3 étapes pour gérer les problèmes d’interférence RF | enterprise.netscout.com

3 étapes pour gérer les problèmes d’interférence RF

    TABLE DES MATIÈRES

  • Introduction
  • STEP ONE: Identifier les interférences qui ne sont pas de type Wi-Fi
  • STEP TWO: Locate non-Wi-Fi interferers
  • STEP THREE: Identify Wi-Fi interferers

INTRODUCTION

Imagine a lecture hall that can hold four hundred students. Then imagine that the lecture hall has been tested for 5 Gbps aggregate throughput. Even at two and a half devices per student, that should be more than enough. Back-of-the-napkin math says that's at least 5 Mbps per device.

Now imagine that the students are in the lecture hall and the Wi-Fi stinks. "It's slow," they say. A peek at the controller shows under 100 Mbps for all the APs in that lecture hall, combined. How is that possible? How can 5 Gbps turn into 100 Mbps so quickly?

The answer is interference. Interference is the common cause of the all-too-common problem of tested wireless networks failing to perform to expectations.

Être conscient que des interférences peuvent causer des problèmes au réseau Wi-Fi est la première étape. That first step, however, is the easy part. Things get more difficult when things need to get more specific. Hard questions need to be answered: What is causing the interference? Est-ce que les interférences peuvent être évitées ? Est-ce que le fait de résoudre le problème d’interférence présent va venir créer de nouveaux problèmes ailleurs ? This paper aims to help in answering those questions using NETSCOUT solutions to illustrate the problem and their resolutions.

NETSCOUT's OptiView XG is an outstanding tool for identifying the reason for interference problems because OptiView XG includes AirMagnet Wi-Fi Analyzer ("Wi-Fi Analyzer") and AirMagnet Spectrum XT ("Spectrum XT") in one elegant package. Wi-Fi Analyzer is a network analyzer, which allows Wi-Fi traffic to be captured and analyzed. Spectrum XT is a spectrum analyzer, which allows radio frequency ("RF") information to be captured and analyzed. By using both a network analyzer and spectrum analyzer, interference challenges from both neighboring Wi-Fi networks and non-Wi-Fi sources can be identified, tracked and solved.



With the integrated capabilities of Spectrum XT and WiFi Analyzer, OptiView XG can be used to identify interference sources in lecture halls—along with numerous other difficult Wi-Fi environments like healthcare, retail, manufacturing, etc.—and lead Wi-Fi professionals towards solutions to interference problems. With OptiView XG and its integrated radios, there are no external adapters to deal with thus it's design and portability is more convenient for mobile operations.

Here, then, is a step-by-step guide to using OptiView XG to identify and resolve interference problems.




STEP ONE: Identifier les interférences qui ne sont pas de type Wi-Fi

It is best to start an analysis with non-Wi-Fi interferers because non-Wi-Fi interferers can make good Wi-Fi impossible. If, for example, a hospital has a DECT phone system (DECT being a separate, non-Wi-Fi technology), it can kill Wi-Fi across the entire 2,4 GHz frequency band. (Modern versions of DECT use different frequencies than 2,4 GHz, but the problem could exist around older DECT systems.) That is because DECT phones don't share. When a DECT phone needs to use wireless, it uses wireless. And many other wireless technologies—from Bluetooth to zigbee devices—work the same way: no sharing.


Wi-Fi interferers are almost always less harmful than non-Wi-Fi interferers because they share. Wi-Fi devices use 802.11 contention, which causes devices to listen and check the channel before transmitting. That checking and listening means that Wi-Fi devices tend to share channels tolerably with each other. In almost all cases, non-Wi-Fi devices do not share as well because they do not use contention.

What, then, can be done about non-Wi-Fi interferers? The best bet is to identify them, locate them, try to determine their impact and then adjust accordingly. That is where OptiView XG can help.


First: identify using Spectrum XT. The default start screen of Spectrum XT shows a list of interferers in the lower left area. More information (including which channels the interferers are using and how strong the received signal strength indicator (RSSI) from the interferer is), can be viewed by going to the bottom pane of Spectrum XT and choosing Spectrum Graphs -> Interferers. The same list of interferers will be displayed, but with additional detail.

One benefit of running Spectrum XT on OptiView XG is OptiView typically will be better able to emulate the experience of the end user as Wi-Fi devices generally have internal radios. If a spectrum analyzer has a similar antenna as a Wi-Fi device, then interference seen in the spectrum analyzer is more likely to be seen by the end user's Wi-Fi device as well. Spectrum analyzers that operate using external antennas may detect interference that does not affect end-users. Investigating interference with no impact wastes valuable troubleshooting time.


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STEP TWO: Locate non-Wi-Fi interferers

Once a non-Wi-Fi interferer has been identified, OptiView XG can be used to find the interfering device's location. From the start screen of Spectrum XT, double-click any interferer that is shown in the lower left area. This will launch the Find tool. Once in the Find tool, click Start and a meter showing the received signal strength of the interference source will activate. With the Find tool started, OptiView XG can be carried around as a tracking device.

Once the interferer has been located, it can be dealt with according to the location's policies. Ideally an interfering device can be disabled, but that is not always possible. The 'Affected Channels' information in Spectrum XT can be used to make deployment adjustments for interferers in the environment that can't be turned off.

This is where the journey with Spectrum XT ends. Spectrum XT has lots of other useful graphs and features and there may be times when it can be valuable to explore those things. But this paper is focused on solving problems, and once we're confident that we understand our non-Wi-Fi interference, then the next step starts with using another native OptiView XG tool that focuses on Wi-Fi issues: AirMagnet WiFi Analyzer.


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STEP THREE: Identify Wi-Fi interferers

Once non-Wi-Fi interference sources have been handled, then nearby Wi-Fi devices should be analyzed. Such analysis requires a network analyzer and OptiView has a network analyzer in AirMagnet WiFi Analyzer.

Calling AirMagnet WiFi Analyzer a network analyzer is, in some ways, selling it short. AirMagnet has unique features like built in device filters, automatic channel adjustments and sortable statistics that can be viewed in a myriad of ways. AirMagnet Wi-Fi Analyzer captures all of the same Wi-Fi traffic that a network analyzer would be expected to, but it also culls information from that traffic in a way that allows for quicker, simpler identification and analysis of Wi-Fi problems. In short, it's fair to say that AirMagnet Wi-Fi Analyzer is the best network analyzer for quickly analyzing interference from nearby Wi-Fi devices.

Before using WiFi Analyzer, one has to know what to look for. Wasted channel time is the greatest killer of Wi-Fi performance because channel time is the one resource that is limited. The number of packets on a wireless channel can increase: if there are fewer packet errors, then there can be more packets. The amount of data (bytes) on a channel can increase: if data rates improve, then more data can be accessed by each device. But one second is one second. If one second —or part of one second—is lost it cannot be recovered. When an OptiView XG running Wi-Fi Analyzer is used to identify interference problems, the wasting of channel time should be the focus of the analysis.

There are several ways that time can be lost. Collisions cause the wireless channel to lose time because data that suffers a collision has to be sent again. That means that the initial transmission of data was a waste of time. Low speeds waste time as well. Data rates are measure by taking data and dividing it by time. If the data rate is lower, then that just means that it will take more time to send the same amount of data. Non-data traffic can also be a waste of time, if it is unnecessary.

AirMagnet Wi-Fi Analyzer can be used to identify all three big Wi-Fi time wasters, and running WiFi Analyzer on an OptiView XG is an especially effective way to do so. OptiView XG uses an internal radio to capture Wi-Fi frames.


First: Collisions. A collision is a failed Wi-Fi data transmission. The way a Wi-Fi collision can be identified in OptiView XG is by looking in AirMagnet Wi-Fi Analyzer for data marked as a Retry. The 802.11 standard (the standard that Wi-Fi is built on) specifies that if data is sent and an acknowledgment is not received (thus indicating that a collision happened), then the device or AP that sent the data must mark the retransmitted data as a Retry. This means that the percentage of Retry data is equal to the percentage of data transmissions that suffer collisions.

Wi-Fi Analyzer not only identifies Retry data, but it allows Retry statistics to be gathered quickly and simply. To see the percentage of collisions over an entire channel, go to the CHANNEL screen of AirMagnet and open the little area in the middle of the screen labeled "Frames/Bytes".


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If the percentage of collisions relating to a single AP is required, that can be found in the INFRASTRUCTURE screen of AirMagnet Wi-Fi Analyzer. After going to the Infrastructure screen, click on an AP on the left hand side and then open up "Frames" or "Frames/Bytes" in the lower right Stats window. Retry statistics for stations can be seen from that same Stats window. Wi-Fi Analyzer allows the user to simply click on any station or AP listed on the left menu of the Infrastructure screen to immediately see statistics for that device.


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In order to put collision statistics to good use, one has to know what is considered a high Retry percentage. A good place to start is at 8% for ordinary Wi-Fi and 20% for difficult Wi-Fi (high density of users, lots of mobility or significant amounts of non-Wi-Fi interference). Once Retry percentages climb above those numbers, it is usually a good idea to set aside some time and investigate why so many retransmissions are happening.

The second big time-waster is low data rates (often called “speeds”). Data rates can be seen in the same general areas that Retry percentages can be seen. The only difference is that in order to see which data rates are being used, the “Speed” tree needs to be expanded after clicking on an AP or station device in the Infrastructure screen.

Determining whether low data rates are a fixable problem can take time and careful analysis. Devices and APs—especially 802.11n/ac devices and APs—routinely use data rates well below their stated maximum rates, even if interference is not significant. In other words, an office worker with an 802.11ac smartphone (maximum rate on a 40 MHz-wide channel: 200 Mbps) connected to an 802.11ac AP might routinely use data rates below 150 Mbps, even if the RF environment is great. 802.11ac (and, to a lesser extent, 802.11n) includes a lot of technologies that are rarely available during typical enterprise usage, even in areas where interference is minimal. For that reason, analyzing data rates to determine interference problems is a task that usually requires some experience.


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Thirdly (and lastly), non-data traffic can be the cause of a Wi-Fi channel losing time. There are lots of different types of non-data traffic, but most of them are mandatory for 802.11 operation, and thus cannot be eliminated. APs and stations have to exchange a number of types of non-data traffic to be able to stay connected, detect collisions and all sorts of other necessities for a wireless network to function.

There are, however, two types of non-data traffic that can sometimes be reduced: Beacons and Probes. Beacons are used by APs to let stations know that a Wi-Fi network is available. The problem is that each Wi-Fi network needs its own set of Beacons. If there are two SSIDs (one guest and one internal), then Beacons will typically take up between 2% and 5% of the available time on a channel. But if the number of SSIDs expands to eight (possibly by having unique SSIDs for different vendors or different groups of internal users), then Beacons will typically take up between 8% and 20% of channel time. That's a big difference. And the problem gets exacerbated if more than one AP is covering the same channel. If a given enterprise tablet can see three APs on channel 11 and all three of those APs are using eight SSIDs, then that makes for 24 sets of Beacons on the channel. That's 24% to 60% of your channel time that would be used for Beacons in that scenario. And all of that Beacon time is channel time that becomes unavailable for what matters most: data.

The place to identify an excess of Beacon traffic is in the CHANNEL screen of Wi-Fi Analyzer. Whenever a channel number is selected, the Channel screen shows how many APs and how many SSIDs in the lower right corner. Use that information to identify whether Beacons are using up channel time that would be better spent on data.


Two important benefits of using OptiView XG to identify Beacon overhead are its portability and its internal Wi-Fi radio. Beacon frames lessen available channel time only if they reach a device's Wi-Fi radio. OptiView XG can be carried to the location where users need Wi-Fi access, thus giving a more accurate measure of whether excess Beacons are affecting performance in a given area.


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Probes are the other type of non-data frame that can waste channel time. Probing is initiated by stations, so that means that the INFRASTRUCTURE screen of WiFi Analyzer is the place to identify whether they are causing a problem. Simply take OptiView XG to the location where Wi-Fi is subpar, navigate to the Infrastructure screen of AirMagnet WiFi Analyzer and click on a station on the left side of the screen. Then look at the Stats box in the lower right corner. In the Stats box, expand "Frames" or “Frames/Bytes” and then expand "Mgmt frames". The number of Probe Request frames sent by the selected station will then be displayed. If the number of Probes keeps increasing, then that device might be taking channel time for Probes that could better be used for data. If there is a Probing problem, try making sure that the station in question has a stable Wi-Fi connection. Modern Wi-Fi devices (smartphones, tablets, laptops, etc.) do very little probing if their Wi-Fi connection has stable access to the Internet.


Handling Wi-Fi interferers has some similarity to handling non-Wi-Fi interferers, but there are also big differences. It is always best to start by identifying and locating the interferer. After a Wi-Fi interferer is identified and located, the problem can often be minimized or eliminated by adjusting the wireless LAN infrastructure. Disabling AP radios, adding new APs in different locations, manually configuring AP channel numbers and setting AP transmit power to levels similar to those of client devices are all activities that can improve an infrastructure of APs and controllers. In contrast, non-Wi-Fi interferers often need to be disabled or avoided in order to get the Wi-Fi working.


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Following these steps using the AirMagnet Spectrum XT and AirMagnet WiFi Analyzer applications native to OptiView XG creates an excellent chance of preventing Wi-Fi interference from becoming a lasting problem. Using a spectrum analyzer and a network analyzer can take some getting used to, but it sure beats blind exercises in trial-and-error. Once you get used to using these tools, you may even be surprised at how fast formerly difficult interference problems get identified and resolved.

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