The IMS: IP multimedia concepts and services in the mobile domain

As the Next Generation Network (NGN) concept becomes increasingly popular, the IP Multimedia Subsystem (IMS) concept has developed rapidly. However, in a narrow sense, the NGN is based on softswitch technology; it pushes the circuit-switch technology to the edge of the whole network. Similarly, the IMS is based mainly on application layer technology such as the Session Initiation Protocol (SIP) and further pushes the softswitch technology to the edge of the whole network. Since the IMS depends heavily on application layer technology, and the lower layer technology is transparent to it, it is easier for the IMS to realize Fixed Mobile Convergence (FMC). On the other hand, since the NGN is not very strictly named, the IMS can also be regarded as a part of NGN in a broad sense. The book The IMS has appeared at an appropriate time to discuss many aspects of the IMS, taking General Packet Radio Service (GPRS) as an IP connectivity access network example. The four authors are all working in the IMS-related standardization arena, so the content is undoubtedly authoritative and accurate. The IMS is suitable for everyone who wants to understand what the IMS is and how it works. The book is based mainly on Universal Mobile Telecommunications System Release 5 (UMTS R5), and some information about UMTS R6 is also presented from time to time. The IMS takes related standards developed by Third Generation Partnership Project (3GPP), Internet Engineering Task Force (IETF), or International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) as the core around which discussions are presented. This kind of arrangement makes readers aware of the current standardization status of each specific aspect of the IMS, wherever the reader is in the book. It also reaches the goal stated in the preface: “the book is also very well suited as a reference” (p. XIX). Nevertheless, instead of reprinting a huge amount of text directly from these standard documents, only the numbers of related standards are provided in the book, pointing out where the information is for interested readers. Since so many standards are mentioned, it would be a good idea to list these standards’ numbers and titles at the end of the book as references. The book is divided into four parts, and the overall structure is very clear and easy to understand. The discussion of IMS concepts and architecture in part one of the book seems somewhat scattered and isolated. However, part two of the book interconnects concepts and architecture in a systematic way so readers can really understand how the IMS works. Part three of the book is about related protocols used in the IMS. Since this part presents basic information about these issues, it may be helpful to read it before part two or even before part one. Of course, if you are familiar with these protocols, you may just skip this part. The IMS may seem like the perfect network, but if it can not provide the public with enough attractive services and applications that are impossible to be deployed in existing networks, its success is not guaranteed. Part four of the book points to this. Based on our own judgment, the application named “phonebook with embedded presence information” is promising. However, too few such applications are mentioned in this book. Figure 2.7 (IMS architecture) is very important for grasping the overall picture of the IMS; it will be referred to from time to time when reading this book. Because there are so many network entities and reference points in the IMS, it is somewhat difficult for a novice to quickly understand what the book is talking about. This phenomenon is normal and many answers will become clearer and clearer gradually during continuous reading through The IMS, so do not give up the book midway. A very important concept of the IMS, which is totally different from 2G mobile networks, is clearly presented in section 2.1.9, namely the home service control; this concept should be kept in mind to aid in understanding the IMS. The book embodies the backward compatibility philosophy of the IMS from time to time. Section 3.4.1.3 (Derived public user identity and private user identity), 3.5.2 (Universal Subscriber Identity Module) and Table 3.3 (AKA parameters) are examples for this philosophy. The book points out that Proxy Call Session Control Function (P-CSCF) is the entry point to the IMS network from other networks, while Interrogating Call Session Control Function (I-CSCF) is the entry point to the home IMS network from the outside of the home network. These two entry points tend to be confused, so extra attention should be paid to them. The IMS benefits from the high flexibility and extensibility of SIP, and the book frequently talks about SIP extensions for the IMS. On the other hand, there are some drawbacks to this book. Throughout The IMS, the contents of SIP/Session Description Protocol (SDP) messages are partially displayed according to specific emphases. However, a complete message example would be appreciated, to give readers the whole picture. It is difficult for people who have little knowledge of security to understand section 3.6, Security Services in the IMS. The book regards the reference point concept and the interface concept as the same thing. In our opinion, they are similar but not exactly the same; reference point pays more attention to the topmost protocol while interface concentrates on the whole protocol stack. Little explanation is given for Fig. 2.2, (IMS/CS roaming alternatives) making it hard to understand. The abbreviation CIMS-MGW (p. 27) should be IMS-MGW (IMS Media Gateway) or even IM-MGW IP MultimediaMedia Gateway. In SGW (Signaling Gateway) in Fig. 2.6, the protocol under M3UA should be SCTP. In Fig. 2.7, the communication relationships between entities are represented in bus form, which cannot clearly indicate which two entities actually need communication with each other. In fact, not all entities have the communication requirements for all the other entities connected to the same bus. For example, the SGW does not intend to communicate with the IM-MGW. So it is better to embody these communication relationships by direct lines. Table 2.1 is never mentioned throughout the book. The end of p. 32, or one of the other places where User Authorization Request (UAR) is mentioned, would be an appropriate place to refer to this table. Table 2.2 and 3.1 are not referred to either. It would be more appropriate for the Mk reference point in the fist line of section 2.3.13 (on the Mk reference point) to be Mi reference point. The Mn reference point is not explained at all; it would be better to include it. In Fig. 3.1 (A high-level IMS registration flow), there are altogether four I-CSCFs. However, the second and the fourth ones from left to right should be Serving Call Session Control Functions (S-CSCFs). On p. 56, the domain part of the private user identity and that of the temporary public user identity should have 234 and 15 exchanged. There are three UTMS in page 88,