erators as a core transport network for data traffic, and the emerging carrier- grade Ethernet, are described. Where appropriate, chapters that were included in the Second Edition of Telecommuni- cations Engineer's Reference Book are reproduced in this volume with changes and updates as needed. I was unfortunately unable to contact four of the previous contributors for their updates. In Part 2 this applies to Chapters 7, 8, and 9 and in Part 3 to Chapter 10. An assess- ment was that these chapters contained a lot of information that is still very important, relevant, and clearly presented. In most cases the changes to the original text are minor. As for Chapter 10, the previous edition's chapter focused on SDH; for this book, I have rearranged the original text slightly and made some additions to highlight the similarities and differences between SONET and SDH. I am greatly indebted to the authors who have contributed to this volume of the Cable and Telecommunications Professionals' Reference. They are all experts in their fields and working at the forefront of technology. I should like to express my thanks to them and to their respec- tive organizations for all the support they have given. Goff Hill GTel Consultancy Ltd. Contributors Tahmina Ajmal, B.Eng., M.Eng., Ph.D. University of Essex Tahmina Ajmal received her Ph.D. from Essex University in 2007, having earlier earned her B.Eng. in 1990 and her M.Eng. in 1993 from Aligarh Muslim University in India. Before starting her doctoral studies at the University of Essex, she was a lecturer at Aligarh Muslim University and was upgraded to the post of reader there in August 2000. Since June 2007, Tahmina has worked as a part-time research officer at the University of Essex on the MUSE (Multi-Service Access Everywhere) project for the development of a low-cost multi-access network. Her research interests include fiber sensors, physical-layer cryptography, and different modulation formats for transmission. J. H. Causebrook, B.Sc., Ph.D., C.Eng., MIEE Independent Consu