Don’t use just because its available

By | February 15, 2016

These days I see a lot of push to use to wider channels as that’s the reason lot of enterprise are buying new AP’s. As marketing told them you will get great speed.

But the big question is,are really giving you those advertised speeds specially when you deploy them in open offices with more than 3 AP’s.

Bottom line is if you can’t have a AP on a same frequencies without causing co-Channel interference you have capacity of one. As 802.11 uses CSMA/CA first part (CS) stand for carrier sense so when a 802.11 transmitter is able to see another transmission in the air on same frequency at 4 db in SNR and transmitter can decode the preamble, it defers its transmission.

Until i tested i did not believe but the signals do travel far even on low power. Below is a Example of a open building which had AP’s deployed on floor 8 and 10 but I could see AP’s from floor 10 on floor 8.

post 2-1         post 2-2

Amount of overlap above was unbelievable.  So best is to start with 40 Mhz and move to 20 Mhz if you can’t reuse your frequency.

In case you want to learn about more details feel to look on Andrews blog safely using 80 Mhz Channels.

Few Reasons you don’t want to use 80 MHz channels

1. Not enough channels available, so we recreate the same issue we have in 2.4 GHz band and causes lot of Co-Channel interference.

2. Your SNR drops by 6 db. As noise floor rises by 3 db and your RSSI drops by 3 db, to achieve higher data rates its very important to have a high MCS rates and when your SNR drops by 6 db to increase your SNR you will need AP’s to get closer and now this makes frequency reuse even harder so you are stuck in a loop.

3. In Wider channels Spectral mask is more so you will experience more adjacent channel over lap

Hard part is to make the nontechnical people understand this concept specially when they are your customers and they have been sold to the new 802.11ac hype,  Now i have a analogy that i tell my customers so far this has been very successful so i would like to share with you all.

Lets say you buy a new Sports car imagine the one you love 🙂 and you know it can do great speeds of 200 miles and hour as advertised and bench tested also by the manufacturer on a racing track but now if you bring the same car on the roads can you do 200 miles an hour, probably not as you will get into an accident or are sure to get a ticket. Similarly in real world using wider channels you will increases you chances to have collisions and over ahead will increase causing degradation of performance on your WLAN.

Does this mean you can never take your car on higher speeds the answers yes of course you can but if the roads and the laws allow it. By this i mean if you are in a less crowded area and no one is using the spectrum and you have only 3-4 Access points to deploy by all means use the wide channels.

Rest if you are looking for more detailed information on using wider channels lot of CWNE’s had written great blogs on this topic few of which i read were from Eddie Forero and Jason Hintersteiner

Gurpreet

 

 

 

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