nt decisions rather than past. but uses a network to transport data to it via long lines from all over The first is the consolidation of vital translation facilities, with the the world. This approach is the simplest for the VAN but the lack of focus moving from the lower levels to the higher. local processing can make traffic between endpoints in the same The second is an extension of the media used to include such remote country uneconomic. things as voice, video etc. To address this the idea of franchising the service was created. The third is the provision of higher level services. These will be This meant in-country operations and local presence, but interna- such things as document conversion, directory services and on-net tional traffic is essentially inter-VAN traffic and often subject to library services where whole document layouts are held on the surcharges. service but only the content is transmitted. This page intentionally left blank Modems TJ Egginton BEng (Sheffield University) Gandalf Digital Communications Ltd Contents 39.1 Introduction 39/3 39.2 Principles of operation 39/3 39.3 Modem standards 39/5 Types of modem 39/9 39.5 Typical modem features 39/10 39.6 Applications 39/12 39.7 References 39/13 This page intentionally left blank Introduction 39/3 39.1 Introduction radically but remain reliable enough to more than keep pace with alternative transmission systems. With a long and illustrious history behind them, one might be forgiven for thinking that the humble modem should be firmly When the CCITT Telecommunications Standards Organisation consigned to the electronic scrap heap. Indeed, it was not SO long introduced the V.32 standard in 1986 for operation at 9000 bit/s, it ago that the majority of industry pundits believed that ISDN would was hailed as the last, great modem standard. By February 1991 the eventually finish off the modem for good. But the modem is a CCITT had approved the V.32bis standard and has a study group survivor and in the 1990s, some twenty years after its first appear- 