a surveillance fence comprising at least two posts each equipped with electro - magnetic radiation transmitters and receivers which establish two parallel communications links . each posts communicate with a surveillance computer at a headquarters . only one post communicates directly via cables with headquarters . data is transmitted over the communication links in opposite directions by modulating the electromagnetic energy beams . a message starts at headquarters and is passed along post to post and back again to headquarters . each post can add information to the message as it is passed along . the interruption of the electromagnetic energy beams signals an intrusion whose location and time of occurence is recorded by a processor which initiates an alarm . if a post can not relay its intrusion data , the data is stored until the communications link is reestablished . the intrusion data stored at a post will not be erased until an acknowledge message is received from headquarters .