Why IMS ?
Early days TDM networks were not robust enough to support emerging technologies and data networking. There was a need to migrate from voic eonly network to Triple play network ( voice , video and data ).
Other factors included :
- rapid service development
- service availiability in both home and roaming network
- wireline and wireless convergence
Due to these above mentioned reasons TDM was outdated and IMS gained support .
What benefits does IMS bring ?
It offers counteless applications around rich multimedia services on wireless , packet swtched and even tradional circuit switched networks.
- Enhanced applications are easier to develop due to open APIs and common network services.
- Third-party developers can offer their own applications and use common network services, sharing profits with minimal risk
- New services involving concurrent sessions of multimedia (voice, video, and data) during the same call are now possible
- Reduced time-to-market for new services is possible because service providers are not tied to the timescales and functions of their primary NEPs
- Better voice quality for business applications, such as conferencing, is possible
- Wireless applications (like SMS, and so on) can be offered to wire line or broadband subscribers.
- Service providers can more easily offer bundled services.
- Cost-effective implementation of services is possible across multiple transports, such as Push-To-Talk (PTT), presence and Location-Based Services (LBS), Fixed-Mobile Convergence (FMC), mobile video services, and so on.
- Common provisioning, management, and billing systems are supported for all networks.
- Significantly lower transport costs result when moving from time-switched to packet-switched channels.
- Service providers can take advantage of competitive offerings from multiple NEPs for most network elements.
- IMS results in reduced expenses for delivering licensed content to subscribers of different types of devices, encodings, or networks.
The reason for widespread adoption of IMS is also that it follows standards and open interfaces from 3GPP and ETSI, also is flexible for policy control , OSS/BSS , Value Added Services etc .
- 2002-03-14 Rel-5 : IMS was introduced with SIP. Qos voice over MGW.
- 2004-12-16 Rel-6 : Services like emergency , voice call continuity , IPCAN ( IP connectivity Access Network )
- 2005-09-28 Rel-7 : Single Radio Voice Call Continuity , multimedia telephony,eCall ,ICS
- 2008-12-11 Rel-8 : IMS centralized services , supplementary services and internetworking between IMS and Circuit Switched Networks,charging , QoS
- 2009-12-10 Rel-9 : IMS emergency numbers on GPRS , EPS(Enhanced packet system) , Custom alert tone , MM broadcast/Multicast
- 2011-3-23 Rel-10 : home NodeB, M2M, Roaming and Inter UE transfer
- 2012-09-12 Rel-11 :-tbd
- 2014-09-17 Rel-12 :- tbd
- 2015-12-11 Rel-13 :- tbd
Majorly IMS is divided into 3 horizontal layers given below :
•Transport / MediaEndpoint Layer
Unifies transports and media from analog, digital, or broadband formats to Real-time Transport Protocol (RTP) and SIP protocols. This is accomplished by media gateways and signaling gateways.
It also includes media servers with media processing elements to allow for announcements, in-band signaling, and conferencing. These media servers are shared across all applications (voicemail, interactive response systems, push-to-talk, and so on), maximizing statistical use of the equipment and creating a common base of media services without “hard-coding” these services into the applications.
•Session and Control Layer
This layer arranges logical connections between various other network elements. It provides registration of end-points, routing of SIP messages, and overall coordination of media and signaling resources.
IMS core which is part of this layer primarily contains 2 important elements Call Session Control Function (CSCF) and Home Subscriber Server (HSS) database. These are explained below
HSS Home Subscriber Server
It is a database of user profiles and location information . It is responsible for name/address resolution and also authorization/authentication .
CSCF Call Session Control Function
Handles most routing, session and security related operation for SIP messages . It is further divided into 3 parts :
- Proxy CSCF: P_CSCF is the first point of contact from any SIP UA. It proxies UE requests to subsystem.
- Serving CSCF: S-CSCF is a powerful part of IMS Core as it decides how UE request will be forwarded to the application servers.
- Interrogating CSCF: I-CSCF initiates the assignment of a user to an S-CSCF (by querying the HSS) during registration.
•Application Services Layer
- Telephony Application Server (TAS) – for defining custom call flow logic
- IP Multimedia Services Switching Function (IM-SSF)
- Open Service Access Gateway (OSA-GW), and so on.
Additional Links :
- To Read about IMS service Switching function and reverse service Switching function click here .
- IP Multimedis subsystem – Detailed : Next part of IMS series , describing IMS components and call flow thoroughly
- Internet Telephony Convergence- JAINSLEE Platform
- IMS in EPC ( Evolved PAcket Core )
Update on IMS :
IMS has been mandated as the control architecture for Voice over LTE (VoLTE) networks. Also IMS is being widely adopted to mange traffic for Voice over WiFi (VoWiFi) systems.