Technology Blog

Hotspot deployments: Part I - Interference sources

by on ‎05-17-2012 03:37 PM - last edited on ‎05-18-2012 09:16 AM by Community Manager

INTRODUCTION

These two Airheads Social posts describe sources of Wi-Fi and non-Wi-Fi interference (Part I), and then methods for mitigating the impact of interference in hotspot and hotzone deployments (Part II). Specifically, these posts offer some guidelines for deploying an Aruba WLAN solution in hotspot and hotzone environments. The term hotzone is typically used to refer to a collection of hotspots that are deployed in a contiguous area such as a downtown or the entire city. While the number of APs may vary, both hotspots and hotzones have to operate in the presence of interference that the operators may not have any control over. For simplicity, the term hotspot is used to refer to both hotspot and hotzone deployments in these posts.

 

INTERFERENCE SOURCES IN HOTSPOTS AND HOTZONES

It is expected that in a hotspot deployment the APs will be operating in a relatively harsh RF environment, where the APs have to co-exist with many other Wi-Fi and Non-Wi-Fi interferers. The possible interference sources in a hotspot environment can be broadly classified as Wi-Fi and non-Wi-Fi interference. The Wi-Fi interference can be further classified into:

  1. Wi-Fi interference from other APs and clients that are part of the hotspot network
  2. Wi-Fi inference from third-party Wi-Fi devices

The non-Wi-Fi interference can be further classified into:

  1. Proprietary and other wireless networks that operate in the same band
  2. Fixed frequency narrowband interferers
  3. Frequency hopping interferers
  4. Wideband interferers
  5. Other generic interferers

Wi-Fi Interference

Wi-Fi APs and clients in the same network

Interference from other Wi-Fi devices includes co-channel and adjacent channel interference. Co-channel interference and adjacent channel interference refer to the interference generated on the same channel and on adjacent channels respectively, by Wi-Fi devices.

 

Third party Wi-Fi APs and clients

Similar to Wi-Fi devices on the same network, third party Wi-Fi APs and clients can also generate co-channel or adjacent channel interference. In addition, some third party devices may exhibit behaviors that result in unfair access to the channel.

 

Non-Wi-Fi Interference

Proprietary and other wireless networks that operate in the same band

There are many proprietary wireless networks that are based on IEEE 802.11 hardware, but use a MAC protocol that does not strictly follow CSMA/CA. These networks are typically deployed outdoors and can cause interference in hotspot deployments. In some cases, the hotspot APs may be able to decode the frames transmitted by these systems, whereas in other cases the frames may not be decodable by the Wi-Fi AP. Other wireless networks such as WiMax can also operate in the 2.4 GHz and 5.8 GHz bands. The Wi-Fi hardware cannot decode these transmissions and therefore these transmissions will appear as interference or noise to the Wi-Fi devices.

 

The duty cycle of these interfering networks vary depending on the protocol and the actual traffic present at any time. Most of the devices in these networks transmit beacons or some form of periodic frames to synchronize master and slave devices. Therefore, a minimum duty cycle is to be expected from any such proprietary or other wireless networks. When these networks are active, Wi-Fi devices will not be able transmit if the signal strength of these transmissions are high enough to trigger the energy detect CCA. If the signal strength of the interference is not high enough to trigger CCA but stronger than the Wi-Fi signal, they may corrupt Wi-Fi transmissions resulting in a high frame error rate.

 

Fixed frequency narrowband interferers

Many fixed frequency narrowband interferers operate in the 2.4 GHz and 5.8 GHz bands. While these interferers may be using only 1 or 2 MHz of the bandwidth, they are capable of blocking Wi-Fi transmissions entirely as they typically have a duty cycle of closer to 100%. Since the frequency of operation is often fixed, these devices typically impact only one Wi-Fi channel at a time. Some examples of such fixed frequency interferers include, wireless video transmitters, wireless cameras, wireless video security systems and some baby monitors.

 

Frequency hopping interferers

There are a number of frequency hopping devices that operate in the 2.4 GHz and 5.8 GHz bands, including Bluetooth, Zigbee, Cordless phones and Xbox. While the signal strengths of these interferers can be quite high, they typically do not have high duty cycle at all times. However, the duty cycle will vary depending on the usage and number of active devices. Unlike the fixed frequency interferers, the frequency hopping devices will use most of the respective 2.4 GHz or 5.8 GHz bands when they are active and therefore affect all the channels within each band. Since these devices hop from one channel to another, they may not be able block the Wi-Fi device from transmitting at all times. However, the packet error rate can go up significantly if the interferer is strong enough to corrupt the Wi-Fi transmissions.

 

Wideband interferers

The wideband interferers include some microwave ovens and similar equipment that may be used in medical and industrial manufacturing facilities. The residential microwave ovens are typically single magnetron devices that are active for 8 ms out of every 20 ms (16 ms, if the AC power is 60 Hz). Therefore, the duty cycle of such a microwave oven is between 40% – 50%. These devices typically operate in 2 – 6 MHz bandwidth between 2450 MHz and 2460 MHz depending on the manufacturer.  However, the emitted energy may spread over a wider bandwidth, potentially affecting channel 6 or 11 or both.

 

The commercial dual-magnetron microwave ovens have a duty cycle closer to 100% and operate in a much wider spectrum (tens of MHz) due to their slow hoping-like behavior. Such ovens may be used in restaurants and cafeterias. The Inverter Microwave ovens, that are more recent entrants to the residential and industrial market, can also have a duty cycle closer to 100%.

 

In addition, there are many other devices used for heating in healthcare and industrial manufacturing that may utilize microwave technology. The duty cycle of such equipment varies depending on the application. The typical microwave is active only for a few minutes at a time. However, the microwaves used in commercial establishments, healthcare and industrial manufacturing may be active for a longer duration making certain channels unusable.

 

Other generic interferers

In addition to the above mentioned interferers, there are some that may not fit into a specific category. These include intentional jammers that operate on all frequencies and other devices such as motion detectors that may be fixed most of the time and change their frequencies based on other factors. These devices may operate in either 2.4 GHz or 5 GHz band.

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