ONLINE PUBLIC ACCESS CATALOGUES 1: NATURE AND DEVELOPMENTS
The paper introduces a series of review articles about OPACs: what they are, how do they function, which problems do they cause for information retrieval, what are the main directions of OPAC research and development.
The first part of the paper deals with origin and evolution of online catalogues from simple circulation or cataloguing systems to complicated information retrieval systems. Three generations of online catalogues according to C. Hildreth's conception are described. Today most of the operating OPACs have only the features of the second generation but there is a number of prototype systems with enhanced retrieval capabilities in many libraries.
In the second part of the paper the conceptual model of an OPAC is introduced as it is presented by R. R. Larson. By Larson, every online catalogue system can be seen as a set of functional layers mediating access between the user and the bibliographic data stored in the database.
These functional layers are: the user interface layer, the DBMS interface layer, the DBMS layer, and the database layer.