Why television is called one way communication?

Because by itself, you can only receive information, there's no facility to reply. Everything is broadcast to everybody, there's no way for the broadcaster to know who has received it, and no way for the receiver to interact with the broadcast.

Example from the receiver's point of view:

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Satellite companies like Sky would love to have a two-way conversation with a digibox to update a change in subscription. But all they can do is regularly broadcast the personal code for your Sky card to every satellite user, which your digibox will receive and update your subscription and decode the signals broadcast to everybody.

Because they don't know if your digibox was switched on, they will keep broadcasting the same codes over and over again for weeks or months. But if you stop your subscription and then leave your digibox turned off for 3 months, the subscription change will probably not be broadcast any more and you'll still have your subscription.

Increasingly, your television set will be able to use both one-way communication signals and two way communication, so you need to understand like this:

• Anything coming through a TV aerial is one-waycommunication.
• Satellite can be two-way... it's technologicially possible. But it's expensive, and isn't used like that for TV transmission at the moment.
• Anything broadcast via an internet system or using your telephone line is two-way communication.