Understanding how channels operate is key to avoiding interference and maximizing the performance/scalability of the WLAN. In radio communication, a wireless station (like a UniFi Access Point) receives a channel assignment and a specific bandwidth over which it transmits and receives signals to and from nearby stations. This channel assignment pertains to the center frequency of the first 20 MHz channel used by the station.
Channel bandwidth refers specifically to the frequency range over which data signals are transmitted. However, the actual transmission signal generated by 802.11 radios looks similar to a volcano, where ‘peak’ power levels are spread across the channel bandwidth, and power levels drop off at the edges of the channel bandwidth near the ‘tail ends.’
The following figure demonstrates two APs in competing WLANs. The 20MHz WLAN (blue channel) is centered at frequency “f”, while the 40 MHz WLAN (yellow channel) actually bonds two 20 MHz channels together. Of the two 20MHz channels, the primary channel (centered at frequency “f”) contains the WLAN beacon announcements, while the secondary channel is optional for compatible, connecting Stations.
The ‘tail ends’ of adjacent channels can incur noise for nearby wireless networks. For this reason, it is very important to apply a channel planning pattern across the WLAN, to avoid co-channel interference (which reduces speeds and limits the scalability of the network).
The yellow WLAN depicted in the chalkboard graphic represents a ‘bonded’ 40 MHz channel (20+20) according to the 2009-802.11n standard. With bonded channels, 802.11n capable stations can communicate at higher data rates, called “High Throughput” (HT) rates. By comparison, the 802.11ac standard supports ‘bonded’ 80MHz channels (20+20+20+20) for “Very High Throughput” (VHT) data rates. A wireless network whose clients all support the same data rates is called ‘Greenfield’. For example, a greenfield VHT network would only be comprised of 802.11ac stations.
Channel availability depends on the world region where the radio will be deployed and is specified in the UniFi Controller under Country Site Settings. In 2.4 GHz deployment scenarios with multiple APs, use only 20 MHz bandwidths on channels 1, 6 and 11, since use of other channels (ex. 3, 5, 9) or larger bandwidths (ex. 40 MHz) overlaps with neighbor channels. In other words, channels 1,6, and 11 allow for proper channel re-use patterns. Contrast this with a channel plan that uses overlapping channels, as illustrated by the image below.
Given its worldwide support of an abundant number of channels, the 5 GHz band allows for more complex 20 MHz channel re-use patterns (as illustrated by the seven neighboring wireless cells. The wider range of available frequencies in the 5 GHz band also permits wider channel assignment (as illustrated in the previous graphic), including 40 and 80 MHz, for greater WLAN throughput. Because wider channel bandwidths require more channel space, be conscious limits the ability of the WLAN administrator to create effective channel re-use patterns across the wireless coverage area.
In order to minimize interference, assign non-adjacent channels to neighboring AP cells. When followed, the WLAN can scale more effectively. When disobeyed, WLANs cannot scale and result in poor performance (higher latency, lower throughput).
Before assigning WLAN channels, conduct site surveys to analyze noise levels across the spectrum. 2nd Generation 802.11ac UAPs feature RF Scan tools to help WLAN administrators decide the best channel, based on all sources of interference, including competing, in-band WLANs, EMI (electromagnetic interference), etc.
