28 Developing SIP and IP Multimedia Subsystem (IMS) Applications
2.1 IMS overview
IP Multimedia Subsystem (IMS) is a set of requirements and specifications
defined by 3rd Generation Partnership Project (3GPP) and 3rd Generation
Partnership Project 2 (3GPP2). 3GPP and 3GPP2 were formed through
collaboration agreements that include a number of regional telecommunication
standards bodies. IMS defines a unifying architecture for IP-based services over
both packet- and circuit-switched networks. It enables the convergence of
different wireless and fixed access technologies for the creation, delivery and
consumption of multimedia services. IMS also supports services integration
through standardized reference points (akin to interfaces and protocols) which
not only makes service creation faster and easier but also leverages the services
available through Internet technologies such as Web Services.
2.1.1 IMS vision and history
IMS is at the center of the 3G (third Generation) initiatives driven by 3GPP and
3GPP2 who are participants in International Mobile Telecommunications-2000
(IMT-2000) the global standard for third generation (3G) wireless
communications, defined by a set of interdependent International
Telecommunications Union (ITU) Recommendations.
The 3GPP was formed as a collaboration of many organizations (includes
Association of Radio Industries and Businesses, China Communications.
Standards Association, European Telecommunications Standards Institute,
Alliance for Telecommunications Industry Solutions, Telecommunications
Technology Association of South Korea and Telecommunication Technology
Committee of Japan) in 1998 to develop the technical specifications for a 3G
network evolving from a GSM network. A similar charter, 3GPP2 (includes
Association of Radio Industries and Businesses, Telecommunication Technology
Committee of Japan, China Communications. Standards Association,
Telecommunications Technology Association of South Korea and
Telecommunications Industry Association), was formed to evolve the North
American and Asian networks from traditional CDMA2000 networks into a 3G
Both 3GPP and 3GPP2 conceptualized their respective IMS (IP Multimedia
Subsystem) architectures to support packet switched communication, in order to
merge the Internet and the cellular worlds. The IMS core network has a common
IP based transport and signalling, which can be accessed by different networks.
IMS common signalling is based on SIP. We presented an overview of SIP in
Chapter 1, “Introduction to Session Initiation Protocol (SIP)” on page 3 as an
end-to-end based session management control protocol. SIP allow applications
Chapter 2. Introduction to IP Multimedia Subsystem 29
to remain agnostic of the access network, which matches the network access
requirements for IMS.
Initial concepts for IMS emerged with the Universal Mobile Telecommunications
System (UMTS) third-generation (3G) specifications in 1998. The first
specification of IMS was published in March of 2003 by 3GPP in UMTS Release
5. UMTS Release 5 provided general description of IMS, SIP and end-to-end
Quality of Service (QoS) as part of an “All IP” network. IMS continues to evolve
with each UMTS release since 2003. New functions and changes to IMS are
introduced through 3GPP approved change requests (CRs) with each release.
3GPP release 6 and 7 added interworking with wireless local area networks
(WLAN) and support for fixed networks, by working together with TISPAN
(Telecoms & Internet converged Services & Protocols for Advanced Networks).
The vision of IMS as the common platform for development and delivery of
diverse multimedia services for a true mobile Internet is based on the set of
requirements set forth in the 3GPP IMS requirements captured in 3GPP TS
The following are the highlights of the key IMS requirements:
IP multimedia sessions
IP multimedia session requirements are focused on the main service
delivered by IMS, it includes support for a variety of media (such as voice,
video and data) and one or more applications that provide the service
experience within sessions. The IP multimedia session requirements call for
media interoperability and per user application flow control.
Quality of Service (QoS)
IMS QoS characteristics are negotiated between end points in IMS sessions.
The parameters include the type of media, the codecs and encoding formats,
bandwidth, delay, delay variation and packet loss. The IMS QoS requirements
include negotiations at session establishment and during the session as well
as for individual media components. IMS requirements also allow for
operators to set policy and control QoS for all or individual users.
Note: Telecoms & Internet converged Services & Protocols for Advanced
Networks (TISPAN) is a standardization body that is working through the
European Telecommunications Standards Institute (ETSI) to define the Next
Generation Network architecture. Some aspects of the TISPAN Release 1
architecture is based on IMS.
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