scope:

INTRODUCTION

This document contains the technical description of the transmission technique, coding lanquage, and user interface for one-way broadcast teletext-service applications in North America.

The first seven chapters in this specification generally correspond to the seven layers in the Open Systems Interconnection (OSI) basic reference model (chapter 1 corresponds to layer 1, etc.), however not all parts of each chapter necessarily correspond to the appropriate layer.

For reference the seven layers, and a summary of their most important functions, are:

1 - Physical Provides the mechanical and electrical (b provisions for the physical interconnection

2 - Link Provides the means for establishing and maintaining data links between network entities.

3 - Network Provides the means for exchange of network service data units over a network

4 - Transport Provides a universal transport mechanism for lower level protocol

5 - Session Provides for the binding and unbinding of two presentation entities

6 - Presentation Provides the method of coding and presenting the information

7 - Application Invokes the protocol directly serving the user

For a full specification of the OS1 model see IS0 7498-1983, and CCITT X.200.

A fundamental concept embodied in this teletext standard is that of maintaining the distinction between the means of transporting the data and the coding of the teletext message. The data transmission system does not impose any restriction on the data which can be transmitted. Similarly, tne coding language is independent of the transmission technique. For example, the presentation technique of this codin9 language may be used with conventional modems on a switched telephone network.

Because of this careful separation of functions, similar to the convention recommended by International Radio Consultative Committee (CCIR) and the International Telegraph and Telephone Consultative Committee (CCITT), the present standard allows advances in technology without making basic level equipment obsolete.

The transmission system is defined in chapters 1, 2, 3 and 4. That part of the standard dealing with the coding and utilization of the teletext message is defined in chapters 5, 6 and 7. Chapter 6 refers to the joint ANSI/CSA standard Videotex/Teletext Presentation Level Protocol Syntax (North American PLPS)Il. Because of the layered structure, the transmission technique defined in levels 1 through 4 can transport any type of digital data, not only those broadcast teletext Records coded according to this standard. Similarly the presentation language can be used without the data-broadcasting technique.

Chapter 8 describes the fundamental service specifications and recommended practices for broadcast teletext.

Terms defined in the body of the specification are identified by capitalizing the initial letters. These terms are also contained in a Glossary in Appendix A. The Glossary gives references to the main text where the complete definitions of the terms may be found.

Any parameters left undefined in this specification are reserved for future standardization and shall not be used for private purposes unless otherwise indicated.

In this specification, use of the words llshalltt,"s hould", and represent requirements, recommendations, and options, respectively.