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Definition of WiFi Interference and Contention

Guest Blogger

As long as I can remember interference of any kind mentioned on a WiFi network was slapped with the label “interference”. Over the years, as I became educated in 802.11 networking and understanding channel contention through the operation of CSMA-CA. I realized something, interference is necessarily the right definition.

 

In 802.11 networking, interference can really be defined in two very distinctive categories. 

 

RF Interference and Channel Contention 

 

By definition both are indeed interference. Definition of interference: the action of interfering or the process of being interfered with

 

RF Interference - Can be defined as non 802.11 interference. In other words something that is on frequency, causing duty and resulting in 802.11 radios to go busy or interference close to a radio causing wave forms to be misinterpreted resulting in bit errors. 

 

The obvious suspects; microwave ovens, cordless phones, cameras, BT, and the list goes on. 

 

Let me give you a real world example. Say for a moment you’re listing to a speaker at a conference and directly behind you a loud conversation or argument is going on. Your ears, receiving radios, are having a hard time understand the speaker distinguishing what he is saying. 

 

Channel Contention - Can be defined as 802.11 mechanisms to gain access to the medium to trigger frames. In other words, CSMA-CA. This contention happens on layer 1 and layer 2. Layer 1 with the use of preambles and PHY headers and layer 2 through the use of NAV.

 

CSMA-CA is the rule book or referee used by WiFi devices to gain access to the medium.

 

Let me give you a real world example. You’re in a conference the speaker is done with his session and opens the floor up to Q&A. You have a question you raise your hand along with 10 other folks. You, like everyone else, needs to wait their turn to ask their question. 

 

 

CCC / CCI - Lets also go down the CCC (Co-Channel Contention) and CCI (Co-Channel Interference) road since we’re on topic. In the industry you hear folks, publications and vendors reference CCI (Co-Channel Interference). In other words, access points on the same channel causing interference with each other. We just covered its not really interference. A better definition is Co-Channel Contention. It’s contention. Because both radios, using CSMA-CA, are backing off to each other causing contention. 

 

 

I would like to hear your thoughts on the subject ! Fire back and let me know what you think! 

 

If you found this blog post educational or helpful support rating system!

 

Comments
MVP

recently also looked into this and found here a pretty clear explanation:

http://www.metageek.net/support/why-channels-1-6-and-11/

 

it seems they mention a third type of interference, the overlapping channel interference. or do you feel that is part of the channel contention?

Guest Blogger

Hello,

 

Ive read this as well. I love the guys at metageek. A few things take issue on with this post:

 

"Let’s get technical for a moment. The 2.4 GHz Wi-Fi (802.11 b/g/n) spectrum is 100 MHz wide and made up of 11 channels centered 5 MHz apart. Each 2.4GHz channel is 20 – 22 MHz wide making the spectrum a little crowded."

 

While they state lets get technical 2.4 GHz is 100Mhz, this isnt true. Actually 2.4 GHz is 2.400 - 4.2835 (83.5 Mhz wide), not 100. 

 

Also its 20-22 Mhz wide channels. Its really 22 Mhz standard calls for 25 Mhz for separation. Now 802.11b was 30 Mhz, as I recall.

 

"If each channel is 20 MHz wide, this means there will be a minimum of 10 MHz of overlap with neighboring channels (overlapping channel interference). For example, if your network is on channel 9, it will overlap with channels 7, 8, 10, 11."

 

Only 5 GHz channel are 20 Mhz wide, so they are wrong again with the mention 2.4 GHz is 20 MHz wide. They just stated it was 20-22 Mhz. 

 

This comes into the 3rd interference. Adjacent channel interference. Yes its true, it does exist, only if you place APs 2 - 3 feet aprt from each other.

 

Thanks for responding ..

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