IP Multimedia Subsystem (IMS)

• Why IMS ?
• What benefits does IMS bring ?
• Features of IMS Network
• IMS Layers
  1. Transport / Media Endpoint Layer
     • Backhaul network
     • Border Gateways
  2. Session & Control Layer
     • HSS (Home Subscriber Server)
     • SCF (Call Session Control Function)
     • MGw (Media Gateway Control Function)
  3. Application Services Layer
     • TAS (Telephony Application Server)
     • IM-SSF ( IP Multimedia Services Switching Function)
     • OSA-GW (Open Service Access Gateway)
• IMS-architecture
  • IMS standalone architecture
  • Interoperable IMS core for heterogeneous access networks

---

IMS is a an architectural framework for IP based multimedia rich communications. It was standardized by a group called 3GPP formed in 1999. It started as an enabler for 3rd generation mobile networks in European market and later spread to wirelne networks too. IMS became the key to Fixed Mobile Convergence (FMC).

Based on IETF Protocols (such as SIP, RTP, RTSP, COPS, DIAMETER, etc), IMS is now crucial for controlling conmmunication in a IP based Next Genration Network (NGN).

Communication service providers and telecom operators are migrating from circuit-switched networks to IMS technology with the increasing bandwidth (5G) and user expectations.

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Due to these above mentioned reasons TDM was outdated and IMS gained support .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.

Easier to Create and Deploy New Applications and Services

• (+)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

Capture New Subscribers,Retain Current Subscribers

• (+) 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.

Lower Operating and Capital Costs

• (+) Cost-effective implementation of services 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.
• (+) Reduced expenses for delivering licensed content to subscribers of different types of devices, encodings, or networks.

The strongest argument for adoption of IMS is that it follows established standards and open interfaces from 3GPP and ETSI. This makes it suited for interoperability, policy control accross networks, streamlined OSS/BSS, Value Added Services etc.

Features of IMS network

1. Abstraction from Underlying Network : IMS is essentially leading towards an open and standardized network and interface,irrespective of underlay network.

2. Fixed /Mobile Convergence : Inter operability with Circuit Switched (CS) Mobile application Part (MAP)

3. Roaming : Location awareness between home and visiting network.

4. Application layer Call Control : IMS application layer has the provision for defining proxy or B2BUA based call flow completion . This leads to operator being able to introduce business logic into call sessions.

IMS is supplemented by SIP (IETF), Diameter (IETF) and H248(ITU-T).The release cycle of IMS is as follows

• 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

IMS Layers

Majorly IMS is divided into 3 horizontal layers given below :

Transport / Media Endpoint 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 & 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

The Application Services Layer contains multiple Application Servers (AS), such as:

• 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.

IMS Architecture

The IMS standalone architecture is suited for an all IP network

---

Interoperable IMS core for heterogeneous access networks

References

• IMS service Switching function and reverse service Switching function read here.

IMSSF and RIMSSF

• IP Multimedis subsystem – Detailed :  Next part of IMS series , describing IMS components and  call flow thoroughly

IP Multimedia Subsystem (IMS) – detailed part2

• Internet Telephony Convergence- JAINSLEE Platform

Internet Telephony Convergence- JAINSLEE Platform

• IMS in EPC ( Evolved PAcket Core )

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.