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February 06, 2011

Dealing with Density

Density Our traffic problems aren’t going to getting any better anytime soon.  

More licensed drivers are taking to the roads than ever before. The logical solution would be to build more and wider roads. But what if we just used the road more efficiently?  Exactly. The same is true with Wi-Fi.

Wi-Fi has speed limits that are set by current technology and the laws of physics. Basically, every Wi-Fi vendor can go the same, maximum speed just like most cars can go the speed limit. So if everyone is limited to the same speed, how can one vendor have an advantage in capacity?

Wi-Fi vendors are constantly asked how many concurrent clients a given AP can handle.  The answer is always the same - about 100 to 150. That's basically not so true. Sure, with enough memory an AP can handle 100+ client associations but that's without any data being sent. So what good is that?

When it comes to high capacity Wi-Fi it really isn't about how many concurrent clients your AP can handle, it's about how effectively a system can manage and optimize the use of the radio spectrum.  That's the real limiting factor when it comes to client density because there's only so much of it.

Spectrum, also known as RF bandwidth, is the amount of frequency allocated for our use. North America has about 83.5 MHz in the 2.4 GHz band (60 MHz of which is actually used) and depending on regulations, around 200 MHz in the 5 GHz band. These are hard limits that cannot be broken by one vendor or another. So we all have the same amount of highway to work with and can’t build any more. 

Dealing-with-density Without explicit control over Wi-Fi signals, vendors then focus on way that help with client density. Consequently there have been good advancements in traffic management that help foster high capacity Wi-Fi networks.  

Air time fairness, client load balancing and band steering are techniques that are all quite useful in easing the density problem. Add to these techniques adaptive, directional antennas and you have something very special for addressing highly-dense user environments.

You see, the Wi-Fi protocol actually does something that you are used to but maybe not conscious of. The better the signal, the faster you go. And it isn’t just your signal that matters, it is the noise.  

Signal strength means nothing without knowing the surrounding noise. The difference between signal and noise is called SNR (signal to noise ratio). Reduced signal and/or more noise equal slow speeds and sometimes complete lack of connectivity.

Wi-Fi is the same except on a smaller scale. You can move 10 feet farther away from the AP or behind a wall and your connection speed can be reduced significantly. As Wi-Fi protocol encounters errors from lower SNR it automatically reduces its speed to ensure that the communication is heard by the recipient. 

This reduction in speed means that it takes longer to say the same thing. And the longer it takes to send a data packet, the longer it is before another device on that same channel can transmit. This is where the beauty lies.  As you increase signal and reduce noise (both accomplished by adaptive antenna arrays) speeds increase, channel capacity goes up - with fewer APs needed to meet the same high density goals.

Adaptive antenna arrays can deliver up to 7dBi more signal gain than standard omni-directional antennas.  So what does 7dB mean in real life?  Try this, find out the Wi-Fi chipset you have in your favorite Wi-Fi device.  Then, look up its RSSI chart. This chart will tell you the minimum signal level needed to achieve a particular data rate. Especially when taking into account the benefits of 802.11n, you will see over a 50% increase in channel capacity just by increasing your signal strength by 7 dB.  

But supporting high capacity in the real world is more than a not-so-well-crafted BLOG entry. Listen to people who've had this problem already: