Wide Area Networks Facilities

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Brian Bramer,
School of Computing and Mathematical Sciences,
De Montfort University, Leicester
(acknowledgements to Ian Vlaeminke for the originals of these notes)
6 November 1993


5 Viewdata Systems

Viewdata or VIDEOTEX is a protocol developed by BT in the early 1970s which permits access to computers by cheap TV-type terminals over telephone lines. Prestel is the tradename of BT's public viewdata system which became operational in 1979. Unlike teletex systems, such as CEEFAX and ORACLE, which are 1-way broadcast mechanisms, Viewdata is a 2-way mechanism in which the user can actively affect the interchange rather than just responding to it.

Viewdata has been very much the poor relation of data communications. It was envisaged as a mechanism to be used in the home but the vast majority of use has been by business (particularly Travel Agents) who recognised its potential as a cheap and simple communications medium for random usage by naive users who are geographically dispersed. A large number of suppliers have developed private Viewdata systems based on the Prestel protocol which can be used within an organisation on its own computers.

Viewdata systems typically run on dedicated minicomputer networks and are characterised by 2 major features:

(a) the terminal is assumed to be a simple low cost device which may be a special purpose VDU, TV set and adaptor or home computer and simple modem. As a minimum there will be a telephone type numeric pad for input.
(b) the organisation of the data into a hierarchy of FRAMES with each frame corresponding to a screen image and identified by a unique numeric reference. A frame may contain information or be an index (menu) frame pointing to an information frame or lower level index.
Users of Viewdata are divided into 3 classes:
  1. information providers who supply data for frames
  2. editors who maintain the database of frames
  3. users who access the frames
It was initially envisaged that users would merely retrieve information ('view the data') and would do this by progressing through a menu hierarchy (pressing key m to select menu item m) until they reached the required frame. This rapidly became tedious for regular users of the same information and a direct access mechanism (by keying *frame-id) was implemented.

It was also recognised that since the communication was 2-way, the system could be used to collect data from users, particularly if they used a full keyboard. A special RESPONSE FRAME was implemented providing a simple form into which a user can input data; this response is sent to a message pool where it can be retrieved by the information provider. The information provider can specify a template for the form and simple edit checks to be carried out; however, the simplicity of the protocol and the physical limitations of a TV screen severely constrain form design.

Editors must have facilities for updating the content of frames and obviously require a full terminal to do this. Updating is typically by bulk batches as the protocol intended; on-line updating is possible but Viewdata is not normally a first choice where up-to-the-minute accuracy must be maintained. Information providers, who may be their own editors, have facilities for displaying database content and structure and for collecting response frames. They may also restrict access to their frames; each user is identified by a terminal id., user id., and password and access may be restricted to a closed user group of specified ids.

There are 3 main classes of Viewdata system usage:

- Prestel or public Viewdata
- private Viewdata
- Prestel gateways
Prestel provides access to a public database of about 270,000 frames of which around 70,000 are index frames. There are about 1200 Information Providers who pay an annual charge of £6000 + £5 per frame. The frames contain general information such as news, sport, etc., services such as timetable, entertainment and result listings provided by general IPs like American Express and Mecca and 'trigger products' such as Micronet and the Nottingham Building Society's Homelink. It also contains a growing number of Closed User Groups. The service is based on dual processors in London, Croydon and Birmingham together with an editing processor in London. Users have local access from 90% of the UK and typically pay the call charge plus 5p per minute for access during normal hours; there are some frames (eg money market rates) which have a charge for each access. There are around 40,000 terminals in the UK and 9 million frame accesses per annum. Private Viewdata utilises the Prestel protocol on an organisation's own machines and accessing its own database of frames. This is becoming an increasingly popular mechanism for information provision within a large organisation and turnkey solutions are available from a number of suppliers. Although it can be used for 2-way communication and has a window which provides access via Viewdata to normal data processing applications, it is more commonly used for information retrieval. Gateways enable users to access 3rd party processors via Prestel terminals and the PSS network. A user's terminal is connected via PSS to an interface minicomputer; the minicomputer acts as a 'front end' to the 3rd party's machine which is called a '3rd party database'. The minicomputer acts as a protocol converter; to the 3rd party processor it emulates a standard compatible terminal and to the user terminal looks like a Prestel database. A gateway facility costs around £7000 per annum.

Viewdata has a number of very attractive features:

- low cost
- speedy implementation
- ease of use without extensive training
It can provide fast update facilities and, if Prestel is used, provides a network of high availability and reliability

There are many applications for which it is plainly unsuited but it should be considered for information handling applications which:

  1. may need to be displayed in many possible locations, at random times to naive users
  2. deal with data which can be grouped in frames and indexed hierarchically, is simple and does not need high resolution graphics.
  3. do not require real-time updating and are suited to bulk updates
  4. have limited processing and hard copy requirements