The third stage of garage door opener technology uses a frequency spectrum range between 300-400 MHz and rolling code (code hopping) technology to defeat code grabbers. In addition to transmitting a unique identifier for the remote control, a sequence number and an encrypted message are also sent. Although an intruder could still capture the code used to open a garage door, the sequence number immediately expires, so retransmitting the code later would not open the garage door. The encryption makes it extremely difficult for an intruder to forge a message with the next sequence number that would open the door. Some rolling code systems are more involved than others. Because there is a high probability that someone will push the remote's button while not in range and thus advance the sequence number, the receiver does not insist the sequence number increase by exactly one; it will accept a sequence number that falls within a narrow window or two successive sequence numbers in a much wider window. Rolling code technology is also used on car remote controls and with some internet protocols for secure sites.
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