with the older DIP switch remote codes, but adding an old technology remote to the security list made the garage door opener vulnerable to a brute force attack to find the DIP switch code. The larger code space approach was an improvement over the fixed DIP switch codes but was still vulnerable to the replay attack. 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. The fourth stage of garage door opener systems is similar to third stage, but it is limited to the 315 MHz frequency. The 315 MHz frequency range avoids interference from the land mobile radio system (LMRS) used by the U.S. military. The following standards are used by units manufactured by Chamberlain (including LiftMaster and Craftsman): Does not apply to keyless entry keypads or universal remotes. Recent Chamberlain garage door openers that have Security+ 2.0 features also use a special serial protocol on wired connections rather than a simple switch closure. The following standards are used by units manufactured by Overhead Door Corporation and its subsidiary The Genie Company: Note: There are no standard color codes for the learn button or LED on units manufactured by Overhead Door or Genie. All accessories made for later versions of Genie Intellicode and Overhead Door CodeDodger are backward compatible with previous generations of Intellicode and CodeDodger. Cloning garage door opener remotes Many garage door