Standards and protocols: Presentation-level protocol for national videotex services

A comprehensive presentation level protocol (PLP) for videotex services is proposed, which, similar to AT&T's PLP, also encompasses mosaic, DRCS, geometric and eventually photographic coding formats, but is in much closer accordance with both the CCITT and CEPT standard recommendations. Based on the German Bildschirmtext network concept, the authors show how this PLP can be implemented in national videotex services, taking into consideration the individual preferences of PTT administrations, the needs of users and information providers and the bounds imposed by existing services. Spectific requirements are derived for three key system elements, namely the PTT centres, acting as transparent gateways, the videotex database software concept and the user terminal family, consisting of basic-mode and advanced-mode terminals. Finally, development status is briefly described for an implementation based on existing services, utilizing a proper subset of the proposed PLP for a 7 bit environment.