Sensor-to-Processor Links

Usually the connection between a sensor (which may have its own miniprocessor) and the alarm system’s processor is wire cable or a radiofrequency signal. In some systems the connection may be optical fiber.

This link must be protected from intentional or accidental physical damage. Cable or optical fiber should be run in metal electrical tubing (thin wall conduit) if it is in a return-air plenum. Outdoors, cable or fiber can be run in PVC or ABS plastic pipe. PVC and ABS plastic pipe cannot be used in a return air plenum because they produce toxic fumes when they burn.

The most frequent threat to inside wired links probably is not an attacker; it is a maintenance person who needs above-ceiling access and who nicks the wire with a tool and doesn’t know about the damage or doesn’t tell you about it.

The greatest threat to outside links, particularly those buried in the ground, is the groundskeeper, landscape contractor, or anyone else with a shovel who digs on the property without first notifying security. The location of buried outside links needs to be documented so that security officials can tell workers where not to dig.

Radiofrequency links are becoming more popular because they offer more flexibility than wire or optical fiber links. Each sensor has its own unique code that identifies it to the master alarm processor. Tamper protection and signal-line supervision can be retained.

Radiofrequency links have some disadvantages.

• Because these links transmit radiofrequency energy with information encoded, the signal can be read, analyzed, and possibly emulated. The danger is not so much that an attacker can fool the master processor into thinking a door or window is still closed. It is that an attacker can generate tamper or nuisance alarms from a remote location outside the secure area. If the attacker has obtained the code and frequency of the sensor he wishes to bypass, the attacker can generate repeated nuisance or tamper alarms that may cause security personnel to become convinced the sensor has malfunctioned and quit responding to it.
  
  
• Because most radiofrequency sensors are stand-alone units, they have their own internal battery power. Batteries must be changed at intervals specified by the manufacturer.
  
  
• The link transmitter may be more difficult to conceal than the sensor itself.