10 myths about WiFi interference

Growing ubiquity of wireless devices combined with the advent of mobility applications requires businesses to be diligent in managing inference throughout their deployments. The many wireless technologies and commonplace electric devices already in use and newly emerging impede wireless performance.

WiFi interference can be a major inhibitor to wireless performance, creating security vulnerabilities and wireless network instability.

This blog post exposes the top 10 most pervasive myths around wireless interference.

1. The only interference problems are from other 802.11 networks.
   

   There are a tremendous number of 802.11 devices out there. It is true that the other 802.11 networks can cause interference with your network. This type of interference is known as co-channel and adjacent channel interference. But since other 802.11 devices follow the same protocol, they tend to work cooperatively-that is, two access points on the same channel will share the channel capacity.
   In reality, the many other types of devices emitting in the unlicensed band dwarf the number of 802.11 devices. These devices include microwave ovens, cordless phones, Bluetooth devices, wireless video cameras, outdoor microwave links, wireless game controllers, Zigbee devices, fluorescent lights, WiMAX, and so on. Even bad electrical connections can cause broad RF spectrum emissions. These non-802.11 types of interference typically don't work cooperatively with 802.11 devices, and can cause significant loss of throughput. In addition, they can cause secondary effects such as rate back-off, in which retransmissions caused by interference trick the 802.11 devices into thinking that they should use lower data rates than appropriate.
   Summary: The unlicensed band is an experiment by the FCC in unregulated spectrum sharing. The experiment has been a great success so far, but there are significant challenges posed by RF interference that need to be given proper attention.

2. My network seems to be working, so interference must not be a problem.
   

   The 802.11 protocol is designed to be somewhat resilient to interference. When an 802.11 device senses an interference burst occurring before it has started its own transmission, it will hold off transmission until the interference burst is finished. If the interference burst starts in the middle of an ongoing 802.11 transmission (and results in the packet not being received properly), the lack of an acknowledgement packet will cause the transmitter to resend the packet. In the end, the packets generally get through. The result of all these hold-offs and retransmissions, however, is that the throughput and capacity of your wireless network are significantly impacted.
   For example, microwave ovens emit interference on a 50 percent duty cycle (as they cycle on and off with the 60-Hz AC power). This means that a microwave oven operating at the same frequency as one of your 802.11 access points can reduce the effective throughput and capacity of your access by 50 percent. So, if your access point was designed to achieve 24 Mbps, it may now be reduced to 12 Mbps in the vicinity of the microwave when it operates. If your only application on the WLAN is convenience data networking (for example, Web surfing), this loss of throughput may not be immediately obvious. But as you add capacity and latency-sensitive applications such as voice over Wi-Fi your network, controlling the impact of interference will become a critical issue.
   Summary: Interference is out there. It's just a silent killer thus far.

3. I did an RF sweep before deployment. So I found all the interference sources.
   

   One of the most troubling issues about interference is that it is often intermittent in nature. The interference may occur only at certain times of day-for example, when someone is operating a device such as a cordless headset-or on certain days of the week. So, unless an initial sweep is done for an extended time, it's very easy to miss sources of interference. And even if the sweep was extensive (for example, making measurement in each area for 24 hours), things change over time. It's very easy for someone to introduce one of the many devices that operate in the unlicensed band into your environment. No amount of periodic sweeping can truly guarantee that you have an interference-free environment.
   Summary: You can't sweep away the interference problem. Microwave ovens, cordless phones, Bluetooth devices, wireless video cameras, outdoor microwave links, wireless game controllers, Zigbee devices, fluorescent lights, WiMAX devices, and even bad electrical connections-all these things can cause broad RF spectrum emissions. These non-802.11 types of interference typically don't work cooperatively with 802.11 devices.

4. My infrastructure equipment automatically detects interference.
   

   Some of the newer, switch-based WLAN infrastructure products provide a level of RF interference management. With their 802.11 chipsets, these solutions detect the presence of non-802.11 signals. And in response to detection, they can change the 802.11 channel of the APs in the area of the interference. An issue with this approach is that it doesn't solve many of the problems that are out there. Some interfering devices-for example, Bluetooth devices, cordless phones, 802.11FH devices, jamming emissions) are broadband, so it's not possible to change channels away from them: they are everywhere in the band. And even for devices that operate on a static frequency, it can be challenging to manage channel assignments in a large, cell-based network. In the end, it's critical that you be able to analyze the source of interference-that is, identify what the device is and where it is located-in order to determine the best course of action to handle the interference. In many cases, this "best action" will be removing the device from the premises. In other cases, the response may be to move or shield the device from impacting the network.
   Summary: Simple, automated-response-to-interference products are helpful, but they aren't a substitute for understanding of the underlying problem.

5. I can overcome interference by having a high density of access points.
   

   The inexpensive nature of 802.11 access points makes it tempting to deploy them with very high density. For example, some networks are being deployed with an AP in every room. This type of deployment has the benefit of greatly increasing the capacity of the network by allowing "spatial reuse" of the spectrum. It seems intuitive that by having more APs spread around, it's more likely that a client will be able to operate successfully even when interference is present.
   Unfortunately, when you deploy a dense network of access points, it's necessary to reduce the transmit signal power of each of the access points. If you don't reduce the power, the access points generate interference to each other, a phenomenon known as co-channel interference. The reduction in the transmit power of the access point exactly offsets the potential benefit of interference immunity. So in the end, the interference immunity of a network with a dense deployment of access points is not significantly better than that of a less dense deployment.
   Summary: It's reasonable to over-design your network for capacity, but a high density of access points is no panacea for interference.

6. I can analyze interference problems with my packet sniffer.
   

   802.11 packet sniffer products suffer from the same problem as WLAN infrastructure equipment: they can see only what the 802.11 chips tell them. They can tell you about secondary indicators of interference, such as increased retransmissions and lower data rates, but they can't analyze interference problems, determine the cause of the interference, and help you find where the interfering device is located.
   A second problem with the data from 802.11 chips is that power measurements are typically uncalibrated. This means that the data you receive from an 802.11 chip about the signal strength of an access point (or other device) can usually not be expressed reliably in absolute dBm units. As a result, it is very difficult to put meaning on the numbers that packet sniffer devices report.
   Summary: You need the right tool for analyzing interference. In the end, it's critical that you be able to analyze the source of interference in order to determine the best course of action to handle the interference. In many cases, the best action will be removing the device from the premises.

7. I have a wireless policy that doesn't allow interfering devices into the premises.
   

   Having a wireless policy is a good first step in tackling the interference problem. But no policy is effective without enforcement. One of the great attributes of unlicensed band wireless devices is that they are inexpensive and widely available. As a result, it's very easy for employees to purchase these devices and bring them to work. In many cases, these employees are not even aware that a particular device may cause interference with your wireless network. And some devices like cordless headsets and microwave ovens may be a necessary part of your business, so they can't be completely disallowed.
   Summary: You have to expect that interfering devices will sneak onto your premises.

8. There is no interference at 5 GHz.
   

   It is generally true that fewer devices currently operating at 5 GHz are causing interference as compared to 2.4-GHz devices. But this will change over time. Just as everyone moved from 900 MHz to 2.4 GHz to avoid interference, the "band jumping" effect will catch up with 5 GHz. Some devices that already exist at 5 GHz include cordless phones, radar, perimeter sensors, and digital satellite.
   Summary: You can run, but you can't hide.

9. I'll hire a consultant to solve any interference problems I run into.
   

   If you have been running a WLAN for some time, you will know that there are frequent instances where your network doesn't operate perfectly. Without having your own visibility into interference, you are left to guess about whether or not interference is the problem. Lack of visibility is an issue for IT personnel, especially when the CEO is asking why he was having trouble yesterday connecting in the conference room. And beyond the issues of control, it's expensive and time-consuming to bring in a consultant to debug these kinds of problems. A single visit and trip report can cost on the order of US $5000 to $10,000.
   Summary: You can't afford to rely on a third party to debug your network.

10. I give up. RF is impossible to understand.
    

    Don't despair. Tools are now available that make RF easier to understand, even for those who consider themselves wired network specialists, not wireless experts. For example,
    

    Oscium WiPry 5x Dual Band Spectrum AnalyzerDual Band Spectrum Analyzer. With this analyzer you can transform a smartphone or tablet into a dual band spectrum analyzer (2.4 & 5 GHz). This perfectly portable tool is the ideal solution for field techs and wireless professionals.
    

    Summary: The cavalry is here!

Source: Cisco