Detection is necessary because a person can climb over a fence. Sandia National Laboratories’ testing has determined that a highly skilled trespasser could get to the other side of a well-designed fence in about 4 seconds. In a high security application, it is accepted that a trespasser will get through a fence within 4 to 40 seconds, so it becomes very important to convey information that someone has gotten over the fence, or is presently attempting to do so. This is where intrusion detection enters the picture.