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UniFi - 802.11 Basic & Supported Rate Controls


Overview


This article discusses 802.11 wireless, basic and supported data rates as well as some details to take into account in high density scenarios.


Table of Contents


  1. Introduction - WLAN Data Rates
  2. Basic & Supported Data Rates
  3. High Density WLAN Considerations
  4. When to Disable Lower Rates and CCK Rates
  5. Related Articles

Introduction - WLAN Data Rates


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

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

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


Basic & Supported Data Rates


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Basic & Supported Data Rates for the wireless local area network (WLAN) are included by the access point in the broadcast beacon, which indicates 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 base station subsystem (BSS), or AP in this case. 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:

Figure A - Data rates depicted for wireless cell coverage. Figure B - Overlapping cells with lowest rates disabled.
test.png rates_channel_plan-good.png

Figure A: The first WLAN consists of a UAP-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-HD can cause the aggregate WLAN speeds to reduce dramatically.


Figure B: The second WLAN consists of two UAP-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-HD.


High Density WLAN Considerations


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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.

To get access to these controls, the Advanced Features must be enabled in the UniFi Controller. Enable them by:

1. Navigate to Settings > Site.

2. Within the Services section, check the box for Enable advanced features. 

3. To configure these settings, now navigate to Settings > Wireless Networks.

4. Either Edit an existing network, or Create New.

5. Under Advanced Options scroll down and click on the + to expand the 802.11 RATE AND BEACON CONTROLS section.

If after Enabling Advanced Features and Applying the changes, you do not see these settings in the Wireless Networks, verify that you are running the most recent firmware (for the Controller, related devices, and Cloud Key if applicable).

Rate_Controls.png


When to Disable Lower Rates and CCK Rates


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  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 (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 close proximity of each other, and therefore, able to 'hear' whenever a transmitter is actively using the wireless channel.

Disabling Complementary Code Keying (CCK) Rates

Complementary code keying (CCK) rates are used in conjunction with 802.11b WLANs. When the "Disable CCK Rates" box is checked in the Wireless Networks settings, CCK data rates 1/2/5.5/11mbps are disabled. This only disables the CCK data rates and does not affect ODFM rates that are in the range of CCK rates. OFDM will still allow for 6/9mbps data rates when CCK rates are disabled. 


Related Articles


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UniFi - High Density WLAN Scenario Guide