2 cissp ® Official Study Guide Eighth Edition

416
Chapter 10 
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Physical Security Requirements
Noise
Noise can cause more than just problems with how equipment functions; it can also inter-
fere with the quality of communications, transmissions, and playback. Noise generated by 
electric current can affect any means of data transmission that relies on electromagnetic 
transport mechanisms, such as telephone, cellular, television, audio, radio, and network 
mechanisms.
There are two types of 
electromagnetic interference (EMI)
: common mode and traverse 
mode. 
Common mode noise
is generated by a difference in power between the hot and 
ground wires of a power source or operating electrical equipment. 
Traverse mode noise
is 
generated by a difference in power between the hot and neutral wires of a power source or 
operating electrical equipment.
Radio-frequency interference (RFI)
is another source of noise and interference that can 
affect many of the same systems as EMI. A wide range of common electrical appliances 
generate RFI, including fluorescent lights, electrical cables, electric space heaters, comput-
ers, elevators, motors, and electric magnets, so it’s important to locate all such equipment 
when deploying IT systems and infrastructure elements.
Protecting your power supply and your equipment from noise is an important part of 
maintaining a productive and functioning environment for your IT infrastructure. Steps 
to take for this kind of protection include providing for sufficient power conditioning, 
establishing proper grounding, shielding all cables, and limiting exposure to EMI and RFI 
sources.
Temperature, Humidity, and Static
In addition to power considerations, maintaining the environment involves control over 
the HVAC mechanisms. Rooms intended primarily to house computers should generally 
be kept between 60 and 75 degrees Fahrenheit (15 and 23 degrees Celsius). However, there 
are some extreme environments that run their equipment as low as 50 degrees Fahrenheit 
and others that run above 90 degrees Fahrenheit. Humidity in a computer room should 
be maintained between 40 and 60 percent. Too much humidity can cause corrosion. Too 
little humidity causes static electricity. Even on antistatic carpeting, if the environment has 
low humidity it is still possible to generate 20,000-volt static discharges. As you can see in 
Table 10.1, even minimal levels of static discharge can destroy electronic equipment.
Ta b l e 10 .1
Static voltage and damage