In the CCNA Wireless Exam 200-355 you will be asked to make dB calculations the simple rules below will ensure you are prepared to answer any question.
There are 3 laws, these are known as the law of 0, 3 and 10.
- Law of Zero – A value of 0dB means that the two absolute power values are equal
- Law of 3s – A value of 3dB means that the power value of interest is double the reference value; a value of -3dB is half
- Law of 10s – A value of 10dB means the power of interest is 10 times the reference
Example
AP 1 is transmitting at 5mW and AP 2 is transmitting at 200mW
How much gain in dB is AP2 over AP1
First you have to calculate how to get from 5 to 200 in multiples of 2 or 10
So 5mW X 10 = 50mW x 2 = 100mW x2 =200mW
So in dB terms it would be a x10 which is 10dB plus a x2 which is 3dB plus another x2 which is 3dB
So 10dB + 3dB + 3dB = 16dB
200mW is a 16dB gain on 2mW
Roger Perkin - CCIE #50038 is a Network Automation Engineer & CCIE Consultant based in the UK, currently working for Softcat Plc as a Senior Network & Security Consultant.
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You can also use the decibel formula 10 * log10 * (P2 / P1) to calculate the gain.
P1 = 200 mw
P2 = 5 mw
10 * log10 (200 / 5) = 16 db
Thanks Tim