Hotspot deployments: Part II – Interference mitigation guidelines

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ArubaOS includes many functions for automatic mitigation of Wi-Fi and non-Wi-Fi interference in hotspot environments. Below is a summary of our recommendations for ways to mitigate these sources of interference:

  1. Deploy dual-band capable APs
  2. Enable band-steering to steer 5 GHz capable clients to the 5 GHz radios
  3. Use the non-overlapping 3-channel plan for 2.4 GHz deployment
  4. Disable 802.11b rates and lower 802.11 a/g OFDM rates if necessary pending validation testing with clients
  5. Minimize the number of SSIDs per radio
  6. Enable one or more of Hide SSID, Deny Broadcast Probes and Local Probe Request Threshold as appropriate (some may not be appropriate for all hotspot deployments)
  7. Leave Disable Probe Retry enabled to avoid probe retries
  8. Enable airtime fairness
  9. Use cell-size-reduction to control the receive sensitivity of APs if possible
  10. Enable hybrid spectrum mode for enhanced non-802.11 interference visibility and troubleshooting
  11. Adjust Non-Wi-Fi Interference Immunity if there is a significant network impact due to non-Wi-Fi interference

Wi-Fi Interference

The Wi-Fi interference from APs on the same network or third party APs can be managed through various mechanisms in ArubaOS. The primary mechanism for controlling such interference is through ARM. ARM constantly monitors the Wi-Fi activity on every channel and estimates the interference levels. Based on the interference measurements, adjustments are made to the channel and/or transmit power.

The Wi-Fi traffic also needs to be managed by other means. Every advertised BSS (SSID) requires a beacon, and every such network generates additional management traffic such as probe requests and probe responses. Therefore, it is important to control the number of such configured networks. The beacons and other management traffic are sent at the robust modulation (lower rates) thus consuming a significant portion of the airtime, if not managed properly. In general, the number of SSIDs per radio should be kept to a minimum to minimize the impact of management traffic.

In 2.4 GHz networks, the lowest 802.11b rates consume more airtime than 802.11g/n rates. If the support for 11b devices is not required, the 11b rates could be disabled, after completion of basic testing to confirm there are no adverse impacts on performance with existing clients. Similarly, some of the lower 11a/g OFDM rates can also be disabled depending on the coverage area and expected client population. The retransmission and rate adaptation techniques used in ArubaOS can also further minimize the impact of Wi-Fi interference by intelligently selecting rates for each transmission and subsequent retransmissions, if any.

Non-Wi-Fi Interference

Fixed frequency interferers can increase the noise floor of the AP. If the signal strength of such an interferer is high, it can affect the ability of the AP to transmit since the media may appear busy at all times. Even if the interferer is farther from the AP, the increase in noise floor will affect the ability of the AP to receive since the SNR will be affected by the increased noise floor. The duty cycle of the interferer will also play an important role in determining the effect on the AP and mitigation methods.

While higher duty cycle interference can significantly affect the AP's ability to transmit and/or receive during the time the interferer is active, lower duty cycle interferers can affect Wi-Fi in other ways. The majority of lower duty cycle interferers are frequency hoppers. While some of the hoppers such as Bluetooth may have relatively lower transmit power, others such as cordless phones can have higher transmit power. Such higher signal strength hoppers can corrupt Wi-Fi transmissions resulting in CRC errors. The CRC errors in turn will increase the number of retries.

Band Selection and Band Steering

It is important to deploy APs capable of both 2.4 GHz and 5 GHz. Many of the new client devices support both 2.4 GHz and 5 GHz operation. When Aruba's Band Steering feature is enabled, clients that may be operating in 2.4 GHz but that are capable of 5 GHz operation are encouraged to move into the 5 GHz spectrum where there is more capacity. Though both 2.4 GHz and 5 GHz bands are license-exempt, there are a large number of unlicensed devices in 2.4 GHz than 5 GHz, leading to more interference. The majority of non-Wi-Fi devices in 5 GHz are also restricted to the upper 5 GHz sub-band (channels 149 through 165). By moving devices capable of 5 GHz devices into this band, there is a considerable boost in performance, both for those legacy devices operating in a less crowded 2.4 GHz band as well as those operating in the less crowded 5 GHz band. There are a few band steering modes available, including "prefer-5GHz" (encourages clients to move but not a hard rule), "force-5GHz" (forces clients capable of 5 GHz operation into that band), and "balance-bands" (attempts to achieve a ratio of client associations across the 5 GHz to 2.4 GHz band of 4:1).

Channel Selection

ARM should be able to select the best channel based on the available channel conditions. In 2.4 GHz, there is an option to select overlapping or partially overlapping channels for deployment. However, in hotspot deployments, non-overlapping channels (i.e.., 1, 6, and 11 in 2.4 GHz) should be selected to minimize the impact of Wi-Fi interference. It is also possible to manually remove channels in either band if they are known to have persistent interference.

Managing Management Frame Traffic

Management frames such as beacons and probe request/responses are sent at the lowest configured rate. In a relatively dense network, such management traffic can constitute a significant percentage of the overall traffic thereby limiting the time available for other traffic in addition to affecting the latency and jitter for other time critical traffic. The number of beacons sent is proportional to the number of BSSs active per radio and the number of APs on the same channel. While the number of APs is determined based on coverage and capacity requirements, the channel selection is done by ARM.

However, the number of BSSs (SSIDs) per radio should be kept to the minimum required, as each new SSID will generate additional beacon and probe traffic. ArubaOS also supports many other mechanisms to manage the probe response traffic. The SSID profile has the following parameters for probe management: Hide SSID, Deny Broadcast Probes, Local Probe Request Threshold and Disable Probe Retry. The Hide SSID feature is used to prevent a particular SSID being broadcast in the beacon, but this may not be a good option for all hotspot deployments, since clients will have to know the SSID by other means prior to sending a probe request with this SSID. When the Deny Broadcast Probes is enabled, the AP will not respond to broadcast probes, which may also have issues with interoperability and certain client devices.

The Local Probe Request Threshold specifies an SNR threshold for the probe requests. If a threshold is specified, the AP will not respond to probe requests received with SNR lower than the threshold. This will of course affect the ability of clients farther from the AP to connect. The Disable Probe Retry, which is enabled by default, prevents the AP from retrying the probe responses when the first transmission of the probe response is not acknowledged by the client. Clients often scan a channel, transmit probe requests and may not wait for the probe responses before moving on to the next channel. Therefore, disabling probe response retries can save precious airtime. In ArubaOS, Disable Probe Retry is enabled by default. One or more of these features can be enabled in a hotspot, depending on the specific requirements of the deployment.

Airtime Fairness

The available capacity on a wireless channel varies depending on many factors. Even in an interference-free environment, the capacity will vary depending on the types of clients connected and the distance between the AP and clients. The legacy data rates take more airtime to transmit compared to the HT (11n) higher modulation rates for the same packet size. The clients that are farther away may also have to use lower data rates (more airtime) compared to the clients that are closer to the AP. Therefore, it is not fair to enforce channel access based on data throughput as the airtime used by different clients will vary. ArubaOS supports airtime fairness capability that can be used to fairly distribute the airtime across the clients and prevent starving of certain clients from accessing the channel. This feature can be controlled from the QoS 802.11a/802.11g traffic management profile, and profiles can be set to default (disabled), fair (fair airtime allocation for all client types), or preferred (prefers 11n to 11a/g to 11b clients in terms of airtime allocation).

Coverage Deployments vs.. Capacity Deployments

While it is important to maximize the range of every AP in some hotspot deployments, not every deployment requires that every AP cover the maximum possible area. In some cases, it would be advantageous to control the coverage area of the APs for capacity and interference reasons. The interference range of any Wi-Fi network is much larger than the communication range. Even if the Wi-Fi transmissions are received with CRC errors due to insufficient SNR, it still affects the ability of the radio to transmit since the radio will try to decode all detected 802.11 carriers. In order to minimize the effect of such Wi-Fi interference, the coverage area of each AP needs to be controlled. ARM automatically adjusts the transmit power if there are multiple APs in the deployment. This would control the transmit range (i.e.., the distance at which clients can receive the AP's transmissions) of the AP. However, the receive range of the AP can be controlled by adjusting the receive sensitivity to minimize the impact of Wi-Fi interference. The cell-size-reduction feature of ArubaOS helps control the receive coverage area.  It is important that the receive coverage area does not shrink below the transmit coverage area or you may end up with clients that hear the network but are unable to connect.  This setting may be helpful in smaller retail hotspots where there is not a lot of square footage to cover or in a dense deployment where channel reuse is important.

Integrated hybrid spectrum mode

ArubaOS supports hybrid spectrum mode on 11n APs, where the APs can serve clients while simultaneously collecting spectrum data on that channel. While spectrum data can be viewed on various spectrum charts, specific alerts can also be configured at the AirWave to alert when certain spectrum events occur. The hybrid mode should be enabled on hotspot deployments.

Non-Wi-Fi Interference Immunity

Non-Wi-Fi Interference Immunity adjusts low level radio settings to mitigate the effect of non-Wi-Fi interference. This value should be changed with caution. Increasing the immunity level in a healthy network will result in severe loss of performance. This should be set to a higher than default level only when there is significant degradation due to non-Wi-Fi interference.