Book Review - The Internet Protocol Journal - Volume 7, Number 4

The IP Multimedia Subsystem

The IP Multimedia Subsystem—Merging the Internet and the Cellular Worlds
, by Gonzalo Camarillo and Miguel A. Garcia-Martin, John Wiley & Sons, 2004. ISBN 0470 87156 3.

The Internet and the cellular telephony system are the two most influential communication systems of the last half century. That the telecommunications industry would attempt to merge them into a single system was inevitable. The potential benefits are compelling—a single packet-based communication system with the capability to carry voice, video and data while providing ubiquitous wireless access and global mobility. The resulting system architecture is called the
Internet Multimedia Subsystem
(IMS) and is described comprehensively in this volume by Gonzalo Camarillo and Miguel A. Garcia Martin.

A "merging" of the two systems is only superficially what has happened. In practice, the IMS is an "embrace and extend" exercise which adapts the IP protocol suite to the existing architecture of the cellular telephony system. The cellular industry has taken a broad collection of IP protocols and mapped them onto their existing architecture, effecting a "protocol transplant" into an environment somewhat different from the Internet. Among the protocols imported are IPv6, SIP, DHCP, DNS, SDP, RTP, IPSec, and DIAMETER. Many are adopted unaltered; some are profiled by introducing new configuration data and rules; others are extended in various ways. The authors navigate their way through the various parts of the system with clarity and confidence. They can speak with authority on the subject—both were major contributors to the design through their key roles in the IETF and 3GPP (
Third Generation Partnership Project
—the standardization body for third generation cellular systems).

The book is clearly written and logically organized. The first part explains the reasoning behind adopting Internet-style packet networking for cellular mobile systems and describes the evolution of the standardization efforts. Although interesting, much of this material can be skimmed by those only interested in the meaty technical material which follows. The authors then explain the general principles behind the IMS architecture, including how various requirements of the cellular telephony industry drove the choices, and particularly the perceived need to extend and adapt the protocols rather than use them as deployed on the Internet. The majority of the book is devoted to explaining in considerable technical depth how the protocols have been modified and how they are intended to work when IMS is successfully deployed. While not for the faint of heart, the writing is extremely clear and logical and hence should be understandable by anyone with a moderate background in the principles of protocol and system design. One aspect of the organization is particularly helpful to readers unfamiliar with some of the protocols in their native Internet instantiation. The authors divide the material into blocks where they first describe the native Internet flavor of the protocol, and then introduce the IMS-specific extensions and modifications.

Much of the volume is devoted to the
Session Initiation Protocol
(SIP) as the core signaling plane for IMS. All aspects of session establishment and management are covered. In addition, the ancillary parts of the control system are covered, including
Authentication, Authorization, and Accounting
(AAA), Security, Session Policies, and Quality of Service. For completeness, the data plane is also covered briefly through a discussion of the 3GPP audio, video, and text encoders, plus material on the media transport protocols.

The book concludes with a substantial section on how services are build on top of the core IMS protocols. Two of the most important,
Instant Messaging
, get comprehensive treatment, with a briefer discussion of the push-to-talk application.

As an old time "IP-head," it is hard to come away from this deep exploration of IMS without a bit of trepidation. The hallmark of IP and the Internet are simplicity and generality. IMS arguably succeeds at the latter, but at the expense of almost numbing complexity. This was perhaps inevitable given that the goal was to adapt Internet packet technology to the cellular system, which is itself quite complex. IMS will be quite a challenge to deploy. It remains to be seen if transplanting IP into a cellular telephony architectural model will result in economically sustainable services for the service providers or if a more native peer-to-peer Internet approach will simply bypass all the fancy IMS elements and just use basic packet transport. Such a market experiment is currently playing out in the broadband access arena with the broadband pipe suppliers offering telephony-oriented services themselves via customized standards like PacketCable, while third parties like Vonage and Skype simply piggyback on basic IP packet transport.

The next few years will be interesting. Whatever the outcome, anyone needing to be technically conversant with the architecture and protocols of IMS will find
The IP Multimedia Subsystem

David Oran

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