IMS - Masterminding the Convergence to IP

1.Motivation for Convergence to IP Convergence was the dream of the telecommunications industry and it has been taken up by them in many forms. But it is close to reality since it is backed up by user, technological and service motivation. Some of the convergence-related services and implementations available already today are seamless WLAN/2G/3G connections, multi-access mobile devices, layered architecture and now the IMS. A new communication culture is emerging, driven by community interaction over the Internet, this trend will prevail and IP paradigm will be, used in almost all areas of communication. Rapid development of radio technology has lead to increased bit rates and support for mobility, and it has completed the formalities that can actually complete the dream of true convergence. So, motivation for convergence to IP is due to lower infrastructure cost, lower maintenance cost, standardized network and protocol specifications; reusable infrastructure with enhancements, reliable and good quality of service.

2.IMS – The Mastermind to the Convergence Now the operators in the network business have realized the need for convergence, and they are faced with significant challenges. Carriers face a market saturated with customers demanding more choices. The carriers want to find new services and values to deliver to their existing customers in an effort to stay ahead of their competitors and also increase their revenue. Carriers seek a solution that allows the migration to All-IP, without losing their original capital expenditure and ensuring portability of their services. The end-users demand for universal access to multimedia services across access devices, they want to be able to carry their personalized service profile as they move from device to device, from home to automobile to office, and wired to wireless networks. Operators who act early and adapt their strategic business plan considering the changing environment, with an early introduction of converged services, will gain a competitive edge. This converged network carrying voice and data bearers offers opportunities for introduction of innovative services. IMS is a cornerstone for efficient converged service offerings that combines the flexibility of IP networks and extensibility of SIP protocol. IMS is defined by 3GPP as a new subsystem, i.e. a new mobile network infrastructure that enables the convergence of data, speech and mobile network technology over an IP-based infrastructure. IMS was specifically architected to enable and enhance real time, multimedia mobile services.

Device Layer The IMS architecture provides a variety of choices for users to choose end-point devices. The IMS devices such as computers, mobile phones, PDAs, and digital phones are able to connect to the IMS infrastructure via the network. Other types of devices, like traditional analog telephone phones, can connect to an IP network via a PSTN Gateway. The function of the devices is to store the SIP user agent, perform Signaling and Media transport, tracking of user’s public and private user id, tracking of User Network address, maintains Security algorithms and keys, Correlate between session control and QoS reservation

Transport Layer The transport layer is responsible for initiating and terminating SIP sessions and providing the conversion of data transmitted between analog/digital formats and an IP packet format. It is also referred to as the transport signalling gateway. The communication between the IMS components is based on IP. Currently the signalling protocol for PSTN is SS7. SS7 has limitations and is not as flexible as IP. The Transport Signaling Gateway (T-SGW) converts SS7 to IP. Soon it may be worked out that the MGCF supports SS7, and then the T-SGW would not be required. IMS devices connect to the IP network in the transport layer via a variety of transmission media, like WiFi, DSL, Cable, SIP, GPRS and WCDMA and PSTN network via the PSTN gateway.

Control Layer This is also referred to as the session control layer. The session control layer is made up of network control servers for managing calls, establishing sessions and making modifications. There are two main elements of this layer: the call session control function (CSCF) and the home subscriber server (HSS).

CSCF handles SIP registration of the end points and process SIP signal messaging of the appropriate application server in the service layer. Other functions of CSCF is interoperating with the access and transport layers to guarantee quality of service (QoS) for all services, process signaling messages for controlling the user’s multimedia session. Another element in the control layer is the Home Subscriber Server (HSS) database that stores the unique service profile for each end user. The service profile may include a user's IP address, telephone records, buddy lists, voice mail greetings and so on. By centralizing a user's information in HSS, service providers can create unified personal directories and centralized user data administration across all services provided in IMS. Each end user service profile is maintained by the HSS database for authentication and authorization purposes.

 In IMS, it is also possible to combine services from several different SIP application servers to create one unified user experience for the end-user. For instance, a user can from a single terminal application, simultaneously combine the services of presence and video calling although the services themselves are located on different SIP application servers. Some application servers are: Presence servers, Group list management server, Instant messaging server. The architectural lay out where IMS “fits in” makes it possible to do it in stages and not an overnight shift.

4.Roadmap to IMS compliance Evolution, not revolution, is the key to taking business-driven steps toward adopting an IMS infrastructure. Operators should evolve network infrastructure to deploy IMS incrementally rather than make drastic changes. Different standardization for different networks makes it all the more complicated in the path to the IMS evolution.
The major driver for this IMS evolution is need for new multimedia services by end-users. The end-user wants services with the same level and quality of service, whether it is via fixed devices or mobile phone. Thus, the focus of the operator has been on the development and identification of services that are network independent. These services will attract more end-users and drive higher data usage.

The evolution path to IMS consists of migration, improvement and eventual replacement of both the Radio Access Network (RAN) and network infrastructure. Most of the service providers are facing increasingly, competitive forces to go beyond VoIP to deliver real-time services now, while they simultaneously plan and execute the evolution of their infrastructures towards full IMS compliance. As a result, many service providers cannot jump directly to IMS, they must proceed in phases.

Phase -I VoIP Transport
The telecom industry has already reached the first stop on the road to IMS adoption. Carriers worldwide have replaced TDM transport networks and Class 4 switches with more economical IP transport and soft switches. A large percentage of long distance voice transport now relies on IP. For end users, however, little has changed; their telephony experience is the same as with TDM transport.

Phase-II Connecting Users to Services
The next step is the “pre-IMS” phase. In this phase, service portfolios expand beyond voice telephony to include video, IM and other applications. In a multimedia service deployment intended for IMS migration, the deployment model would implement the layered architecture with SIP as the dominant protocol. The model that can be adapted at this point is:

Application Layer VoIP, video, IM, presence, servers
Control Layer The SBC as the Control Function
Transport Layer Switches, routers, gateways

Certain IMS elements like the Home Subscriber Server (HSS) may also be deployed. Phase 2 is still pre-IMS delivering the sort of real-time multimedia experience that IMS is intended to support, without the benefit of the full IMS control structure.

Phase-III Standard application and Session Control
This phase involves the integration of SIP and IMS into the carrier infrastructure. Universal use of SIP puts an end to vendor-specific protocols and promotes application and endpoint interoperability. It also breaks down barriers between disparate networks, facilitating fixed-mobile convergence and other advanced capabilities. The layered IMS framework accelerates service creation and delivery by enabling consistent behavior across diverse access networks, IMS increases the reach and productive lifespan of new multimedia services.
Phase-IV IMS Compliance

This is the transition from Pre-IMS to IMS. Many fixed-line and mobile operators are already in the pre-IMS phase and few enterprises are deploying and managing large scale SIP-based services with a foresight of piloting the move to IMS compliance in the near future. The steps mentioned are suggestive for enterprises planning to deploy IMS in the near future. A comparative analysis with IP network with and without IMS is shown in the next section this will give a better idea about the benefits in the move to IMS compliance.

5.Conclusions 6.References
Books 1.Black, U., “Voice over IP”, Prentice Hall, 7/99
2.Berliner Solomon., "SIP/IMS standard view: SIP IMS Specifications For Dummies" ,self published by Sipknowledge
White Papers
3.Calhoun Pat “AAA requirements for IP Telephony/Multimedia” SIP WG, IETF.
4.Dutta Ashutosh, Ling Yibei, Chen Wai, Chennikara Jasmine, “Multimedia SIP sessions in a Mobile Heterogeneous Access Environment,”,.Telcordia Technologies Inc. white paper
5.Dutta A., Vakil. F, Baba S., Schulzrinne H., “Application Layer Mobility Management Scheme for Wireless Internet”,at 3G Wireless 2001, San Francisco.
6.Ericsson – ‘Combinational services – the pragmatic first step toward all-IP, Ericsson Review No.2, 2003
7.Ericsson – “Efficient Softswitch” white paper, Revision A, August 2006
8.Ericsson – “Mobile multimedia, the next step in richer communication” white paper
9.Ericsson – “Introduction to IMS”., March 2007 , White paper
10.Ericsson – “Mobile Multimedia” – White Paper
11.Ericsson - IMS - IP Multimedia Subsystem, White Paper.
12.Handley Mark, Schooler Eve, Schulzrinne H, Rosenberg Jonathan.,“Session Initiation Protocol”, RFC 2543
13.Kuthan Jiri, Sisalem Dorgham., “SIP: More Than You Ever Wanted To Know About”, Tekelec., March 2007,White Paper.
14.Perkins C., “IP mobility support”, RFC 2002, IETF October 1996”
15.Technical Specification Group Services and System Aspects, “IP Multimedia Subsystem (IMS)”, Stage 2, V5.15.0, TS 23.228, 3rd Generation Partnership Project 2006
16.The 10 that Established VoIP (Part 2: Level 3), iLocus (July 13, 2007).
17.3G Americas – IP Multimedia Subsystem (IMS) overview and applications white paper
18.3GPP TS22.228, Service requirements for the Internet Protocol (IP) multimedia core network subsystem (IMS); Stage 1
19.3GPP TS23.228, IP Multimedia Subsystem (IMS); Stage 2
20.3GPP TS23.279, Combining Circuit Switched (CS) and IP Multimedia Subsystem (IMS) services
21.Ubiquity., “Understanding SIP” White paper
22.Vakil. F, et. Al., “Host Mobility Management Protocol for 3G network ”., Internet Draft –White paper

Web sites
1.http://www.arnnet.com.au- Arnnet Magazine
2.http://www.cs.columbia.edu/~hgs/sip/ -SIP Home Page
3.http://www.ietf.org IETF web site
4.http://www.ibm.com/developerworks/webservices/library/w-soaipmultisub1 IBM web site
5.http://www.imsforum.org - IMS Forum
6.http://www.openimscore.org/- IMS open core
7.http://www.sipcenter.com- SIP Center
8.http://www.sipforum.org/.-SIP Forum
9.http://www.sipknowledge.com/IMS_Specs.htm -
10.http://www.ss7.net/ss7-wiki/index.php/Ims-failure - wiki for source material critical of IMS
11.http://www.3gpp.org -3GPP home page
12.http://www.tech-invite.com/ SIP/IMS Technical Portal
13.http://www.tmcnet.com/ims-TMCNET–IMS Magazine issues from 2005 to 2006
14.http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem"

Contributed By: Ms Vijayalakshmi Ravi, Lecturer, SIES( Nerul) College of Arts, Science & Commerce, Nerul, Navi Mumbai 400706 I am a professor, Qual - MPhil (Computer Science), interested in technical writing from home. Guide: Dr. Santosh Mohanty, Global Practice Director, TCS Mumbai. viji.ra