IP Multimedia Subsystem


The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is a standardized architectural framework for delivering IP-based multimedia services. Historically, mobile phones have provided voice call services over a circuit-switched network, rather than over an IP-based packet-switched network. Various VoIP technologies are available on smartphones; IMS offers a standardized protocol across different vendors.
IMS was originally designed by the wireless standards body 3rd Generation Partnership Project, as a part of the vision for evolving mobile networks beyond GSM. Its original formulation represented an approach for delivering Internet services over GPRS. This vision was later updated by 3GPP, 3GPP2 and ETSI TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed lines.
IMS uses IETF protocols wherever possible, e.g., the Session Initiation Protocol. According to the 3GPP, IMS is not intended to standardize applications, but rather to aid the access of multimedia and voice applications from wireless and wireline terminals, i.e., to create a form of fixed-mobile convergence. This is done by having a horizontal control layer that isolates the access network from the service layer. From a logical architecture perspective, services need not have their own control functions, as the control layer is a common horizontal layer. However, in implementation this does not necessarily map into greater reduced cost and complexity.
Alternative and overlapping technologies for access and provisioning of services across wired and wireless networks include combinations of Generic Access Network, softswitches and "naked" SIP.
Since it is becoming increasingly easier to access content and contacts using mechanisms outside the control of traditional wireless/fixed operators, the interest of IMS is being challenged.
Examples of global standards based on IMS are MMTel which is the basis for Voice over LTE, Wi-Fi Calling, Video over LTE, SMS/MMS over WiFi and LTE, Unstructured Supplementary Service Data over LTE, and Rich Communication Services, which is also known as joyn or Advanced Messaging, and now RCS is operator's implementation. RCS also further added Presence/EAB functionality.

History

  • IMS was 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, as part of their standardization work for 3G mobile phone systems in UMTS networks. It first appeared in Release 5, when SIP-based multimedia was added. Support for the older GSM and GPRS networks was also provided.
  • 3GPP2 based their CDMA2000 Multimedia Domain on 3GPP IMS, adding support for CDMA2000.
  • 3GPP release 6 added interworking with WLAN, inter-operability between IMS using different IP-connectivity networks, routing group identities, multiple registration and forking, presence, speech recognition and speech-enabled services.
  • 3GPP release 7 added support for fixed networks by working together with TISPAN release R1.1, the function of AGCF and PES are introduced to the wire-line network for the sake of inheritance of services which can be provided in PSTN network. AGCF works as a bridge interconnecting the IMS networks and the Megaco/H.248 networks. Megaco/H.248 networks offers the possibility to connect terminals of the old legacy networks to the new generation of networks based on IP networks. AGCF acts a SIP User agent towards the IMS and performs the role of P-CSCF. SIP User Agent functionality is included in the AGCF, and not on the customer device but in the network itself. Also added voice call continuity between circuit switching and packet switching domain, fixed broadband connection to the IMS, interworking with non-IMS networks, policy and charging control, emergency sessions. It also added SMS over IP.
  • 3GPP release 8 added support for LTE / SAE, multimedia session continuity, enhanced emergency sessions, SMS over SGs 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, enhancements to IMS emergency sessions.
  • 3GPP release 11 added USSD simulation service, network-provided location information for IMS, SMS submit and delivery without MSISDN in IMS, and overload control.
Some operators opposed IMS because it was seen as complex and expensive.
In response, a cut-down version of IMS—enough of IMS to support voice and SMS over the LTE network—was defined and standardized in 2010 as Voice over LTE.

Architecture

Each of the functions in the diagram is explained below.
The IP multimedia core network subsystem is a collection of different functions, linked by standardized interfaces, which grouped form one IMS administrative network. A function is not a node : An implementer is free to combine two functions in one node, or to split a single function into two or more nodes. Each node can also be present multiple times in a single network, for dimensioning, load balancing or organizational issues.

Access network

The user can connect to IMS in various ways, most of which use the standard IP. IMS terminals can register directly on IMS, even when they are roaming in another network or country. The only requirement is that they can use IP and run SIP user agents. Fixed access, mobile access and wireless access are all supported. Other phone systems like plain old telephone service, H.323 and non IMS-compatible systems, are supported through gateways.

Core network

HSS – Home subscriber server:

The home subscriber server, or user profile server function, is a master user database that supports the IMS network entities that actually handle calls. It contains the subscription-related information, performs authentication and authorization of the user, and can provide information about the subscriber's location and IP information. It is similar to the GSM home location register and Authentication centre.
A subscriber location function is needed to map user addresses when multiple HSSs are used.
User identities:

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

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

The IP Multimedia Public Identity is used by any user for requesting communications to other users. Also known as Address of Record. There can be multiple IMPU per IMPI. The IMPU can also be shared with another phone, so that both can be reached with the same identity.
Globally Routable User Agent URI:

Globally Routable User Agent URI is an identity that identifies a unique combination of IMPU and UE instance.
There are two types of GRUU: Public-GRUU and Temporary GRUU.
  • P-GRUU reveal the IMPU and are very long lived.
  • T-GRUU do not reveal the IMPU and are valid until the contact is explicitly de-registered or the current registration expires
Wildcarded Public User Identity:

A wildcarded Public User Identity expresses a set of IMPU grouped together.

The HSS subscriber database contains the IMPU, IMPI, IMSI, MSISDN, subscriber service profiles, service triggers, and other information.

Call Session Control Function (CSCF)

Several roles of SIP servers or proxies, collectively called Call Session Control Function, are used to process SIP signaling packets in the IMS.
  • A Proxy-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 or in the home network. Some networks may use a Session Border Controller for this function. The P-CSCF is at its core a specialized SBC for the User–network interface which not only protects the network, but also the IMS terminal. The use of an additional SBC between the IMS terminal and the P-CSCF is unnecessary and infeasible due to the signaling being encrypted on this leg. The terminal discovers its P-CSCF with either DHCP, or it may be configured or in the ISIM or assigned in the PDP Context.
  • * It is assigned to an IMS terminal before registration, and does not change for the duration of the registration.
  • * It sits on the path of all signaling, and can inspect every signal; the IMS terminal must ignore any other unencrypted signaling.
  • * It provides subscriber authentication and may establish an IPsec or TLS security association with the IMS terminal. This prevents spoofing attacks and replay attacks and protects the privacy of the subscriber.
  • * It inspects the signaling and ensures that the IMS terminals do not misbehave.
  • * It can compress and decompress SIP messages using SigComp, which reduces the round-trip over slow radio links.
  • * It may include a Policy Decision Function, which authorizes media plane resources e.g., quality of service over the media plane. It is used for policy control, bandwidth management, etc. The PDF can also be a separate function.
  • * It also generates charging records.
  • An Interrogating-CSCF is another SIP function located at the edge of an administrative domain. Its IP address is published in the Domain Name System of the domain, so that remote servers can find it, and use it as a forwarding point 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, in which case it's called a Topology Hiding Inter-network Gateway. From Release 7 onwards this "entry point" function is removed from the I-CSCF and is now part of the Interconnection Border Control Function. The IBCF is used as gateway to external networks, and provides NAT and firewall functions. The IBCF is a session border controller specialized for the network-to-network interface.
  • A Serving-CSCF is the central node of the signaling plane. It is a SIP server, but performs session control too. It is always located in the home network. It uses Diameter Cx and Dx interfaces to the HSS to download user profiles and upload user-to-S-CSCF associations. All necessary subscriber profile information is loaded from the HSS.
  • * it handles SIP registrations, which allows it to bind the user location 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 the SIP message will be forwarded, in order to provide their services
  • * it provides routing services, typically using Electronic Numbering lookups
  • * it enforces the policy of the network operator
  • * there can be multiple S-CSCFs in the network for load distribution and high availability reasons. It's the HSS that assigns the S-CSCF to a user, when it's queried by the I-CSCF. There are multiple options for this purpose, including a mandatory/optional capabilities to be matched between subscribers and S-CSCFs.