Electromagnetic Interference Shielding

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Can you answer this sample Security+ question?

Q. Which of the following is an environmental control?

A. EMI shielding

B. Fencing

C. Video surveillance

D. Motion detection

More, do you know why the correct answer is correct and the incorrect answers are incorrect? The answer and explanation is at the end of this post.

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EMI and RFI Differences

Although you may see EMI and RFI in the same category as EMI/RFI, they are different.

  • EMI comes from different types of motors, power lines, and even fluorescent lights.
  • RFI comes from radio frequency (RF) sources such as AM or FM transmitters.

However, shielding used to block interference from both EMI and RFI sources is often referred to as simply EMI shielding. Shielding is an environmental control that helps prevent EMI and RFI from interfering with normal signal transmissions.

Attackers often use different types of eavesdropping methods to capture network traffic. If the data is emanating outside of the wire or outside of an enclosure, attackers may be able to capture and read the data. EMI shielding fulfills the dual purpose of keeping interference out and preventing attackers from capturing network traffic.

Using Shielding Cables

Twisted-pair cable, such as CAT5e and CAT6 cable, comes in both shielded twisted-pair (STP) and unshielded twisted-pair (UTP) versions. The shielding helps prevent an attacker from capturing network traffic and helps block interference from corrupting the data.

When data travels along a copper wire (such as twisted-pair), it creates an induction field around the wire. If you have the right tools, you can simply place the tool around the wire and capture the signal. The shielding in STP cable blocks this. Fiber-optic cable is not susceptible to this type of attack. Signals travel along a fiber-optic cable as light pulses, and they do not create an induction field.

Remember this

EMI shielding is an environmental control that prevents outside interference sources from corrupting data and prevents data from emanating outside the cable.

Cables and Electromagnetic Interference Sources

Physical security includes planning where you route cables and how you route them. Skilled network administrators can cut a twisted-pair cable, attach an RJ-45 connector to each end, and connect them back together with an adapter in less than 5 minutes. I recently taught a CompTIA Security+ class to some Verizon Fiber Optic Services FiOS technicians, and they said they can do the same thing with a fiber-optic cable within 10 minutes.

If an attacker did this, he could connect the cut cable with a hub, and then capture all the traffic going through the hub with a protocol analyzer. This represents a significant risk.

One method of reducing this risk is to run cables through cable troughs. A cable trough is a long metal container, typically about 4 inches wide by 4 inches high. If you run data cables through the cable trough, they aren’t as accessible to potential attackers. In contrast, many organizations simply run the cable through a false ceiling or a raised floor.

In addition to considering physical security, it’s important to keep the cables away from EMI sources. As an example, if technicians run cables over or through fluorescent lighting fixtures, the EMI from the lights can disrupt the signals on the cables. The result is intermittent connectivity for users.

For example, the following graphic shows EMI interfering with a signal run through a fluorescent light, but no EMI when the cable is run through a cable trough.

emi

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Faraday Cage

A Faraday cage is a room that prevents signals from emanating beyond the room. It includes electrical features that cause RF signals that reach the boundary of the room to be reflected back, preventing signal emanation outside the Faraday cage.

Electromagnetic interference

In addition to preventing signals from emanating outside the room, a Faraday cage also provides shielding to prevent outside interference such as EMI and RFI from entering the room.

At a very basic level, some elevators act as a Faraday cage (though I seriously doubt the designers were striving to do so). You may have stepped into an elevator and found that your cell phone stopped receiving and transmitting signals. The metal shielding around the elevator prevents signals from emanating out or signals such as the cell phone tower signal from entering the elevator.

On a smaller scale, electrical devices such as computers include shielding to prevent signals from emanating out and block interference from getting in.

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Q. Which of the following is an environmental control?

A. EMI shielding

B. Fencing

C. Video surveillance

D. Motion detection

Answer is A. Electromagnetic interference (EMI) shielding is an environmental control that provides protection against interference from electromagnetic sources such as fluorescent lights.

Admittedly, the title of this post gave you a good clue to the answer.  However, it’s also good to know the difference between physical security and environmental controls. Other environmental controls including heating, ventilation, and air conditioning (HVAC) systems, fire suppression systems, and temperature and humidity controls.

Fencing, video surveillance, and motion detection are all examples of physical security controls.

See Chapter 9 of the CompTIA Security+: Get Certified Get Ahead: SY0-401 Study Guide for more information on environmental controls.

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