UniFi - WLAN Rate Configuration

What are WLAN Data Rates?

In 802.11 wireless, PHY Data Rates (e.g., 300Mbps) constitute all the 1 and 0 bits in wireless communication including:

  1. Data payload (i.e., actual user data like HTTP), and,
  2. MAC/PHY overhead (i.e., 802.11 bits, such as Preambles, Error Checking, etc.)

Compared to PHY Data RatesThroughput represents the actual, achievable TCP data pushed between two wireless stations.

What are Basic & Supported Data Rates?

Basic & Supported Data Rates for the WLAN are included by the AP in the broadcast beacon, which indicate to potential clients whether or not they can associate with the advertised wireless network, as well as the speeds at which particular data can be sent (basic = broadcast/multicast). By default, the UniFi Controller disables the slowest 2G rates, which prohibits slower, legacy clients from joining the WLAN, while also discouraging high-airtime consumption transfers (e.g., broadcasts) due to low data rates. By default, UniFi leaves all of the basic 5G data rates enabled.

Keep in mind, although wireless clients should remain connected as they roam across the ESS, it is detrimental to the aggregate WLAN speeds if low-rate and high-rate clients co-exist on the same BSS (AP). Ideally, low-rate clients should be forced to join another AP in order to prevent them from consuming too much airtime on the WLAN. The following example describes the impact of a low data rate client in two WLAN scenarios:

  1. The first WLAN (Figure A) consists of an UAP-AC-HD capable of up to 1733 Mbps PHY rates with high SNR, VHT clients (i.e., the laptop clients). By consequence of supporting the lowest data rates, a weak signal client (the mobile client) connected to the UAP-AC-HD can cause the aggregate WLAN speeds to reduce dramatically.

  2. The second WLAN (Figure B) consists of two UAP-AC-HD, strategically placed to provide comparable coverage but with stricter data rates enabled. By disabling the lowest data rates, clients are more likely to remain connected to the AP with the best signal possible, resulting in maximum allowable throughput at the UAP-AC-HD.
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Figure A Figure B

Consideration for High-Density WLANs

However, in High-Density WLANs, where airtime efficiency is critically important to the overall performance of the wireless network, carefully consider adjusting the 802.11 RATE AND BEACON CONTROLS—albeit with extreme caution. Improper configuration of this advanced feature, can lead to disruptive performance or widespread connectivity problems for clients throughout the WLAN.

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Disabling lower rates benefits WLANs (especially High-Density) in a few important ways:

  1. Forces “sticky” roaming clients to re-associate to another AP. If the WLAN is well-designed/implemented, the new AP to which the client roams will offer a better signal and have a twofold impact on improving that client’s speeds as well as the aggregate performance of the previous BSS, since low-rate clients no longer drag down the airtime.

  2. Reduces the likelihood of channel contention (i.e., the probability of signal collisions at receivers in the WLAN) since there is less overhead from associated clients within the BSS. With low data rates disabled, clients transfer data as quickly as possible—then remain quiet. This frees up the WLAN for other clients that need to use the channel.

  3. Lessens the probability of hidden nodes on the network since associated clients are within a closer proximity of each other, and therefore, able to 'hear' whenever a transmitter is actively using the wireless channel.
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