If a radar signal is detected, then the AP must cease transmissions on the channel within the “Channel Move Time”, which is 10 seconds in the EU/UK.Īt the end of this period, the AP will have ceased transmissions and moved to a new channel. The AP must continuously monitor its channel for the presence of radar signals. This is known as “In-Service Monitoring”. Once an AP is operating on a DFS channel, it has to monitor for the presence of radar signals appearing on that channel. This extended checking period is due to weather radar restrictions on those channels. This means that the 5GHz radio is not available until 10 minutes after the access point has booted up. In some regions, where channels 120 – 128 are allowed for use by Wi-Fi networks, there may be an increased channel availability check of 10 minutes. This is due to the AP performing its channel availability check, if the AP is trying to use a DFS-impacted 5GHz channel. When powering up an AP that uses a DFS channel, you will see that the 2.4GHz radio becomes available as soon as the AP has completed its boot sequence, but the 5Ghz radio may not available for another minute. If no radar is detected, then the channel is designated as being an “Available Channel”. The AP will listen for 60 seconds for the presence of radar signals. This mechanism is known as Dynamic Frequency Selection (DFS) and is designed to mitigate interference to 5GHz radar by WLANs.īefore an AP will use a channel that may be impacted by radar, it will perform a “Channel Availability Check” to check for radar signals on that channel. However, due to the coexistence of both radar and Wi-Fi networks in the same area of spectrum, the Wi-Fi standard (IEEE 802.11) was designed to incorporate a spectrum sharing mechanism on 5GHz to ensure that Wi-Fi networks do not operate on frequencies (hence causing interference) that are used by nearby radar stations. This doesn’t apply to all radar stations that have been deployed there are many radar installations do not use 5GHz. This means that they may use some of the same frequencies that are used for Wi-Fi networks. Some types of radar installation operate in the 5GHz band that is used by Wi-Fi network. One particular service that shares spectrum with wireless LANs is radar. channel widths used, spectral masks etc.).Įven though they may be licence-exempt, Wi-Fi networks are still subject to restrictions to minimize their impact on other wireless services and equipment in the same areas of spectrum used by WLANs. This may include restrictions on parameters such as RF transmit power levels and limiting the spectral characteristics of transmitted signals (e.g. This is in contrast to many other areas of the radio spectrum that generally require some form of (paid-for) licence to operate radio equipment.Īll wireless services are generally subject to a range of enforceable technical restrictions to ensure they operate in a manner that will minimize interference to other wireless services. Wi-Fi networks operate in areas of RF spectrum that require no licence to operate. However, beneath the headline of “2.4Ghz = bad, 5Ghz = good”, there lurks a shadowy figure that can be troublesome if you’re not aware of its potential impact: DFS. 5GHz has far more channels available, a corresponding lower number of devices per channel, and generally suffers much lower non-Wi-Fi interference. Many wireless LAN professionals will generally advise that you put your “important stuff” on the 5GHz band whenever possible. The 2.4GHz band has a reputation of being something of a “sewer” of a band, due to its limited number of usable channels, the number of Wi-Fi devices already using the band, and the high levels of non-Wi-Fi interference that it experiences. Wi-Fi networking provides us with 2 bands for the operation of wireless LAN networks: the 2.4Ghz band and the 5GHz band.
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