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With the popularity of wireless devices and the increasing requirements for mobile applications, enterprises must diligently manage and plan the entire deployment. However, some wireless technologies and common electronic devices that have been put into use or emerging have affected the performance of wireless networks.

Among them, RF interference is the main reason that affects the operation of wireless network, it will affect the security and stability of wireless network.

This article lists the 20 most common misstatements about wireless interference.

Myth # 1: "the only interference comes from other 802.11 networks."

802.11 devices are numerous, and other 802.11 networks are sure to interfere with your network. This kind of interference is known as same-frequency or near-frequency interference, but because 802.11 devices follow the same protocol, they can cooperate with each other-that is, two wireless access points on the same frequency share channel capacity. But in fact, in a band without license restrictions, the number of other models of devices far exceeds the number of 802.11 devices. Other devices include microwave ovens, cordless phones, Bluetooth devices, wireless cameras, outdoor microwave links, wireless game controllers, Zigbee devices, fluorescent lights, WiMAX, etc., and even a bad electrical connection can produce a very wide RF spectrum. These non-802.11 types of interference do not work well with 802.11 devices, and they can greatly reduce throughput. In addition, they will also have secondary effects, such as reducing the rate, and data retransmission caused by interference will mislead 802.11 devices to use low numbers.

According to the rate rather than the appropriate rate.

Summary: the unlicensed band is an experiment of FCC in the shared irregular spectrum. The experiment has been a great success so far, but RF interference still poses a great challenge and needs proper attention.

Myth # 2: "my network seems to be working, so interference is not a problem."

802.11 protocol is designed to resist interference to some extent. 802.11 the device detects interference before data transmission, and it pauses transmission until the interference disappears. If interference occurs during transmission (and the interference causes the packet not to be received properly), confirming that the packet is not received will cause the data to be retransmitted.

Eventually, all the packets will pass. However, pausing or retransmitting data can seriously affect the capacity and performance of your wireless network.

For example, microwave ovens produce interference with a duty cycle of 50% (because they are based on the alternating current cycle of the 60-Hz). This means that the microwave oven is at the same frequency as your 802.11 wireless access point, reducing network capacity and performance by 50%. So if your wireless access point rate design is 24Mbps, there may be only 12Mbps around the microwave oven you are using. If the applications on your wireless network only receive data (such as browsing the Internet), the loss of throughput may not feel obvious. But if you use capacity-and delay-sensitive applications, such as wireless voice, controlling interference can become an important issue.

Conclusion: interference is everywhere, it is just a silent killer at the moment.

Myth # 3: "I did a RF cleanup before deployment, so I found all the sources of interference."

One of the biggest headaches is that interference is always intermittent. Interference may only occur at certain times of the day or week-such as when someone is using a wireless headset. So, unless you keep cleaning up, it's easy to miss some sources of interference. Even if you keep cleaning up (such as 24-hour testing in each place), things around you will change over time. Some devices that work in the unlicensed frequency band can easily enter your network environment. No matter how many periodic cleanups you have, there is no guarantee that you will have a completely interference-free environment.

Conclusion: it is impossible to clear the interference problem. Microwave ovens, cordless phones, Bluetooth devices, wireless cameras, outdoor microwave links, wireless game controllers, Zigbee devices, fluorescent lights, WiMAX, etc., and even bad electrical connections-all of which can generate RF spectrum. None of these non-802.11 devices can collaborate with 802.11 devices.

Myth # 4: "my infrastructure device automatically detects interference."

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Some new, switch-based wireless infrastructure products can manage RF interference to some extent.

The 802.11 chip can detect a non-802.11 signal and then change the 802.11 channel of the wireless access point in the interference area. But there is a problem with this function, that is, it can not solve problems outside the area. Some jamming devices-- such as Bluetooth devices, cordless phones, 802.11 FH devices, human interference signals, etc.-- are broadband and ubiquitous, so changing channels can't avoid them. Even for a device running at a fixed frequency, it is difficult to manage the frequency in a large cellular-based network. Therefore, the most important thing is to analyze the interference source, determine the nature of the equipment, the location and then determine how to solve the interference. In many cases, the best solution is to take the device away. Sometimes, it is also a good choice to move or shield the device to avoid interfering with the network.

Summary: simply, it is very useful for products that can automatically respond to interference, but you also need to have some understanding of potential problems.

Myth # 5: "I can overcome interference as long as I use a high-density wireless access point."

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Because 802.11 wireless access points are not expensive, many people tend to deploy them very densely. For example, in some networks, there is a wireless access point in each room. This deployment method greatly improves the capacity and performance of the network through the spatial reuse of the spectrum. It seems that the more wireless access points around, even if there is interference, the device can operate very normally.

In fact, if you deploy wireless access points too densely, you have to reduce the power of signal transmission. If the power is not reduced, the wireless access points will influence each other, which is what we are all familiar with as co-channel interference. Reducing the power of wireless access points does counteract the potential benefits of interfering immunity. So the network of high-density wireless access points is not necessarily better than low-density deployment.

Summary: it makes sense to design your network for capacity, but high-density wireless access points are not a panacea for interference.

Myth # 6: "I can use packet detectors to analyze interference problems."

802.11 packet detectors face the same problem as infrastructure devices in wireless networks: they can only see what the 802.11 chip tells them. They can tell you about the secondary problems of interference, such as increased retransmissions, low data rates, etc., but they can't analyze the interference, determine the cause of the interference, or help you locate the jamming device.

The second problem with 802.11 chips is that power measurements are usually not accurate enough. This means that the signal strength of wireless access points (or other devices) cannot be accurately and reliably judged based on the dBm data received on the 802.11 chip. Therefore, it is difficult to judge the situation based on the number of packet detectors.

Summary: you need the right tools to analyze interference problems. Finally, it is very important to be able to analyze the interference source in order to determine the best way to solve the interference. In many cases, the best way is to remove the interfering device.

Myth # 7: "I have a wireless policy that does not allow interference devices to enter."

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Defining a wireless policy is a good first step in solving interference problems. But if it is not implemented, it is futile to have a strategy. One of the most important features of the unlicensed band is that it is inexpensive and widely used. So, it's easy for employees to buy these devices and take them to the workplace. In many cases, employees simply do not realize that they are bringing devices that may cause interference to the wireless network. Some devices, such as cordless headphones and microwave ovens, are office necessities and cannot be banned altogether.

Conclusion: don't expect interfering devices to disappear in your environment.

Myth # 8: "there is no interference in the 5GHz band."

Compared with 2.4-GHz devices, it can cause interference.

It is true that 5GHz has fewer devices, but that will change. Just as everyone moves from 900 MHz to 2.4 GHz to avoid interference, the same goes for 5GHz. Cordless phones, radars, peripheral sensors, digital satellites and other devices already use the 5GHz band.

Conclusion: you can run, but you can't hide.

Myth # 9: "I can find a consultant to solve all the interference problems I encounter."

If you use the wireless network for a while, you will find that your network does not work well enough from time to time. Without seeing it with your own eyes, you can only guess whether it is a matter of interference. One of the problems faced by IT staff was that they couldn't see it with their own eyes, especially when CEO asked why there was a problem with the Internet connection in the conference room yesterday. In addition to being out of control, it takes time and money to find a consultant to diagnose these problems. A door-to-door service and a business trip report costs about $5000 to $10000.

Conclusion: it is too expensive to find a third party to solve the network problem.

Myth # 10: "I give up. RF doesn't understand."

Don't despair. There are tools to make RF easier to understand, even for those who think they are wired experts but not wireless experts. For example, Cisco Spectrum Expert Wi-Fi can classify interference, so you don't need to read those curves. When we determine the types of interference, we will help you find and remove them.

Conclusion: the solution to the problem lies with Cisco.

Myth # 11: "Wi-Fi interference does not occur very often."

More and more evidence shows that Wi-Fi interference is actually very common and difficult to solve. Here are some recent examples:

Technical engineers from Wi-Fi infrastructure vendors reported to Cisco that they recently conducted a phone call back to a major customer and found that the customer had found about 20 sources of interference, 50% of which were from the customer's own Wi-Fi network. A large wireless service outsourcing manager reported to Cisco that one out of every three Wi-Fi questions his technician received was about interference.

A recent survey of 300 of their customers by a large supplier of Wi-Fi tools showed that diagnosing interference problems was their biggest challenge in managing Wi-Fi networks.

Jupiter Research reports that 67% of residents have Wi-Fi problems related to interference devices, including cordless phones, baby monitors and microwave ovens.

Conclusion: please don't run away: Wi-Fi interference does exist.

Myth # 12: "I need to check for interference only when I rule out the possibility of other problems."

The physical layer of any network system is fixed. If the physical layer does not work properly, the high protocol layer will be inefficient or abnormal. For this reason, we usually confirm the situation at the physical layer before checking for problems at a higher level.

And so on, when your computer is plugged into an Ethernet cable, but the network does not work, the first step to check is to see if the light on the Ethernet adapter is on. If the lights are not on, there is no need to further check for network configuration problems, because there is something wrong with your physical layer connection.

The potential physical layer problem in Wi-Fi is more serious than Ethernet. The question of whether the physical layer is connected or not should only be considered when the Ethernet cable is plugged in for the first time. If it can work properly on the first day, it should work normally in the future. But in the RF environment, the quality of physical layer connections varies from hour to hour because other devices are artificially brought in to affect the network environment.

Summary: to avoid wasting time, first check the RF physical layer.

Myth # 13: "even if I find interference, I can't do anything."

The most common way to solve the interference is to replace or remove the interference source. For example, you can replace the old microwave oven with a microwave leak or replace the 2.4-GHz cordless headphones with other models that do not work in the Wi-Fi band. Most of the time, interference is caused by well-intentioned employees. A Wi-Fi administrator found an employee sitting with his back to the door with a wireless camera so he could see what was behind him. But this camera works in 2.4GHz. In this case, there is a need to develop a policy to prohibit such devices from appearing in the campus.

Another way is to move the wireless access point around the interfering device, or change the operating frequency of the wireless access point to a frequency that is not affected by the device. Once you know the location and frequency parameters of the jamming device, this solution is easy to do. It must be noted that some devices (such as Bluetooth devices) are frequency hopping, so it is impossible to change the working frequency to reduce interference.

The final solution is to remove or shield the source of interference. For example, in a hospital, devices that produce RF interference can be isolated in a specific room where there is no Wi-Fi network requirement. If it cannot be isolated, then electromagnetic interference (EMI) shielding equipment can be used to limit the interference to a small area. You can use a grounded shield or add metal foil (essentially Faraday cages) or insulating paint to the wall for shielding purposes.

Conclusion: as long as you know the source of the interference, you will always find a solution.

Myth # 14: "only some devices that are easy to find will interfere with my Wi-Fi network."

There are countless wireless devices in the unlicensed band, and it is no longer obvious what devices are the source of interference-wireless connections are now found in watches, shoes, MP3 players and many small consumer goods. In some cases, some devices have been upgraded to use RF technology. A good example is the motion detector, which is used in many offices as part of voice-activated lights. A new hybrid motion detector uses passive infrared sensor (PIR) and 2.4-GHz radar to detect motion. These devices were installed with good intentions at the beginning, but now there is a great interference to the Wi-Fi network.

Some inadvertent transmitters are also difficult to find. The faulty ballast on the fluorescent lamp will also produce broadband RF interference, which will affect the Wi-Fi network. You can't find a problem simply by checking the equipment. "Hidden devices" are becoming more and more common. We see a lot of examples, such as the installation of hidden cameras in the security department-- the network department doesn't know-- that these devices unwittingly affect Wi-Fi.

Summary: you need the right tools to quickly find interference, not a magnifying glass.

Myth # 15: "when interference occurs, the impact on the data is usually very slight."

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The interference of an interference source to the data throughput (or data capacity) of the Wi-Fi network can be very surprising.

There are three main elements to determine the impact of interfering devices:

Output power. The greater the output power, the larger the physical "interference area" produced by the interference device.

The time characteristic of the signal behavior. Analog devices, such as cameras and old cordless phones, have a constant signal that is online. Digital products, such as digital cordless phones, tend to turn signals on and off. The intervals between online and offline signals are different for different devices. In general, the greater the percentage of time spent on the online signal and the more frequently it is sent, the greater the impact on throughput.

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The frequency characteristics of the signal behavior. Some devices operate at a fixed frequency, affecting a specific Wi-Fi frequency. Some devices jump in multiple frequencies, affecting each channel, but the impact is relatively small. Some devices, such as microwave ovens and interfering transmitters, quickly scan the entire spectrum, causing brief but serious interruptions to many frequencies.

In a recent study by Farpoint Research, the impact of different jamming devices on Wi-Fi data throughput is measured. A microwave oven 25 feet from the wireless access point or client reduces data throughput by 64%, while placing a frequency hopping phone in the same location reduces data throughput by 19%, and analog phones and cameras lead to a 100% reduction (that is, cannot be connected).

Summary: interference does affect the data throughput of Wi-Fi networks.

Myth # 16: "the rate of voice transmission is very low, so the impact of interference on Wi-Fi voice should be minimal."

With modern voice coding, personal voice calls use a data rate of 8Kbps. This is nothing compared to the maximum throughput of the Wi-Fi network. So it seems that a Wi-Fi wireless access point can easily carry multiple VoIP phones at the same time.

However, many factors can affect the number of phones carried by wireless access points. First, the packet header with a large number of VoIP protocol layers will increase the amount of traffic that the data flow reaches 100Kbps. In addition, there is an additional protocol packet header for Wi-Fi.

Second, voice traffic is very sensitive to jitter and delay, which requires a lot of bandwidth to be reserved on the network to reduce network congestion. The recommended number of voice phones on one Wi-Fi wireless access point is 15. If there is interference, the number of phones available will be reduced accordingly.

In addition, a small amount of interference will seriously affect the quality of wireless voice calls.

In a recent study by Farpoint Research, testing the impact of different interference devices on the MOS value of wireless voice phone call quality found that voice quality was almost unacceptable when there were microwave ovens, cordless phones, cameras or same-frequency Wi-Fi devices within 25 feet of a wireless access point or wireless phone. More importantly, interference will create a hole in signal coverage and the voice will be interrupted. An indoor study shows that when there is an interference source (cordless phone or camera) within 75 feet of the wireless access point, it reduces the effective range of wireless voice by 50%. A 50% reduction in the range of wireless voice is equivalent to more than 75% of the space on the entire floor, creating a signal coverage hole.

Conclusion: can you hear me? Wireless voice and interference cannot coexist.

Myth # 17: "interference affects performance, not security issues."

If a network worm breaks through your firewall, takes up 50% of the network bandwidth and spreads from computer to computer, do you think this is a security issue or a performance issue? The point is that any problem that affects the core IT network system is security. As corporate Wi-Fi networks become increasingly critical, any interfering device-- whether malicious, such as interfering with a transmitter, or accidental-- should be seen as a potential security problem. In addition to RF denial of service, there are other threats related to non-Wi-Fi RF devices, including:

. Multiprotocol equipment.

Wi-Fi networks usually have secure access control, but devices running on non-Wi-Fi networks, such as Bluetooth devices, do not have secure access protection. A laptop with a Wi-Fi and Bluetooth connection will act like a bridge, allowing devices to enter a local area network or wireless LAN. To prevent an insecure network from accidentally connecting to the corporate network, you need to:

1) Control the configuration of wireless network interface based on client-side tools.

2) RF detector, which is used to check for suspicious non-Wi-Fi activities that may cause bridging.

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Rogue devices that are not Wi-Fi. Most enterprises use Wi-Fi 's rogue wireless access point detection device to discover unauthorized (and insecure) wireless access points in the corporate network. However, non-Wi-Fi devices, such as Bluetooth access points, can cause similar security vulnerabilities. Like Wi-Fi 's rogue devices, these devices must be detected and erased.

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Sensitive data leak. Some non-Wi-Fi devices, such as cameras, wireless phones can take sensitive data out of the restricted area after bypassing the company's security policy. If such a problem is involved, the operation of the wireless network must be restricted in the area, and the area must carry out spectrum monitoring to identify unauthorized devices.

Summary:

The security issues of RF will not stop because the Wi-Fi network stops. Do you know who is using your spectrum?

Myth # 18: "802.11n and its antenna system can operate under any interference."

Systems that use multiple antennas or smart antennas can improve interference immunity by strengthening useful signals on the receiver. If the useful signal is strong, then the signal-to-noise ratio (SNR) will also increase, which can effectively reduce the interference area of the jamming device. However, the gain gained by smart antenna systems usually only increases the signal strength by 10 dB. This means that compared with the traditional antenna system, the range of interference will be reduced by two times, but it is still a long way from solving the interference problem. For example, if a jamming device used to have an impact 80 feet from the receiver, it is now within 40 feet.

Then there is still a problem of interference in the area of 5000 square feet in the floor space.

Summary: the antenna can alleviate the problem, but it is not the solution.

Myth # 19: "my site survey tool can be used to find interference problems."

The standard Wi-Fi site survey tool is used to measure the coverage of Wi-Fi signals. When you walk around the building,

The Wi-Fi chip can measure the strength of the wireless access point signal. But the Wi-Fi chip can only be used to watch Wi-Fi signals and can't tell you the interference signals from non-Wi-Fi devices. (the Wi-Fi packet analyzer has the same problem).

The Wi-Fi site survey tool can detect the approximate location of non-Wi-Fi signals, but it cannot help you determine the characteristics, equipment type or location of the interference source. So your problem still can't be solved. You do need a RF layer tool to diagnose interference. The good news is that many of the next-generation Wi-Fi site survey tools will more fully integrate RF layer tools to provide a complete solution.

Summary: site survey tools measure signal coverage, but do not meet your RF level needs.

Myth # 20: "RF analysis tools are too big and too expensive."

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Many RF analysis tools, such as large and expensive spectrum analyzers, are not suitable for enterprise applications. But Cisco's RF spectrum analysis tool is designed to meet both your desired shape (a small card inserted into a laptop) and your IT budget. Even better, Cisco's smart spectrum solutions allow you to solve interference problems without having to be an RF expert.

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