If you are like me, you probably saw the issues Steve Jobs had this week with his wireless demos during the Keynotes presentations. For those who missed this great moment, Apple CEO was trying to demonstrate some great features of the new iPhone… but could not access some web pages using the phone WiFi antenna. The blame was put on the 570+ WiFi networks set up by the audience. Many of them were Verizon’s MiFi, a device that creates a portable hot spot that can be shared by up to 5 users, in other words a sort of mini access point.
How can too many access points prevent connectivity, and how to fix this? Here are a few tips (for next time)!
One limitation of Wireless networks is that it can use only 2 bands: the 2.4 GHz band, and the 5 GHz band. The first one offers 3 non-overlapping channels, and the second 23. This means that if you put 4 access points or more on 2.4GHz, at least 2 of them will be on the same channel. Is it an issue? Not always. WiFi devices tell the others, upon sending, how long they are going to send for. The others just wait, then send in turn. So where is the problem? It lies in two words: exposed node.
Each access point has zones, in which your WiFi device can communicate at certain speed. For example, if you are close to the access point, you can send traffic at 54 Mb/s. If you go farther away, you get to the 48 Mb/s zone, then the 36, 24, 18, 12, 9 and 6 Mb/s zones. Beyond a certain distance, you are out of range and cannot communicate with this access point anymore.
Now imagine the situation below.
In this scenario, A and D are access points, on the same channel, but far away from each other. B and C are WiFi laptops. Imagine that C is Steve Job’s iPhone. C tries to receive a web page from the access point D. “Tries”, because at the same time, B, one of the bloggers in the room, sends something to the MiFi access point A. B is too far from Steve Job’s access point (D) to hear its “hey, I am sending for this amount of time” signal. So B ignores it and sends its own packets. B is too far from D.. but close to Steve Job’s iPhone (C)! So C receives both B signal and D signal, at the same time, on the same channel. And this is just like when you hear two radio stations on the same frequency: they jam each other and you cannot understand a word of what is said.
How do you fix that? Of course, Steve Job’s solution (shut down all wireless devices in the room) does work. Simple. No need to explain.
The other solution is to provide a better wireless coverage. I am far from saying that the wireless coverage was poor, but the way this issue is usually fixed in public, densely crowded environments, is to provide a wireless blanket, that is coverage for the users/bloggers, so they do not need to use their own system. The access points are quite close to one another, with power set to a low value, so each covered area is small. This reduce the exposed node issue considerably.
The Cisco solution is very efficient for this type of environment, because the Wireless LAN controllers dynamically adapt the power of the access points to reduce it when needed. Cisco also has a feature called CCX, available on most WiFi devices (for free, yes, for free!) that allows the access point to dynamically reduce the WiFi device as well, thus creating a better, quieter, wireless environment, for everyone’s benefit. The foundation is of course a good site survey, to put the right access points at the right place, to get the right coverage….
Oh, did I tell you that Fast Lane offers Wireless Site Survey courses? This awesome course is part of the CCNP Wireless curriculum… in case you know someone in Steve Job’s team… 
Tags: CUWSS, exposed node, near/far issue, site survey, WiFI, wireless
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