Introduction to IMS

1.1 What is IMS?
IP Multimedia Subsystem (IMS) is a set of specifications that describes the Next Generation Networking (NGN) architecture for implementing IP based telephony and multimedia services. IMS defines a complete architecture and framework that enables the convergence of voice, video, data and mobile network technology over an IP-based infrastructure. It fills the gap between the two most successful communication paradigms, cellular and Internet technology. Do you ever imagine that you can surf the Web, play an online game or join a videoconference no matter where you are using your 3G handheld device? This is the vision for IMS; to provide cellular access to all the services that the Internet provides.
1.2 History of IMS
IMS was initially defined by the 3rd Generation Partnership Project (3GPP), which is a collaboration agreement among a number of telecommunications standards bodies, as part of their standardization work for supporting GSM networks and radio technology evolution. IMS was first introduced in 3GPP Release 5, where "Session Initiated Protocol" (SIP), defined by the Internet Engineering Task Force (IETF), was chosen as the main protocol for IMS. It has been further enhanced in Releases 6 and 7 of 3GPP to include additional features like presence and group management, interworking with WLAN and CS based systems, and Fixed Broadband access.
Another standards body, 3rd Generation Partnership Project 2 (3GPP2), also standardized their own IMS. 3GPP2 was born to evolve North American and Asian Cellular Radio-telecommunication Intersystem Operations into a third-generation system. The initial release of 3GPP2 specifications on IMS largely adopts from 3GPP Release 5. The two IMS network defined by two organizations are fairly similar but not exactly the same. 3GPP2 added appropriate adjustments for their specific issues. Nevertheless, the purpose of both organizations is to ensure the IMS applications will work consistently across different network infrastructures.
In addition to the 3GPP and 3GPP2, Open Mobile Alliance (OMA) plays an important role on specifying and developing IMS service standardization. The services defined by OMA are built on top of IMS infrastructure, such as Instant Messaging (IM), Presence service, and Group Management Service.
1.3 Benefits of IMS
We've discussed the idea of IMS as a way to offer Internet services everywhere using cellular technology. You may already be very familiar with accessing Internet services like Web access, email, or instant messaging via a 2.5G and 3G cellular phone. As a result, you may wonder why we need IMS.
The benefits of IMS over the existing cellular network infrastructure can be demonstrated in the following four aspects.

• IMS provides a common platform to reduce time-to-market for rolling out new multimedia services: One of the biggest challenges in today's communication network is to improve the long and costly process for creating a new service. Service providers are looking for ways to reduce the time-to-market for rolling out new multimedia services. The IMS infrastructure solves this problem by providing the standardized platform and reusable components. The standardized interface and common features provided by IMS infrastructure enables service providers to easily adopt a service created by third parties and create a service that integrates with many services effectively. In addition, with the standardized interface provided by IMS, the service is no longer solely provided by a single provider; any provider who implements the standardized interface can provide the service. The multi-vendor service creation industry leads to an open market, and allows service providers to choose the most effective way to roll out new services.
• IMS provides multimedia services with Quality of Service (QoS) enablement:: Although the dramatically increased bandwidth in 3G cellular networks provides a much faster and more reliable Internet access compared with 2.5G cellular network, there are no guarantees about the quality of the services. A 3G cellular network provides what is known as "best effort, which means the network will do its best to ensure the required bandwidth, but there is no guarantee it will remain at the same level. Consequently, the bandwidth of a particular connection can vary significantly over time. In order to solve this problem, Quality of Service (QoS) mechanisms were developed in order to provide certain guarantee levels of network bandwidth during transmission instead of the so called "best effort". IMS specifies enablement of Quality of Service within the IP network and takes advantage of th QoS mechanism to improve and guarantee the transmission quality.
• IMS allows operators to charge multimedia session appropriately: If a user uses videoconference over the 3G cellular network, there is usually a large data transfer that consists of audio and video. This is usually expensive since the operator will generally charge by the number of bytes transferred. On the other hand, if the operator is willing to provide a different charging scheme based on the actual service type, it may be more beneficial to the users. The advantage of IMS is that it provides information about the service type being invoked by the user and thus allows the operators to determine how to charge the users based on service types, i.e. they can choose to charge user by the number of bytes transferred, by the session duration (time-based), or perform any new type of charging.
• IMS allows all services to be available irrespective of the users' location: A typical, and particularly annoying problem when working with cellular technology is that some of the services will not be available when the user is roaming in another country. To resolve this problem, IMS uses Internet technologies and protocols in order to allow users to move across the countries and still be able to execute all the services as if they were from their home networks.

1.4 IMS architecture
IMS architecture supports a wide range of services that are enabled based on SIP protocols. As you can see from Figure 1-1 below, an IMS architecture delivers multimedia services that can be accessed by a user from various devices via an IP network or traditional telephony system. The underlying network architecture can be divided into three layers (Device Layer, Transport Layer, and Control Layer) plus the service layer and will be introduced from bottom to top respectively.

• 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, such as traditional analog telephone phones, although they are not able to connect to an IP network directly, are able to establish the connection with these devices via a PSTN Gateway.
• 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. IMS devices connect to the IP network in the transport layer via a variety of transmission media, including WiFi (a wireless local area networks technology), DSL, Cable, SIP, GPRS (General Packet Radio Service is a mobile data service), and WCDMA (Wideband Code Division Multiple Access, a type of 3G cellular network). In addition, the transport layer allows IMS devices to make and receive calls to and from the PSTN network or other circuit-switched networks via the PSTN gateway.
• Control Layer: The Call Session Control Function (CSCF), which is a general name that refers to SIP servers or proxies, is one of the core elements in the control layer. CSCF handles SIP registration of the end points and process SIP signal messaging of the appropriate application server in the service layer. 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.
• Service Layer: On top of the IMS network architecture, we have the service layer. The three layers described above provide an integrated and standardized network platform to allow service providers to offer a variety of multimedia services in the service layer. The services are all run by application servers. The application servers are not only responsible for hosting and executing the services, but also provide the interface against the control layers using the SIP protocol. A single application server may host multiple services, for example, telephony and messaging services run on one application server; one advantage of this flexibility is to reduce the workload of the control layer. There are many application servers providing different services, and three core application servers of IMS will be highlighted below.
  • Presence server: A "Presence server" provides the services to collect, manage and distribute the real time availability and the means for communicating among users. It allows users to both publish their presence information and subscribe to the service in order to receive notification of changes by other users.
  • Group List Management server: A "Group List Management server" provides services that allow users or administrators the ability to manage, create, modify, delete and search the network-based group definition and the associated lists of members. It also maintains the access permissions and other specific properties associated with the groups and the members. It is also used to provide buddy lists for instant messaging or other services.
  • Instant Messaging Server: An "Instant Messaging server" provides a communication service that allows users to send and receive messages instantly. Users are able to deliver the messages containing rich text, images, audio, video, or the combination of these over an IP network. It has been widely used in today's Internet community, and IMS will bring the same service experience into the mobile world.

Figure 1-1. IMS Architecture Diagram

Service providers are eager to allow their customers to be able to develop and implement services that leverage the existing the service resources described above. However, many enterprise application developers may have an IT background but are not familiar with those complex telephone protocols (e.g. SIP, ISDN, SS7 etc.); and they need a simple API for services creation and development. It then falls to Parlay X SOA (Service-Oriented Architecture) Web services, which have been defined by Parlay Group in 2003 in order to provide a set of simple-to-use, high-level, telecom-related Web services. The idea with Parlay X is to provide Web services in a context that is already widely adopted and understood by a large number of developers and programmers, and to do so in an environment where there are a variety of development tools available. With the Parlay X SOA Web services interfaces, the application developers can access and leverage the existing IMS services more easily through Web services. The Parlay X SOA Web services are connected to the telecommunication network either via the Open Services Access - Gateway (OSA-GW) or directly through data service components over IP Protocols.