WARNING: HOT WI-FI

“Just max out the power. Or, just add another AP.” If you’re the lucky engineer tasked with supporting your company’s Wi-Fi, there’s a good chance you’ve heard this more than once from different people – from your director who was an engineer three jobs and15 years ago to the finance guy who thinks he’s a guru because he worked at Geek Squad while he went to college.

And while this can still be valid advice in some older, low-speed environments like sparsely deployed warehouse WLANs using only 802.11b handheld barcode scanners, it’s a big problem in today’s denser, more Wi-Fi-dependent world.

Below I’ve used analogies to explain three real-life examples of what your users experience when the wireless runs too hot – and what you can do to avoid them.

The picture below covers all three: 1) Near-far, 2) co-channel, and 3) oversubscription.

A near-far scenario occurs when a client can hear the AP better than the AP can hear the client. This is because client devices typically transmit at around 30mW or lower while APs can transmit at as loud as 100mW (or more depending where you are in the world). When network admins set the APs to max power, it’s like handing a megaphone to a speaker so they can have a conversation with more people (laptops) in an extremely large space (coverage area). The people can hear the speaker perfectly, but the speaker can barely hear them. So they have to yell and repeat themselves many times before they can finish their thoughts (data retransmission due to missed acks). Additionally, while some people speak only a few words at a time, others have a lot to say (send large email attachments). Because they have to repeat themselves so often, it takes longer for them to shut up, which further delays everyone else’s turn to speak. The end result is congestion to the point that nothing is sent or received.

Further affecting the conversation between the speaker and the people is co-channel interference. This is when multiple transmitters are heard on the same channel. Think of the picture above and imagine two people in the crowd trying to talk to each other while the whole megaphone thing is going on. They have to raise their voice, and you’ll still hear “what – what – what?” because of the rest of the people and the guy with the megaphone talking around them. They have to repeat themselves because, as they’re speaking a sentence, the megaphone goes off or someone in the crowd yells back at the speaker interrupting the two people mid-statement. This problem becomes worse as more people try to have their own conversation. And no, you cannot just crank them up and “move to 5Ghz only” because there are limits there as well, especially if you’re using channel bonding. But I’ll get into that in another post.

Oversubscription is when an AP has more clients associated than it can handle to maintain good network communication. The limits are not only the AP hardware but RF environment as well – things like channel utilization and noise floor play into this. Again, referring to the image of the speaker and the crowd, imagine if there were 20 speakers with megaphones on that floor. The crowd will tend to migrate to the one that interests them the most. (In a laptop, the NIC makes the decision about which AP it will associate to.) If all of the people on that floor can hear all the megaphones, there’s a chance that there will be uneven distribution and one AP will be overloaded with users.

Now combine all three scenarios above and imagine this happening throughout the entire work day. That’s what happens when you decide to crank APs to max power without understanding the environment.

Now what can you do to avoid this?

• Understand the density of APs, the speed requirements, and attenuating factors before modifying AP power levels. (The goal is to have some overlap between each AP coverage area, but not too much. So clients associate and roam to ideal APs on their own.)

• Strive in an office environment for AP Min/Max Tx power levels that are close to the Tx levels of the clients they’re serving.

• Disable some of the lower data rates if clients in your company do not require them.

• Last but not least, contact us at American Digital to see how we can increase your workforce efficiency by assessing, optimizing, and/or upgrading your WLAN.