3G的IMS子系统

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Foreword by Stephen Hayes
3GPP, or the 3rd Generation Partnership Project, was formed in late 1998 to specify the
evolution of GSM into a 3rd generation cellular system. Although much focus was placed on
new higher bandwidth radio access methods, it was realized that the network infrastructure
must also evolve in order to provide the rich services capable of taking advantage of higher
bandwidths. The original GSM network infrastructure was very much circuit and voicecentric.
Although data capabilities were added over time the system retained much of its
circuit-switched heritage and its inherent limitations. A new approach was needed.
IMS, or the IP Multimedia Subsystem, represented that new approach. The development
of IMS was very much a collaborative effort between the leading cellular standards
organization (3GPP) and the leading Internet standards organization (IETF). IETF provided
the base technology and protocol specifications, while 3GPP developed the architectural
framework and protocol integration required to provide the capabilities expected of a worldclass
mobile system, such as inter-operator roaming, differentiated QoS, and robust charging.
Since the initial specification of IMS, IMS has been adopted by 3GPP2 (the other major
cellular standards organization) and is the leading contender as the base of the ITU work on
Next Generation Networks. In the upcoming decades an understanding of IMS will be as
important a fundamental for the well-rounded telecom engineer as ISUP knowledge was in
previous decades.
IMS is a system. It is designed to provide robust multimedia services across roaming
boundaries and over diverse access technologies. To understand IMS, you must understand
both the underlying protocols and how IMS uses them within its architectural framework.
This book facilitates that understanding by explaining first the underlying protocols, such as
SIP, and then explaining how IMS makes use of those protocols. This approach allows the
user to easily grasp the complex relationship between the protocols and entities as developed
in the IETF and their usage and extensions as defined in IMS.
The two authors are uniquely qualified to explain not just the inner workings of IMS but
also the rationale and tradeoffs behind the various design choices. Miguel Angel Garc´ıa-
Mart´ın was and still is a key contributor within 3GPP. He was one of the principal designers
of IMS and authored the initial protocol requirements draft as well as other 3GPP-specific
SIP drafts and RFCs. Gonzalo Camarillo was similarly a key contributor within IETF where
he is currently a SIPPING WG co-chair. He has written many RFCs that are key components
of IMS. Both authors have been involved with IMS since its inception and do a good job of
explaining not only what IMS is but also how it came to be.

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