Wi-Fi communication

Under what conditions and over what range.

There is nothing intrinsic to these applications that would make it unreliable.

Always consider safety.

What would happen under various fault conditions, ie lost power to the control side of the rf link, emi, etc.

Absolutly, but only if properly integrated into an appropriate application.

Wi-Fi communication for plc or ddc is reliable its ok fine but in case of network failure due to any cause,then can we give redundancy for that ? How?

For redundant communication you would use more than one communication link. The system should be configured to transfer in the event of a failure.

As an example. You could use redundant pairs of wireless bridge /AP clients as part of a redundant ethernet turbo ring then your layer two devices would handle the redundant communications for you.

http://www.moxa.com/product/AWK-1100.htm