If terms like 802.11ac, MIMO, channel bandwidth and frequency overlap are confusing, read on!
My family and I recently moved into a new house. Well, that's only partially true. We moved into a 63-year-old house, but it's new to us. One of the first things I did in the “new” house was to set up Wi-Fi. Thanks to the multiple floors and old plaster and lath walls, setting up a reliable wireless network proved to be quite challenging. I learned a lot along the way and figured it would be a perfect topic for an Open-Source Classroom series. So in this article, I discuss the technologies themselves. It's important to understand how Wi-Fi works so you know what sort of setup you want to install. Is 802.11ac worth the cost? Will it make a difference? Should you abandon the 2.4GHz frequency range altogether? Those questions will be easier to answer once you understand how the various standards work.
There currently are three common frequency ranges used in wireless networking. The most common is the 2.4GHz range. The newer 802.11ac standard (and oddly, the mostly failed 802.11a standard) uses the 5GHz range. And, a handful of devices currently are utilizing the 900MHz range of the spectrum for networking. The latter isn't used very commonly, but that likely will change with the ratification of 802.11ah, which will standardize the spectrum for some really interesting use cases. For now, however, Wi-Fi access points are either 2.4GHz, 5GHz or both.
The frequencies between 2.4 and 2.5GHz are broken up into 14 channels, all 5MHz apart. Channel 1 starts at 2.412GHz, and channel 14 is actually 12MHz above channel 13 (Figure 1). Every country handles the availability of those channels differently, but in the US, channels 1–11 are available for public use. If wireless networks used only 5MHz of bandwidth to transmit and receive data, that would mean we'd have 11 channels to use, and we could have up to 11 access points in one place without worrying about interference.
Unfortunately, that's not how Wi-Fi works. Every wireless device using the 2.4GHz frequency range actually uses 22MHz of bandwidth when sending and receiving data. That increased bandwidth allows for more data to get through, and 22MHz was decided to be the standard. That means if a user transmits on channel 1, it bleeds 11MHz above and 11MHz below 2.412GHz. If you do the math, that means channel 1 actually bleeds over channel 2 and channel 3! Since each channel bleeds over its two adjacent channels, the only usable channels in the 2.4GHz range are channel 1, channel 6 and channel 11. (Again, see Figure 1 for a visual representation.) In Japan, channels 12–14 are available for public use, and that makes channel 14 an option, but only barely. If you look at the graph, you can see channels 13 and 14 bump right up against each other.
In order to be complete, I also should mention that 802.11n allows for 40MHz of bandwidth per channel, even in the 2.4GHz frequency range. That increased bandwidth per channel means more throughput, but it comes at a terrible cost. Since the entire usable spectrum between channels 1 and 11 is 55MHz, there's really room only for a single device in the 2.4GHz spectrum to operate at 40MHz! It might seem like you could fit two channels in that space, but there's just not enough space in the legally usable spectrum to fit more than one channel that wide. So although it's technically possible to use 40MHz-wide channels with 2.4GHz using 802.11n, by default, it has to use 20MHz. Even when changing the default setting to force 40MHz, devices must scale back to 20MHz if they sense interference. There are a few notable products that ignore that requirement (the original Apple Airport Extreme for instance), but it's just not wise to waste all the available spectrum on a single channel! If you want to use 40MHz-wide channels with 802.11n, the answer is to use 5GHz.
The world of 5GHz networking is an interesting one. All the way back in 1999, 802.11a was released that functioned in the 5GHz range. The spectrum at that frequency is far less busy, and using a different signal modulation, it allowed for up to 54Mbps speeds. 802.11a used 20MHz of bandwidth, and since there is much more room in the 5GHz spectrum range, overlapping channels were far less of an issue. Unfortunately, 802.11a suffered from range and interference problems. The higher frequency doesn't penetrate walls as well, and as such, it wasn't as reliable in a building environment. Because it offered more potential bandwidth (54Mbps over 802.11b's 11Mbps), it was popular in corporate environments, but it was outmoded quickly by 802.11g. In 2003, 802.11g took the frequency modulation technology from 802.11a and applied it in the 2.4GHz spectrum. That allowed the best of both worlds, because it achieved up to 54Mbps while penetrating walls just as good as 802.11b did.
I mentioned that 802.11n can use 2.4GHz, but it also can use 5GHz for transmitting and receiving. The main problem is that not all devices that claim to be 802.11n are equipped with 5GHz radios. They're technically 802.11n-compatible, but only if that 802.11n is using the 2.4GHz spectrum. It's very frustrating. Most people end up implementing 802.11n on both 2.4GHz and 5GHz spectra. Thankfully, the 5GHz spectrum is both bigger and less busy, because not as many devices currently are utilizing it. Also, thanks to MIMO technology (more on that later), the penetration and interference isn't as much of an issue as it was with 802.11a.
Figure 2 shows the 5GHz band along with available channels. If you use the same 20MHz channel width used by the 2.4GHz channels, that means you have upwards of 25 non-overlapping channels from which to choose! The reality isn't quite that rosy, however, because more than half the available frequencies are called DFS channels, or Dynamic Frequency Selection channels. They work, but if your hardware finds them being used by military or weather radios, it is forced to switch to another channel, even if you assigned the DFS channel manually. It's nice that we're able to use those frequencies, but we're truly second class citizens, because we always get moved if the channels are officially in use.
For most people, that means using the nine available non-overlapping channels that are outside the DFS range. You can see that channels 36, 40, 44, 48, 149, 153, 157, 161 and 165 are available to use without concern of getting bumped by weather or military. Things do become a little more complicated if you want to use more bandwidth per channel, however. Remember that 802.11n can use 40MHz per channel, which you see in Figure 2 will limit your available channels to four non-DFS, or 12 if you include DFS. Still, four completely open 40MHz channels is far, far better than the single 40MHz channel available in the 2.4GHz spectrum!
The new kid on the block is, of course, 802.11ac. This new standard uses only the 5GHz spectrum, and it allows a wide variety of channel widths. The old standard of 20MHz is allowed, but also widths of 40, 80 and even 160MHz are allowed for some serious throughput! As channel width increases, the number of non-overlapping channels decreases, of course, so Figure 2 shows how many options you have in each scenario. I don't think there are any devices currently on the market that support 160MHz channels, but I could be wrong.
You might be wondering why 802.11ac is touted as so much faster than even 40MHz-wide 802.11n access points. The answer is similar to why 802.11n is so much faster than 802.11g. You probably noticed that 802.11n and 802.11g both use the 2.4GHz spectrum (Table 1), and yet although 802.11g is limited to 54Mbps, the “n” standard can achieve speeds up to 300Mbps. That's because with 802.11n, Multiple Input Multiple Output (MIMO) technology allows multiple antennas to transmit and receive at the same time.
Table 1. Details on the Various Standards
|Standard||Frequency||Channel Bandwidth||MIMO Streams||Max Speed|
|802.11n||2.4GHz/5GHz||20/40MHz||up to 4||up to 150Mbit (per stream)|
|802.11ac||5GHz||20/40/80/160MHz||up to 8||up to 866Mbit (per stream)|
You can shop at the store and see 802.11n access points and routers boasting speeds of 150Mbps and others boasting 300Mbps. That's a multiplier based on the number of antennas the access points use at once. (Note: some vendors claim 802.11n access points can achieve 600Mbps, but they're just using fancy marketing to sound faster than they really are. They're simply adding 300Mbps transmit and 300Mbps receive to get 600; you can't really get 600Mbps transfer rates.) The 802.11n spec allows for four data streams at once, which is what makes it so much faster than 802.11g, which uses only a single data stream.
With 802.11ac, that number has been increased to a possible eight simultaneous data streams, meaning the potential throughput is more than 2Gbps. No hardware currently supports that many data streams, nor the channel width required to achieve it, but the standard supports a very fast future for 802.11ac.
The original problem faced by 802.11a's use of the 5GHz spectrum has been solved by several means. First, far more channels are available, so an increased number of access points can be near each other without concern of overlapping each other. That means a higher density of access points and a higher density of supportable clients.
The other “solution” to the 5GHz propagation problem is that with MIMO technology, a weaker signal is still very fast, because multiple antennas can work together even on a degraded connection. With 802.11a, there was only a single stream of data, so if the signal got weak, the throughput slowed way down. With MIMO, you get the advantage of multiple weaker signals, which add up to a decent speed even in an area of weak reception.
Although it would be nice simply to decide to go 5GHz only and buy the newest, fastest 802.11ac access point you can, it's not really a practical idea. Many devices still contain only 2.4GHz 802.11n radios, so you have to provide something on the overly saturated 2.4GHz spectrum. Thankfully, most access points provide dual-band support, and you can have both 5GHz and 2.4GHz options available.
Sometimes it's better to have separate 2.4GHz and 5GHz access points, because you might need more 5GHz APs to cover the same area. Sometimes devices that support 5GHz networks will default to 2.4GHz if both are available with the same SSID. It can be very frustrating to figure out how best to support your devices!
In my next article, I'll tackle the actual planning, purchasing and installation of a wireless network in your home or business. Thankfully, having a firm understanding of the technology makes the next part a lot more fun. In the meantime, I recommend checking out your wireless devices to see what type of radios they have. Do you have anything that will take advantage of a 5GHz network? Anything that is 802.11ac-compatible? Do you want to future-proof your wireless network? Answer those questions to prepare for Part II of this series.