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dB Laws – CCNA Wireless

Home » CCNA » CCNA-Wireless

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

Category: CCNA-Wireless
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Reader Interactions

Comments

  1. Tim

    April 22, 2017 at 3:53 pm

    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

      May 11, 2017 at 11:04 am

      Thanks Tim

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