The first edition of this book covered signaling for telecommunications through the early 1990s. Telecommunication technology has continued its remarkable progress in the years that followed. New technologies and services have come into use, and are supported by a number of new or expanded signaling systems. The aim of this second edition is to cover some of the most important of these new technologies and their signaling. Six new chapters, briefly outlined below, have been added for that purpose.
Access systems (Chapter 8) have become a part of the local network architecture. Several access systems (AS) surround a local exchange, each one serving the analog and digital lines of a group of nearby subscribers. Such systems, once called 'remote line concentrators', used to have proprietary interfaces to the local exchanges. Interfaces and signaling have now been standardized, allowing a telecom to purchase access and switching hardware equipment from different product suppliers.
In code-division multiple access (CDMA) wireless systems (Chapter 13) all traffic channels in a cell are carried in two common (forward and reverse) frequency channels. Individual traffic channels can be separated because the data stream from the user is encoded at the sending end with a special bit sequence that identifies the channel. To recover this information, the receiving end decodes the sender's data stream with the same sequence. Regardless of the number of cells, a CDMA system thus needs only one pair of frequency bands. This is an important advantage over AMPS and TDMA systems, which manage interference by assigning different RF bands (up to seven pairs) to adjacent cells.
INAP (Chapter 18) is the ITU-T-sponsored architecture and protocol for remote operations, equivalent to, but much more ambitious and complex than, the Advanced Intelligent Network described in Chapter 17. INAP standards are based on a sophisticated multi-level abstract view of features and services.
The first edition of this book did not cover networks for data communication, which made their appearance around the 1970s. Data communication
(Chapter 20) has expanded tremendously over the last 30 years, reflecting the enormous increase in the availability of computers and in the number of applications that require distributed data processing. A data message consists of a sequence of 'packets', short data bursts that have two parts: the 'header', which contains signaling information used by intermediate nodes to route the packet to its destination, and a second part that carries user information. Among the most important innovations in communications is the convergence of voice and data in packet networks, popularly known as Voice over IP (VoIP). The door to convergence was opened in the 1960s with the introduction of digital voice transmission. Digitally coded speech can be transported in a data network as a sequence of packets, each one carrying a number of speech samples. This technology is revolutionizing telecommunications and the driving factors for that are discussed in Chapter 20.
The most popular and widely deployed signaling protocols for VoIP, such as H.323, SIP, H.248, and BICC are discussed in Chapter 21.
ATM, or asynchronous transfer mode (Chapter 22), closes the book with a discussion of a packet technology that supports broadband multimedia communication, and facilitates the interworking of packet-switched and circuit- switched networks. Although widely deployed in carrier backbone networks and in ADSL access networks, ATM appears to have lost momentum to packet technology based on TCP/IP protocols.
Chapters 11 (ISUP), 14 (Introduction to Transactions), and 16 (TCAP) have been rewritten to cover the newest developments in ITU-T standards. Chapters 1 through 7 have also been updated to reflect current standards.
We would like to extend our grateful thanks to former colleagues from Lucent Technologies who were invaluable in providing advice and information, as well as in reviewing parts of the manuscript: Donald W Brown, James R Davis, Thomas S Hornbach, Konstantin Livanos, Alan J. Mindlin, Leon J Peeters, and Makoto Yoshida.
We cannot close this preface without also thanking our wives, Maria and Elizabeth, without whose patience, understanding, and loving support this second edition could not have been written.
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