1. Field of the Invention
The present invention relates to security systems and more particularly but not exclusively to a security system for protecting consumer apparatus from theft.
2. Related Art
In our co-pending European Patent application No. 94302211.1 (Publication No EP 0675626A) there is described a security system which utilises an interface between a domestic power supply and the public switched telephone network (PSTN) to provide identity signals derived from the calling line identity (CLI) of the customers PSTN telephone line. The identity signals are requested by a security module in apparatus such as televisions, video recorders, hi fi equipment and the like.
The system is arranged such that each time mains electrical power is connected to protected apparatus the security module requests (by way of the electrical power circuit) an identity code. The interface to the telephone network recognises the request from the mains supply circuit and causes a telephone call to be established via the PSTN to the security control centre which returns an identity code. The identity code is compared with a previously stored identity code in the protected apparatus, non-identity being used to prevent power being supplied to operational circuits.
It has been realised that while the security system previously described provides a high level of protection (by making stolen goods unattractive to thieves since they will not work away from their proper location), since connection of any of several items of protected apparatus will result in a corresponding number of PSTN calls, high levels of PSTN traffic may be generated resulting in network congestion and higher consumer costs.