FAQ: Network Intrusion Detection Systems version 0.8.1 – (Updated Jan 17, 2000) This FAQ has lots of excellent info on IDS systems, with a good focus on how they work.
You can download it from the following link: https://packetstormsecurity.com/files/download/13843/network-intrusion-detection.htm
Source: https://packetstormsecurity.com/files/13843/network-intrusion-detection.htm.html