In the past, wireless local area networks (WLANs) were a luxury and were used for a specific purpose, such as bar code scanners in manufacturing plants. Today, WLANs are everywhere and companies, schools, and even your home is dependent upon the existence of a functional wireless network.
Schools in particular are feeling the pressure of the expanding need to have a robust and properly designed wireless network. Some school districts have implemented a 1:1 device policy for their students. This means some schools are guaranteed to have upwards of 1000 devices in use at any one time. Because of this, the old “coverage model” of Wi-Fi, where access points were installed in hallways and serviced just a few devices in multiple classrooms, is no longer acceptable. Instead, the schools must now consider the capacity their existing wireless design can support. Today each classroom can have anywhere from 20-60 devices in it. If one access point in the hallway provides wireless coverage for 4 classrooms, the access point now has to service 80-240 wireless clients. Even at home, you no longer have just a laptop on your wireless router. Homes now can have 5 – 10 wireless devices, and most of the devices will be streaming video. The increased demand for fast, reliable Wi-Fi means that proper wireless network design is more important than ever. Designing a wireless network is very complex and requires a working knowledge of RF (radio frequency), since this is the medium in which wireless data is transmitted. There are many factors that go into this, but in this article we will look specifically at proper channel planning.
Before we define what a channel is, we need to understand the two bands currently in use for Wi-Fi. They are the 2.4ghz band and the 5ghz band. The 2.4ghz band is commonly used with baby monitors, cordless phones and countless other wireless devices. The 5ghz is used by some cordless phones and a specific range of the 5ghz channels are used by airports and Doppler radar. In the USA, we are allowed to use channels 1-13 in the 2.4ghz band and 36-64, 100-144 and 149-165 in the 5ghz band. A channel is essentially a number that is given to represent a specific range of RF signal. For instance, channel 1 in the 2.4ghz band actually represents frequencies 2.402 – 2.422, but it is listed as 2.412, which is the center frequency in which all channels are listed. Choosing the correct channels in your wireless deployment is paramount to having a functioning Wi-Fi network that will handle a heavy device load. To ensure this, you must avoid using adjacent channels next to each other and also do not reuse a specific channel too often, as this can cause issues known as ACI and CCI.
ACI, also known as adjacent channel interference, is a common issue in Wi-Fi. It is created when adjacent channels are used next to one another. For example, channel 1 is used on an access point and the neighboring access point is using channel 2, or to a lesser extent when channels 36 and 40 are used near each other. This should be avoided because adjacent channels actually overlap; the data sent over these overlapping channels can cause the frames to become corrupt and the AP will ask the client to retransmit the data all over again. Since Wi-Fi is a half-duplex medium, this is a huge deal! The more retransmissions on your wireless network, the worse the wireless experience will be for the end user. It is very easy to check your wireless environment for ACI using applications such as inSSIDer or Acrylic Wi-Fi. As shown below, the access point on channel 9 overlaps the access points on channel 6 and 11.
The same issue can happen on the 5ghz band, but with the sheer amount of channels available, a proper channel reuse pattern can negate it. Also, because of the shape of the spectral mask for 5ghz channels, the overlap only occurs at high power. Since high power is not used in high density deployments, this should also negate the issues seen due to ACI. There are mechanisms in place to handle CCI, but ACI can wreak havoc on your wireless network if not designed properly. In addition to schools, this can also cause issues at homes and businesses due to their location being in close proximity to other buildings with wireless networks. Most wireless routers provided by internet service providers, will be setup on overlapping 2.4ghz channels and sometimes will use 80mhz channel width on the 5ghz band. The use of the larger channel width can cause numerous issues as well. In high density deployments, it cuts the number of available channels dramatically. This means you are using each channel more often, which will result in CCI. If you use wide channels at home, you may be interfering with your neighbor’s wireless.
Co-channel interference, or co-channel contention, is caused when multiple access points are on the same channel within a certain proximity. Co-channel contention is a more accurate way to describe this issue, as there is not actual interference happening. Instead, the access points and the wireless clients compete for the use of a given channel. Each channel can only be accessed by one device at a time; think 2 way radio. On a 2 way radio, if I hold my button down to talk, the person at the other end cannot hold their button down to talk at the same time. To combat this, the use of words such as copy, roger, over were implemented to signal when a person was finished speaking and/or was expecting a reply. The 802.11 wireless standard defines mechanisms Wi-Fi devices must use to request, reserve and use the wireless medium. The more devices you have requesting to use the same channel, the busier the wireless medium is. If the medium becomes too busy, such as the example of one access point servicing 4 classrooms, the result is, best case – a slow user experience, or worst case – client devices get disassociated from the access point. Of course in areas where there is not a high density of wireless devices, such as a small office, this issue can still exist, but is not very noticeable. In areas of high client density, it becomes more important to minimize the effects of CCI, although it may be impossible to avoid it completely. As you can see below, there are numerous wireless networks on channel 1 and a couple on channel 6. The 4 access points shown on channel 1 all must compete to use that channel.
Obviously, there are many things that determine whether your users get a good experience from the wireless network, but the single most important thing you can do is to ensure you have a good channel reuse plan. A high percentage of all wireless issues come back to channel planning and it’s the first thing I check for when troubleshooting. Typically the biggest source of Wi-Fi interference comes from your very own wireless network, due to channel overlap and transmit power on the access points. With the right tools and a knowledgeable wireless engineer, you can fine-tune an existing wireless network or design it from scratch. The RF environment in your building is never static. It changes multiple times a day due to wireless printers, security cameras, toys, microwaves, baby monitors, wireless audio, etc. that can affect your wireless network at any given time. Some cars even have Wi-Fi built into the now! This makes Wi-Fi one of the most difficult things to troubleshoot, but with a well-designed channel plan, you have a solid foundation that will put you in the best possible place to have happy users.