IMS, introduction IMS, introduction

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IP Multimedia Subsystem
From Wikipedia, the free encyclopedia
 
The IP Multimedia Subsystem (IMS) is a standardised Next Generation Networking (NGN) architecture for telecom operators that want to provide mobile and fixed multimedia services. It uses a Voice-over-IP (VoIP) implementation based on a 3GPP standardised implementation of SIP, and runs over the standard Internet Protocol (IP). Existing phone systems (both packet-switched and circuit-switched) are supported.
 
The aim of IMS is not only to provide new services but all the services, current and future, that the Internet provides. In this way, IMS will give network operators and service providers the ability to control and charge for each service. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals, IMS uses open standard IP protocols, defined by the IETF. So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols. This is why IMS truly merges the Internet with the cellular world; it uses cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services.
 
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), as part of their standardization work for 3G mobile phone systems in UMTS 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.  
Early IMS was defined to allow for IMS implementations that do not yet support all "Full IMS" requirements.  
3GPP2 (a different organisation) based their CDMA2000 Multimedia Domain (MMD) on 3GPP IMS, adding support for CDMA2000.  
3GPP release 6 added interworking with WLAN.  
3GPP release 7 added support for fixed networks, by working together with TISPAN R1.  
 
 
Basic Principles
Access independence: IMS will eventually work with any network (fixed, mobile or wireless) with packet-switching functions, such as GPRS, UMTS, CDMA2000, WLAN, WiMAX, DSL, cable, ... Older circuit-switched phone systems (POTS, GSM) are supported through gateways. Open interfaces between control and service layers allow elements and calls/sessions from different access networks to be mixed.  
Different network architectures: IMS allows operators and service providers to use different underlying network architectures.  
Terminal and user mobility: The mobile network provides terminal mobility (roaming), while user mobility is provided by IMS and SIP.  
Extensive IP-based services: IMS should make it easier to offer just about any IP-based service. Examples include voice over IP (VOIP), Push to talk over cellular (POC), multiparty gaming, videoconferencing, Messaging, community services, presence information and content sharing.  
 
 
Fixed/Mobile Convergence
IMS was originally designed for mobile networks, but with the addition of TISPAN in release 7, fixed networks are supported too. This is called Fixed/Mobile Convergence (FMC), which became one of the key trends of the telecommunications industry in 2005.
 
The vision is for people to use one phone with one number, address book and voicemail bank, taking advantage of cheap, high-speed connectivity in their fixed-line home or office setting, while enjoying mobility outside in the wide-area mobile phone network. It also includes a seamless handover of calls between fixed-line and mobile networks.
 
Telecommunications operators can provide services to users irrespective of their location, access technology, and terminal. IMS guarantees interworking with existing phone systems, while providing an upgrade path for modern multimedia sessions (like a videophone).
 
Critics say that fixed operators are mainly interested in expanding their services in the area of mobile operators (and vice versa), while lowering their operational costs at the same time by using Voice over IP technology.
 
Architecture
The IP Multimedia Core Network Subsystem is a collection of different functions, linked by standardized interfaces. A function is not a node (hardware box) : an implementer is free to combine 2 functions in 1 node, or to split a single function into 2 or more nodes. Each node can also be present multiple times in a network, for load balancing or organizational issues.
 
 
Access Network
The user can connect to an IMS network using various methods, all of which are using the standard Internet Protocol (IP). Direct IMS terminals (mobile phones, PDAs, computers, ...), can register directly into an IMS network, even when they're roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in 'Early IMS') and are running SIP User Agents. Fixed access (e.g., DSL, cable modems, Ethernet, ...), mobile access (W-CDMA, CDMA2000, GSM, GPRS, ...) and wireless access (WLAN, WiMAX, ...) are all supported. Other phone systems like the POTS (the old analogue telephones), H.323 and non IMS-compatible VoIP systems are supported through gateways.
 
 
Core Network
 
User Database
The HSS (Home Subscriber Server) is the master user database that supports the IMS network entities that are actually handling the calls/sessions. It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the physical location of user. It's similar to the GSM HLR and AUC.
 
An SLF (Subscriber Location Function) is needed when multiple HSSs are used. Both the HSS and the SLF implement the DIAMETER protocol (Cx, Dx and Sh interfaces).
 
 
User identities
In normal 3GPP networks, the following identities are used;
 
International Mobile Subscriber Identity (IMSI)  
Temporal Mobile Subscriber Identity (TMSI)  
International Mobile Equipment Identity (IMEI)  
Mobile Subscriber ISDN Number (MSISDN)  
IMSI is a unique user identity that is stored in the SIM. To improve privacy, a TMSI is generated per geographical location. While IMSI/TMSI are used for user identification, the IMEI is a unique device identity and is phone specific. The MSISDN is the telephone number of a user.
 
With IMS, the following additional identities are implemented:
 
IP Multimedia Private Identity (IMPI)  
IP Multimedia Public Identity (IMPU)  
Both are not phone numbers or other series of digits, but 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).
 
The IMPI is unique to the phone, and you can have multiple IMPU per IMPI (often a tel-uri and a sip-uri). The IMPU can also be shared with another phone, so both can be reached with the same identity (for example, a single phone-number for an entire family).
 
The HSS user database contains, but is not limited to, the IMPU, IMPI, IMSI, and MSISDN.