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**

Tim

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

Roger Perkin

Thanks Tim