Monthly Archives: May 2013

IP Multimedia Subsystem ( IMS )

 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.
ims layers

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Due to these above mentioned reasons TDM was outdated and IMS gained support .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


IMS features

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 / 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

 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.

Additional Links :

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.

IMS in EPC ( Evolved Packet Core )

Timeline of Evolution 

GSM  : calls  on circuit switching ( CS ) between 2 parties for communication. Dedicated circuits are used for voice and SMS.

GPRS : packet switching (PS) is introduced for data services

UMTS / 3G : network elements begin evolving into PS . No changes to core .

EPC / LTE/VOLTE : No circuit switched domain at all .

IP Multimedia SubSystem was originally meant for evolved UMTS network to provider IP communication to mobile UAs. Today IMS is gaining plenty of attention due to oncoming

More information on IMS and IP Communication go here 

What is IMS or IP Multimedia System ?

•A standardized IP-based architecture that allows the convergence of fixed and mobile communication devices and multimedia applications.
•Using IMS, applications can combine voice, text, pictures, and video in call sessions, offering significant ease-of-use to subscribers and allowing service providers to drive branding through a common interface
•Defined by the Third Generation Partnership Project  (3GPP) and supported by major Network Equipment Providers and service providers.
•The standard supports multiple access types – including GSM/GPRS/EDGE, WCDMA, CDMA2000, wireline broadband access and wireless LAN.

ims arc1

 LTE ( Long Term Evolution )

 LTE evolved its precursor Universal Mobile Telecommunication System (UMTS), which in turn evolved from the Global System for Mobile Communications (GSM). It defines the access layer of Telecom architecture. EPC is the core of LTE system .
LTE is often linked with evolved UMTS terrestrial radio access (E-UTRA) and
evolved UMTS terrestrial radio access network (E-UTRAN). With this eco-system Evolved NodeB (eNodeB) is the base station for LTE radio.

What is EPC (Evolved Packet Core )  ?

EPC is a core network architecture framework by 3GPP.

 Primarily EPC has 4 sub parts

Serving Gateway
PDN gateway
MME (Mobility Management Entity)


Unified Service Delivery Stack

The Unified Communication Solution leads to Network Agnostic, Agile, Cost Effective  & Customer Experience Centric Services Platform.

unified ccommunication


The Way from Copper -> Fiber -> 2nd Generation -> 3rd Generation -> LTE , depicts evolution of Telecommunications over the decades , in the Network layer Infrastructure area

The Sevice Layer Infrastructure is built  using techniques as Switching , Home Location Register (HLR) ,  Authetication (  AuC) etc  . The Services vary over ranges such as IN , Voice , SMS , VOIP , IM , IPTV , IMS  , Presence , MMS etc .

Top of this lies the Harmonization layer that performs the inter networking between different platforms and protocols .

The Application Layer consists of various usecases as Enhanced Screening , Social Networking Integration , Education Trade etc .

Mobicents SIP server platform

The Mobicent server bears 50% resemblance to Rhino TAS .
So far , I have successfully done the following

1. installed the Mobicent platform on Linux machine
2. set up the environment to build and deploy the applications .
3. Ran simple sip b2bua call using sipp .

Attached are the screen shots of the same .

1. User agent client (UAC) Dialog
2. User agent server (UAS) Dialog
3. Mobicent slee management console
4. Joboss status console
5. Admin console depicting applications installed .

IMS , the revolution ahead

vision :
To make a model that separates the services offered by
fixed-line (traditional telcos),                mobile (traditional cellular),            and            converged service providers (cable companies and others who provide triple-play — voice,  video, and data — services) from the access networks used to receive those services.
Layers :
 IMS architecture is broken into distinct layers:
Screenshot from 2013-05-16 18:37:09
Drivers :
Revenue streams for plain vanilla voice services are sharply falling and the need of the hour is to propose smart intuitive and creative service to kep up the Telecom market alive .