Prestel

Prestel (abbrev. from press telephone), the brand name for the UK Post Office's Viewdata technology, was an interactive videotex system developed during the late 1970s and commercially launched in 1979. It achieved a maximum of 90,000 subscribers in the UK and was eventually sold by BT in 1994.

The technology was a forerunner of on-line services today. Instead of a computer, a television set hooked to a dedicated terminal was used to receive information from a remote database via a telephone line. The service offered thousands of pages ranging from consumer information to financial data but with limited graphics.

Prestel was created based on the work of Samuel Fedida at the then Post Office Research Station in Martlesham, Suffolk. In 1978, under the management of David Wood the software was developed by a team of programmers recruited from within the Post Office Data Processing Executive. As part of the privatisation of British Telecom, the team were moved into a "Prestel Division" of BT.

Prestel databases is commonly referred to as the ‘tree structure’. The structure is shown pictorially as an inverted tree with the data considered as ‘leaves’ of the tree, accessed via ‘branches’ which serve as a means of classifying the information. There exists quite a lot of jargon regarding such structures but in order to appreciate the concept it is necessary to mention just the node, page and frame. Nodes are the junction pages in the tree at which a number of choices can be made leading to other nodes or to the information itself. Pages are the final levels in the tree and contain the actual data-these may be divided into frames which are really screenfuls of information.

The public Prestel database consisted of a set of individual frames, which were arranged in 24 lines of 40 characters each, similar to the display used by the Ceefax and ORACLE teletext services provided by the BBC and ITV television companies. Of these, the top line was reserved for the name of the Information Provider, the price and the page number, and the bottom line was reserved for system messages. Thus there remained 22 lines (of 40 characters each) in which the IP could present information to the end user.

A page should be considered as a logical unit of data within the database and the frame a physical unit.Unfortunately the terms node, page and frame are often used synonymously which may lead to some confusion. To the user of course a node is the same as a page and they are both identified by a ‘page’ number. To access a particular item of information, a simple progression down through the nodes to the page is all that is required, and then the frames of that page can be stepped through. This is facilitated by each node displaying up to ten choices, one of which may be taken by the user responding with the appropriate digit from 0 to 9. This simple method of access may be thought of as a question and answer session: the computer displays a question ‘Which of the ten choices do you want to make?’ and the user replies with the appropriate digit. A choice of 9 at node 17 moves the user to page 179. The flexibility of this logical access method is increased firstly by allowing cross-referencing from one branch of the tree to another and secondly by providing a few simple commands available to the user for accessing certain pages directly. While this principle had considerable advantages in user simplicity and computer efficiency over the “keyword/thesaurus principle” used in many other systems, it has two very real disadvantages which have now been recognized: lack of flexibility; slowness.

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