Wireless Channels

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The concept of channels is one of the most important things to understand about wireless networking. Channels are individual, smaller sections of the overall frequency band used by a wireless network. Channels are super important because they help address a very old networking concern, collision domains. You might remember that a collision domain is any one network segment where one computer can interrupt another. Communications that overlap each other can’t be properly understood by the receiving end. So when two or more transmissions occur at the same time, also called a collision, all devices in question have to stop their transmissions. They wait a random amount of time and try again when things quiet down. This really slows things down. The problem caused by collision domains has been mostly reduced on wired networks through devices called switches. Switches remember which computers live on which physical interfaces. So traffic is only sent to the node It’s intended for. Wireless networking doesn’t have cables, so there aren’t physical interfaces for a wireless device to connect to. That means, we can have something that works like a wireless switch. Wireless devices are doomed to talk over each other. Channels help fix this problem to a certain extent. When we were talking about the concept the frequency bands, we mentioned that FM radio in North America operates between 80 megahertz and 108 megahertz. But when we discuss the frequency bands we use by Wi-Fi, we just mentioned 2.4 Gigahertz and five Gigahertz. This is because that’s really just shorthand for where these frequency bands actually begin. For wireless networks that operate on a 2.4 Gigahertz band, what we really mean is that they operate on roughly the band from 2.4 Gigahertz to 2.5 Gigahertz. Between these two frequencies are a number of channels, each with a width of a certain megahertz. Since different countries and regions have different regulatory committees for what radio frequencies might be used for what, exactly how many channels are available for use depends on where in the world you are. For example, dealing with an 802.11b network, channel one operates at 2412 megahertz, but since the channel width is 22 megahertz, the signal really lives on the frequencies between 2401 megahertz and 2423 megahertz. This is because radio waves are imprecise things. So, you need some buffer around what exact frequencies a transmission might actually arrive on. Some channels overlap but some are far enough apart so they won’t interfere with each other at all. Let’s look again at 802.11b network running on a 2.4 Gigahertz band, because it’s really the simplest and the concepts translate to all other 802.11 specifications. With a channel width of 22 megahertz, channel one with its midpoint at 2412 megahertz, is always completely isolated from channel six with its midpoint at 2437 megahertz. For an 802.11b network, this means that channels one and six and 11 are the only ones that never overlap at all. That’s not all that matters, though. Today, most wireless networking equipment is built to auto sense what channels are most congested. Some access points will only perform this analysis when they start up, others will dynamically change their channel as needed. Between those two scenarios and manually specified channels, you can still run into situations where you experience heavy channel congestion. This is especially true in dense urban areas with lots of wireless networks in close proximity. So, why is this important in the world of I.T. support? Well, understanding how these channels overlap for all of the 802.11 specifications is a way you can help troubleshoot bad wireless connectivity problems or slowdowns in the network. You want to avoid collision domains wherever you can. I should call out that it’s not important to memorize all of the individual numbers we’ve talked about. The point is to understand how collision domains are a necessary problem with all wirel

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