ABOUT THIS BOOKThere has been unprecedented development in data communications and services since the first edition of this book was published in 1986. In less than a decade the technology has advanced beyond all recognition and the first edition is now really no more than an interesting historical record. The second edition, published in 1989, reflected some of these developments and introduced the then emerging proposals for an integrated services digital network (ISDN). Since that date ISDN has become a fact and has already begun to be superseded by proposals and standards for a broadband ISDN (B-ISDN), offering greatly enhanced and flexible data rates over a public network based mainly on optical fibre transmission. Optical fibre technology is also being used in wide-area private digital networks and for high-capacity internetworking operations.