high-speed Internet service to large geographical areas. The 2005 WiMAX revision provided bit rates up to 40 Mbit/s with the 2011 update up to 1 Gbit/s for fixed stations...
) are all supported. Other phone systems like
plain old telephone servicePlain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
(POTS—the old analogue telephones),
H.323H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...
and non IMS-compatible VoIP systems, are supported through
gatewaysIn telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
.
HSS – Home Subscriber Server
The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), is a master user
databaseA database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...
that supports the IMS network entities that actually handle
callsTelecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
. It contains the subscription-related information (subscriber
profilesA user profile is a collection of personal data associated to a specific user. A profile refers therefore to the explicit digital representation of a person's identity...
), performs
authenticationAuthentication is the act of confirming the truth of an attribute of a datum or entity...
and
authorizationAuthorization is the function of specifying access rights to resources, which is related to information security and computer security in general and to access control in particular. More formally, "to authorize" is to define access policy...
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 FunctionThe subscriber location function, or SLF is an entity within an IP multimedia subsystem that provides information about the home subscriber server that is associated with a particular user profile. It is generally implemented using a database...
(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 IdentifierIn computing, a uniform resource identifier is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network using specific protocols...
(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 cardBusiness cards are cards bearing business information about a company or individual. They are shared during formal introductions as a convenience and a memory aid. A business card typically includes the giver's name, company affiliation and contact information such as street addresses, telephone...
). 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,
IMSIAn International Mobile Subscriber Identity or IMSI is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field in the SIM inside the phone and is sent by the phone to the network...
, and MSISDN, subscriber service profiles, service triggers and other information.
CSCF – Call Session Control Function
Several roles of
Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(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
A session border controller is a device regularly deployed in Voice over Internet Protocol networks to exert control over the signaling and usually also the media streams involved in setting up, conducting, and tearing down telephone calls or other interactive media communications.SBC's original...
(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 additional SBC between the IMS terminal and the P-CSCF as such pointless and also not feasible due to the signaling being encrypted on this leg. The terminal discovers its P-CSCF with either DHCPThe Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
, or it may be configured (e.g. during initial provisioning or via a 3GPP IMS Management Object (MO)) or in the ISIMAn IP Multimedia Services Identity Module is an application running on a UICC smart card in a 3G mobile telephone in the IP Multimedia Subsystem . It contains parameters for identifying and authenticating the user to the IMS...
or assigned in the PDP Context (in General Packet Radio ServiceGeneral packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...
(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
Internet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
or TLSTransport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...
security association with the IMS terminal. This prevents spoofing attackIn the context of network security, a spoofing attack is a situation in which one person or program successfully masquerades as another by falsifying data and thereby gaining an illegitimate advantage.- Spoofing and TCP/IP :...
s and replay attackA replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and retransmits it, possibly as part of a masquerade attack by IP packet...
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, 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
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(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
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
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
High availability is a system design approach and associated service implementation that ensures a prearranged level of operational performance will be met during a contractual measurement period....
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
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
is published in the Domain Name SystemThe Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
(DNS) of the domain (using NAPTRA Name Authority Pointer is a type of resource record used in the Domain Name System .- Rationale :Uniform Resource Names are a subset of Uniform Resource Identifiers used for abstract identifiers, such as a person's name or their telephone number. For URNs to be meaningful, they must be mapped...
and SRVA Service record is a specification of data in the Domain Name System defining the location, i.e. the hostname and port number, of servers for specified services. It is defined in RFC 2782, and its type code is 33...
type of DNSThe Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...
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
In computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device....
and firewall functions (pinholingIn computer networking, the term firewall pinhole is used to describe a port that is opened through a firewall to allow a particular application to gain controlled access to the protected network....
). The IBCF is practically a Session Border ControllerA session border controller is a device regularly deployed in Voice over Internet Protocol networks to exert control over the signaling and usually also the media streams involved in setting up, conducting, and tearing down telephone calls or other interactive media communications.SBC's original...
specialized for the NNIIn telecommunications, a network-to-network Interface is an interface which specifies signaling and management functions between two networks. NNI circuit can be used for interconnection of either signalling , IP In telecommunications, a network-to-network Interface (NNI) is an interface which...
.
Application servers
SIP
Application serverAn application server is a software framework that provides an environment in which applications can run, no matter what the applications are or what they do...
s (AS) host and execute services, and interface with the S-CSCF using
Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP). An example of an application server that is being developed in 3GPP is the
Voice call continuityThe 3GPP has defined the Voice Call Continuity specifications in order to describe how a voice call can be persisted, as a mobile phone moves between circuit switched and packet switched radio domains ....
Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA (
user agentIn computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
) 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
The CAMEL Application Part is a signalling protocol used in the Intelligent Network architecture. CAP is a Remote Operations Service Element user protocol, and as such is layered on top of the Transaction Capabilities Application Part of the SS#7 protocol suite...
to communicate with CAMELA camel is an even-toed ungulate within the genus Camelus, bearing distinctive fatty deposits known as humps on its back. There are two species of camels: the dromedary or Arabian camel has a single hump, and the bactrian has two humps. Dromedaries are native to the dry desert areas of West Asia,...
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.
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 switchedCircuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
(CS) networks. For signalling, CS networks use
ISDN User PartThe ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
(ISUP) (or BICC) over
Message Transfer PartThe Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
(MTP), while IMS uses
Session Initiation ProtocolThe Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...
(SIP) over IP. For media, CS networks use
Pulse-code modulationPulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
(PCM), while IMS uses
Real-time Transport ProtocolThe Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...
(RTP).
- A Signalling Gateway (SGW) interfaces with the signalling plane of the CS. It transforms lower layer protocols as Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(SCTP, an Internet ProtocolThe Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) protocol) into Message Transfer PartThe Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
(MTP, an Signalling System 7Signalling System No. 7 is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls...
(SS7) protocol), to pass ISDN User PartThe ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
(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
A Media gateway is a translation device or service that converts digital media streams between disparate telecommunications networks such as PSTN, SS7, Next Generation Networks or PBX...
(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 codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
s don't match (e.g. IMS might use AMRThe Adaptive Multi-Rate audio codec is a patented audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1999 and is now widely used in GSM and UMTS...
, PSTN might use G.711G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...
).
Media Resources
Media Resources are those components that operate on the media plane and are under the control of IMS Core functions. Specifically,
Media ServerA media server refers either to a dedicated computer appliance or to a specialized application software, ranging from an enterprise class machine providing video on demand, to, more commonly, a small personal computer or NAS for the home, dedicated for storing various digital media .-Purpose:By...
(MS) and
Media gatewayA Media gateway is a translation device or service that converts digital media streams between disparate telecommunications networks such as PSTN, SS7, Next Generation Networks or PBX...
(MGW)
NGN interconnection
There are two types of
Next Generation NetworkingNext-generation network is a broad term used to describe key architectural evolutions in telecommunication core and access networks. The general idea behind the NGN is that one network transports all information and services by encapsulating these into packets, similar to those used on the...
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
Coix is a genus of grass in the Poaceae family.-External links:*...
): 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 layerIn intelligent networks and cellular networks, service layer is a conceptual layer within a network service provider architecture. It aims at providing middleware that serves third-party value-added services and applications at a higher application 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 chargingOnline charging system is a system allowing a Communications service provider to charge their customers, in real time, based on service usage-Unified charging engine for all services:...
, 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 roamingIn wireless telecommunications, roaming is a general term referring to the extension of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device is kept connected to the network, without losing the...
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.
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 UE 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 :
- 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 |
| 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. |
SIP |
| Ma |
I-CSCF <-> AS |
Main functions are to:
Forward SIP requests which are destinated 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 |
| 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 |
Used to exchange messages between CSCFs |
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, AS (SIP AS, OSA SCS, IM-SSF) |
Facilitates the management of subscriber information related to services and settings |
HTTP(s), XCAP |
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.
Security aspects of early IMS and non-3GPP 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/
ISIMAn IP Multimedia Services Identity Module is an application running on a UICC smart card in a 3G mobile telephone in the IP Multimedia Subsystem . It contains parameters for identifying and authenticating the user to the IMS...
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.
CableLabsFounded in 1988 by cable television operating companies, Cable Television Laboratories, Inc. is a not-for-profit research and development consortium that has cable operators as its members. System operators from around the world are eligible to be members. Members dues are based on revenue...
in PacketCable#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, also
TISPANThe Telecoms & Internet converged Services & Protocols for Advanced Networks is a standardization body of ETSI, specializing in fixed networks and Internet convergence...
did a similar effort given their Fixed Networks scopes, yet the procedures are different. To compensate for the lack of
IPsecInternet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
capabilities,
TLSTransport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...
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.
See also
- 4G
In telecommunications, 4G is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G families of standards. In 2009, the ITU-R organization specified the IMT-Advanced requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s...
- Softswitch
A softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a general-purpose computer system...
- Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
- Mobile VoIP
Mobile VoIP or simply mVoIP is an extension of mobility to a Voice over IP network. Two types of communication are generally supported: cordless/DECT/PCS protocols for short range or campus communications where all base stations are linked into the same LAN, and wider area communications using...
- SIMPLE
SIMPLE, the Session Initiation Protocol for Instant Messaging and Presence Leveraging Extensions, is an instant messaging and presence protocol suite based on Session Initiation Protocol managed by the IETF...
- 3GPP Long Term Evolution
3GPP Long Term Evolution, usually referred to as LTE, is a standard for wireless communication of high-speed data for mobile phones and data terminals. It is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using new modulation techniques...
, UMBUMB was the brand name for a project within 3GPP2 to improve the CDMA2000 mobile phone standard for next generation applications and requirements...
(4G network efforts that will use technologies like IMS)
- Mobile Broadband
Mobile broadband is the marketing term for wireless Internet access through a portable modem, mobile phone or other mobile device.-Description:...
- Peer-to-peer video sharing
Peer-to-peer video sharing is a basic service on top of the IP Multimedia Subsystem .Early proprietary implementations might also run a simple SIP infrastructure, too.The GSM Association calls it "Video Share"...
- Video share
Video Share is an IP Multimedia System enabled service for mobile networks that allows users engaged in a circuit switch voice call to add a unidirectional video streaming session over the packet network during the voice call. Any of the parties on the voice call can initiate a video streaming...
- Image share
Image Share is a service for sharing images between users during a mobile phone call. It has been specified for use in a 3GPP-compliant cellular network by the GSM Association in the PRD IR.79 Image Share Interoperability Specification....
- Text over IP
- Multimedia Telephony (MMTel)
- Voice call continuity
The 3GPP has defined the Voice Call Continuity specifications in order to describe how a voice call can be persisted, as a mobile phone moves between circuit switched and packet switched radio domains ....
- Push to talk
Push-to-talk , also known as Press-to-Transmit, is a method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode....
- IMPS
- Rich Communication Suite
Rich Communication Suite is an industry effort focused on the use of IMS for providing mobile phone communication services. "Rich Communication" in itself is meaningless jargon, which refers to the use of more than just voice for communication, but has long been touted as a benefit of IMS...
- Service Capability Interaction Manager
A Service Capability Interaction Manager orchestrates service delivery among application server platforms within the IP Multimedia Subsystem architecture.A key requirement for the Service Broker functionality, is to be able to bridge between the existing legacy networks, and the next-generation...
External links
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)