Alarm systems, as a core component of a physical Intrusion Detection System (IDS), are designed to detect unauthorized presence or actions. They achieve this by using a variety of sensors. Laser beams (more formally, photoelectric beam sensors) are frequently used to establish an invisible perimeter; an alarm is triggered when the beam is interrupted. Similarly, touch or pressure-sensitive sensors can be deployed on specific assets like windows, doors, or safes to detect physical contact or attempts to breach them. The primary function of these sensors is to provide an input signal to the alarm control panel.

A. CCTV: Primarily uses image sensors in cameras for video surveillance, not laser beams or touch sensors as its main detection mechanism.

B. Server: A server is a computational device for processing, storing, and managing data; it does not inherently use physical intrusion sensors.

D. Backup device: This is a data storage medium used for disaster recovery purposes and has no function that requires physical detection sensors.

1. Geng, H. (2014). Data Center Handbook. John Wiley & Sons. In Chapter 35, "Data Center Security," Section 35.3.2, "Intrusion Detection Systems," the text describes various sensors used in data centers, including "Photoelectric beam detectors" which create a "fence-like barrier of light beams."

2. Whitman, M. E., & Mattord, H. J. (2016). Principles of Information Security. Cengage Learning. Chapter 9, "Physical Security," discusses intrusion detection and monitoring equipment, explicitly mentioning photoelectric sensors that use an infrared beam to detect motion or intrusion.

3. Peltier, T. R. (2013). Information Security Fundamentals. CRC Press. In Chapter 10, "Physical and Environmental Security," the section on "Intrusion Detection Systems" details the use of various sensors, including photoelectric systems (light beams) for perimeter protection.