Abstract
Although promised for some time now, advanced multimedia services for mobile devices were not yet a reality. With IMS (IP Multimedia Subsystem), the fundaments for advanced multimedia services based on IP will be laid. However, there are still a lot of confusions about IMS that may hinder its success. In this chapter, a comprehensible presentation of IMS together with its potential regarding the development of advanced services is given. The chapter starts with a justification of the existence of IMS. The necessary adaptations of SIP (Session Initiation Protocol) are explained. A concise description of IMS will be provided. Next, the deployment of IMS in fixed – mobile environments will be examined thoroughly. The heart of the chapter is the presentation of the strengths of IMS, that is, what can IMS be used for. A few advanced multimedia service scenarios are given as illustration. The chapter will also consider the most important but yet neglected component of the whole IMS, namely the IMS client. The chapter concludes with a summary of the challenges that must be resolved.
TopThe Need Of Ims
Short about the Session Initiation Protocol
With the advent of the Internet, the users get the opportunity to experience both fancy and useful services such as email, messenger, information services, banking, ticketing, e-commerce, and so forth. As the popularity of the Internet increases, the need for telephony or more generally communication services on the Internet arises.
To meet this, the Session Initiation Protocol (SIP) (IETF, 2003) allowing the establishment of soft real time communication sessions between two or several parties has been specified. As known the Internet is a packet-switched network meant for computer communications. Computers are equipped with functions to receive packets whenever they arrive. Users as human beings are not. In order to offer telephony or voice communication, users need to have assistance from SIP user agents. The agents have the responsibility to communicate with each other and with the users to establish a session between the users.
If the agents know the IP address that is assigned to its counterpart they can communicate directly with it and invite for a session. But, telephony is supposed to be available for any user and it is therefore not possible for a SIP user agent to know the IP address of all other SIP user agents in the world.
As shown in Figure 1 a SIP server is introduced to provide the mapping between users and the IP addresses of the user agents. A user is allowed to move and make use of one or several devices. With SIP, personal mobility, also called user mobility is supported. To make a call or to receive calls, a user may register to one or more devices. When moving the user may later on deregister and register to new devices. Several users may also register themselves to the same device.
Figure 1. Overview of the SIP architecture
However, SIP is not made for mobile networks and does not support terminal mobility, that is, enable calls when a terminal or device is moving. Indeed, the device may move to another IP sub-network and acquire a new IP address without the awareness of the user agent and the SIP server.
Key Terms in this Chapter
Home Subscriber Server: HSS is the central repository of all subscriber-specific authorizations and service profiles and preferences.
Session Initiation Protocol: SIP is an IETF signalling protocol used for establishing sessions in an IP network. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session.
Web 2.0: A term coined by Tim O’Reilly referring to the usage of the Web as a platform.
Push-to-Talk (PTT): A method of conversing on half-duplex communication lines, including two-way radio, using a momentary button to switch from voice reception mode to transmit mode.
Call Session Control Function: CSCF consists of several types of SIP servers and process all the SIP signaling in the network. The three most common SIP servers in the CSCF are: 1) Proxy-CSCF —First point of contact for device and controls authentication; 2) Interrogating-CSCF —Entry point of all SIP messages; 3) Serving-CSCF —Manages all session control functions
Presence: The information about the availability of a person for communication. It is more than just “online,” “off-line” and may include states like “present” “busy” “at meeting” “sleeping,” and so forth.
IP Multimedia Subsystem: IMS is an architectural framework for delivering Internet protocol (IP) multimedia to mobile users. It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), and is part of the vision for evolving mobile networks beyond GSM. Its original formulation (3GPP R5) represented an approach to delivering “Internet services” over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed line.