adapt to become more "data friendly." Transport networks and the services they carry are therefore adopting packet principles to a far greater extent than they did 10 years ago. Preface Part 1 examines the way voice, data, and video sources are digitized and coded preparation for transmission over telecommunications networks. This provides a solid intro- duction to the way signals are carried over transport networks. Part 2 deals with the physical media commonly used in core transport networks- namely optical fiber and radio. Optical fiber carries the vast majority of telecommunications traffic at regional, national, and international levels. The basic properties of optical fiber itself are discussed, and the principle characteristics and limiting factors of high-capacity, long-range optical systems are explained. Radio systems are used to a lesser extent in core networks. Very good discussions of these were included in the earlier Telecommunication Engineer's Reference Book, and selected chapters are reproduced here, with only minor addi- tions, on the grounds that the principles of radio propagation have not changed. Part 3 presents the main network technologies used today in core transport networks. While the trend is toward data-networking technologies, such as IP/MPLS and carrier-grade Ethernet, the concept of "next-generation networks" is not specific about which technologies should predominate, other than that a packet-networking technology should act as a con- vergence layer. Indeed, a key principle is that converged traffic can be carried over any underlying transport technology. For example, while SDH is generally considered to be a legacy technology now, its installed base is SO huge that it will continue to be a key transport technology for many years to come. Moreover, its life is effectively extended with the appli- cation of Next Generation SDH. Other "legacy" technologies are also described, including ISDN, leased lines, Frame Relay, and ATM. In conclusion, IP/MPLS, which is now favored by many op