12.1 Introduction

The basic principle of services such as television or pay-TV is that the content is distributed via a broadcast network to the end user. When these types of services are taken into account, television can be considered a passive medium. Interactive television implies that in communication the end user is able to control and influence the subjects of communication, with the control and influence taking place via an interaction network. Examples of interactive services are Internet services that are provided via the television medium, video mail, and interactive teletext. If interactive services are provided on the basis of CA, one can, for example, think of pay-per-view or video-on-demand.

Before developing technical standards for interactive services, DVB first specified the commercial requirements ^[1]. One of these requirements is that the specifications must be compatible with different types of networks. DVB decided not to specify an interaction channel solution associated with each broadcast system. Hence, in the higher layers DVB developed network independent protocols for interactive services ^{[2] [3]}. However, at the transport and physical level, several standards have been specified for the different networks. One of these standards concerns broadcasting via CATV networks, where the interaction channel(s) can be embedded in the CATV network itself. In case of the DVB satellite and terrestrial broadcasting systems, however, it is specified that the interaction channels are accommodated in a separate network. The application layer, as well as the hardware and software of the end user terminal, are left up...