Wireless Network

Why Wireless Interference is an important consideration in Wi-Fi networks

Unlike a Wired Network, where adding more network switches gives better performance, a wireless network cannot be optimized for performance by adding more access points/ denser deployment of access points – mainly due to the Wireless Interference. In this article, we’ll try to understand frequency bands, interference, interference from 802.11 Wi-Fi enabled devices, interference from Non-Wi-Fi devices and how to identify and mitigate wireless interference.

Understanding Wireless Frequency Bands:

You might be familiar with the concept of frequency tuning in radio. When you tune your receiver to a certain frequency, you are able to hear the programs from a particular channel. So, when you use an analog rotary tuner to switch channels, you might have noticed that as you rotate the tuner, first a faint sound appears, then you get a strong signal, and then the signal weakens. So, the signals are received (with varying range of amplitudes) over a range of frequencies. When you consider many channels, the used range of frequencies becomes wider.

Similarly, Wireless (Wi-Fi networks) operate mainly in two major frequency bands (ranges) – 2.4Ghz and 5 Ghz. Both are unlicensed ISM band frequencies (Industrial, Scientific and Medical RF band) – Which means, any device / technology can use that band for communications.

2.4 Ghz & 5 Ghz are frequency bands (range of frequencies). The actual communications happen in sub-frequencies called channels, within each spectrum (frequency band). For example, in the 2.4 Ghz spectrum, Channel center frequencies might be like : Channel 1 – 2.412 Ghz; Channel 2 – 2.417 Ghz…… Channel 13 – 2.472 Ghz, etc. A Wireless Radio (on wireless access point) & client radio (wireless client on a laptop) operates in one of these channels to transmit information between them.

Every channel (sub-frequency) overlaps with its adjacent channels. So, Channel 6 for example, might overlap strongly with channels 5, 4 but weakly with channels 3, 2. In the 2.4 Ghz spectrum, Channels 1,6 & 11 are non-overlapping channels. That brings us to the next topic – Interference.

Wireless Interference:

Consider that there are three operational access points situated at a distance of 1 meter from each other (for example). If they operate in channels 1, 2 & 3 (respectively) or channels 1, 1 & 1 (respectively) – there would be a lot of interference that will affect all the clients connecting to these three access points. That’s because, generally access points and clients receive all the communications that are transmitted and reject those that are not in its frequency (channel) of operation. But if different access points operate in same channels (or) adjacent channels, they get confused if messages sent to them were meant for them or not!

But if the three access points are operating in channels 1, 6 & 11 (respectively), even if they are placed very close to each other, there would not be much interference because, the sub-frequencies used by each channel are far apart. In other words, these three channels are non-overlapping channels.

Interference might not allow you to connect to a wireless access point/ network, disconnect you from an existing connection (requiring you to re-connect to the network) or might slow down/ choke the wireless connectivity. Wireless Interference causes noticeable problems with real time applications like voice/ video transmitted over the wireless network. Interference is both a performance issue and a security concern (Rogue Access Points, Wireless DOS attacks, etc).

There are two types of wireless interference – Interference from Wi-Fi (802.11) Sources & Interference from Non-Wi-Fi Sources.

Interference from Wi-Fi (802.11) Sources:

Wi-Fi devices that interfere with the wireless network are – Access Points that are in the range of each other (and operating in overlapping channels); Neighboring Access Points that might be operating in overlapping channels & Wireless Jammers that intentionally operate in overlapping channels.

So, when two access points operate in same channel/ adjacent channels, and are in the range of each other, there would be interference. With 802.11 Wi-Fi based networks and devices, people might still be accessing and working on the wireless network even if there is considerable interference but with reduced throughput levels. 802.11 networks are resilient enough to retransmit the lost packets, but that might reduce the total available bandwidth.

Similarly, the access points across the street or in neighboring office, might as well be operating in the same channel, causing some interference. There are certain wireless jammers which cause interference in the network with the intention of disrupting wireless services.

Since the latest 802.11n network and devices use multiple antennas, they might be in a slightly better position to reduce interference by comparing the received signals from multiple antennas and averaging out the interfering signals.

Interference from Non-Wi-Fi Sources:

Since 2.4 Ghz and 5 Ghz are unlicensed frequency bands (spectrum), a lot of other technologies like Bluetooth, Zigbee & lot of devices like microwave ovens, wireless cameras, wireless speakers/mic, cordless phones, wireless headsets, wireless device controllers, gaming consoles (like Xbox), baby monitors, Jammers, etc. operate in these frequency bands as well, thereby causing interference to Wi-Fi network communications.

Microwave ovens operate in multiple frequencies (wideband) and consistently interfere with the Wi-Fi devices. Wireless Cameras operate in narrow band and hence interfere on particular Wi-Fi frequencies, Bluetooth headset keeps hopping across the frequency band but still causes interference temporarily.

Even if a complete site-survey is done prior to the implementation of Wi-Fi network, it is still difficult to find out the Non-Wi-Fi sources of interference because, newer/smaller wireless devices are appearing in the market which could be brought by the employees at any time, thereby causing (unintentional) disturbance to the corporate Wi-Fi network.

Detecting and Mitigating Wireless Interference:

5 Ghz is a relatively clean spectrum without much interference from non Wi-Fi sources. But most of the commercially available Wi-Fi network devices operate in the more popular 2.4 Ghz spectrum. It might be better to implement Wi-Fi networks to operate in 5 Ghz frequency band (For this, both the client adapter on the laptops and access point should support 5 Ghz operation), especially with 802.11n high performance networks.

Some vendors fit sensors on access points that detect interference in their channel of operation (if any) and switch to other channels. But this may not be a solution for interference from non Wi-Fi sources. Its possible to reduce the chances of interference by controlling (reducing) the (transmission) power levels of access points. Using multiple/ multi-sector antennas might also improve the SNR.

Wi-Fi Sources: The interference from other Wi-Fi sources are relatively easier to detect, and in some cases even mitigate. The basic principle with Wi-Fi sources is to avoid any neighboring access points operating in the same channel (and adjacent channels). As far as possible, neighboring access points need to operate in non-overlapping channels (Like 1,6,11).

Its quite difficult to monitor each access point manually, and change the frequency of operation manually for all access points (though its possible). Even if they are set manually, if an access point reboots (due to power loss etc), it will choose an arbitrary frequency (channel) which may not be the same as manually set frequency. So, the process (assigning channels manually) needs to be repeated.

To automate this process, a Wireless Controller, that provides centralized management can be used in a network to continuously gauge the channel of operation for all the neighboring access points and adjust their channel settings dynamically. Most of the Wireless Controllers can manage only their own make of access points, but there are wireless management softwares available to manage multi-vendor access points/ controllers.

Non Wi-Fi Sources: The normal Wireless management softwares/ controllers may not detect interference from non Wi-Fi sources (some of them do) but there are specialized spectrum analyzers that can be employed for this purpose. But unlike the Wi-Fi sources of interference, simply changing the frequency channel of operation of access points may not be a solution for non Wi-FI based interference and hence the best way to tackle them might be to physically remove the sources / shield the sources from spreading out, hence restricting them to a certain area.

There are certain open source based spectrum analyzers which can be used for detecting interference like Netstumbler, Kismet, inSSIDer etc. Commercial spectrum analyzers are also available for the same.

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