Техническое описание: Ключевые вопросы для поддержки BYOD | enterprise.netscout.com

Техническое описание: Ключевые вопросы для поддержки BYOD

This paper is designed to help networking people understand some of the key issues when supporting BYOD. The paper will cover important — and sometimes overlooked — aspects of supporting a variety of Wi-Fi devices, as well as specific solutions and strategies for BYOD support, including those offered by NETSCOUT's wireless LAN analysis solutions.

    TABLE OF CONTENTS
  • Introduction
  • Different Devices, Different Capabilities
  • MIMO Support
  • Diversity Antennas
  • Transmit Power
  • Roaming Behavior
  • Remember the LAN
  • Conclusion


INTRODUCTION

When a salesperson sells something, it's often a win-win. Both parties — buyer and seller — walk away happy. The person who likes mint chocolate chip ice cream enters the ice cream shop, sees his or her favorite flavor, and gleefully chooses. The buyer gets ice cream, the seller gets money and both parties leave satisfied. Salespeople get to enjoy win-win transactions all the time.

Networking professionals, on the other hand, are often faced with win-lose situations when working with users. Users often want something that makes life more difficult to networking folk. Users may want faster Internet access, but networking people understand the cost. Users may want unfettered access, but networking people need to keep it secure. Users may want to be able to use their own devices, but networking people know that BYOD (bring your own device) makes managing Wi-Fi more challenging.

In many cases it is fruitless to fight BYOD, because the users have already won. For many companies, once they see the benefits of BYOD — higher productivity, happier users, less money spent on device acquisitions — having networking people who can support BYOD becomes essential.

This paper aims to guide network professionals through some of the do's, don'ts and decisions that often come up when BYOD is deployed. For network folks who have already deployed BYOD, we've got you covered, too. The practices covered in this paper — which include tips specific to NETSCOUT's OptiView XG tool — can also be used to optimize a BYOD-supporting wireless network as well.


A Master Craftsman Needs a Master Tool

Much like a sheet metal worker carrying a toolkit to a construction site, a Wi-Fi worker should enter a BYOD environment equipped to do the job. In many cases, that can mean a confusing array of software, devices and adapters.

An effective Wi-Fi troubleshooting toolkit requires three distinct tools. A network analyzer like NETSCOUT's AirMagnet WiFi Analyzer is necessary for troubleshooting performance issues. A spectrum analyzer like NETSCOUT's AirMagnet Spectrum XT is great for gauging interference levels. A third tool is usually needed to monitor received signal strength (RSSI) and identify areas where APs have been over-deployed. (The third tool is needed because AirMagnet WiFi Analyzer is most commonly run using a USB or ExpressCard adapter. Internal Wi-Fi adapters are necessary when looking for low RSSI or AP over-deployments because users' devices use internal Wi-Fi adapters.) Using a toolkit with all of these different devices and adapters can be cumbersome, especially since proper BYOD surveying, maintenance and troubleshooting requires that these tools be used in the locations where the users are.

OptiView XG is a three-in-one Wi-Fi analysis tool. AirMagnet WiFi Analyzer and Spectrum XT come pre-installed, and both applications operate using internal radios within the OptiView XG. That means there's no need for a separate USB and/or ExpressCard adapter. In addition, a third tool used for identifying low RSSI or AP over-deployments may be unnecessary because OptiView XG has an internal Wi-Fi radio. (We say "may" because some internal Wi-Fi adapters may act very differently than others. We recommend testing the OptiView XG's internal Wi-Fi radio in comparison to a user device's internal Wi-Fi radio before using the OptiView XG to identify areas with low RSSI or an over-deployment of APs.) Combined with OptiView's network troubleshooting features, it makes for the most elegant Wi-Fi troubleshooting solution available today.



Different Devices, Different Capabilities

The most important thing to keep in mind when supporting BYOD is that different Wi-Fi devices behave differently. All Wi-Fi devices support the 802.11 standard, but there are many variations allowed within that standard.

Differences between different Wi-Fi devices may include the following:

Standards support: 802.11ac is the most modern Wi-Fi standard, but an enterprise BYOD network is almost certain to have to support other standards. 802.11ac is exclusively a 5 GHz standard, while 802.11n, which made its debut in devices in 2007 and was officially approved by the institute of electrical and electronic engineers (IEEE) in 2009, is the most modern standard for Wi-Fi in the 2.4 GHz frequency band. Modern enterprise wireless LANs typically support both the 2.4 GHz and the 5 GHz frequency bands simultaneously, and most Wi-Fi devices used in enterprises support 802.11n, 802.11ac or both. The 802.11a and 802.11g standards, which were common in the mid-00's, may also need to be supported on wireless LANs. Some wireless LANs have non-traditional devices that are replaced less frequently than typical smartphones, tablets and laptops. The 802.11b standard, which dates back to the 1990's, may have to be supported in some cases, but is increasingly eschewed in modern enterprise Wi-Fi deployments.

The beauty of OptiView XG is that it includes AirMagnet WiFi Analyzer, along with a 3-stream, 802.11ac radio that is pre-configured for Wi-Fi capture. OptiView XG runs AirMagnet WiFi Analyzer in a way that allows for instant detection and classification of Wi-Fi devices that connect to the network. This capability allows OptiView XG users to authorize these devices, quickly troubleshoot and resolve issues caused by these devices as well as determine performance and security impact to the WLAN network.

Using AirMagnet WiFi Analyzer on an OptiView XG to identify standards support in a BYOD environment is a simple process. After opening AirMagnet WiFi Analyzer on an OptiView XG, the Infrastructure screen can be accessed via navigation buttons in the lower-left corner. In the Infrastructure screen, all devices are accompanied by a one or two letter sequence indicating which standard the device supports. WiFi Analyzer culls this information from management frames that are captured from Wi-Fi device transmissions. The 3-stream, 802.11ac radio of OptiView XG ensures that all management frames can be captured, since 802.11ac is backwards compatible with all earlier Wi-Fi standards; 802.11a, 802.11b, 802.11g and 802.11n.


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MIMO support

Multiple-input, multiple-output (MIMO) antennas are antenna systems that allow devices to either use data rates that are two or three times higher or improve consistency and range. Device support for MIMO may be revealed in a device's data sheet, but in some cases a protocol analyzer, like the AirMagnet WiFi Analyzer that comes pre-installed on OptiView XG, may be required to identify a device's MIMO capabilities.

MIMO antenna systems can improve Wi-Fi connections in one of two ways: by improving data rates or by enhancing consistency and range. What MIMO technology does is it allows for two or three streams of data to be transmitted and received simultaneously. Essentially, the same frequencies are being re-used two or three times at the exact same moment by two or three different antennas.

One way that MIMO antenna systems can benefit Wi-Fi devices is by using data streams that are unique. In that case, data rates are doubled (when using two streams) or tripled (when using three streams). This is called spatial multiplexing.

If MIMO data streams are identical, then consistency and/or range may be improved. A number of MIMO-based technologies use identical data streams, including maximal ratio combining (MRC), space-time block coding (STBC) and transmit beamforming (TxBF).

The bottom line is that device support for MIMO is good for enterprise wireless LAN performance. Either the device will use higher rates (thus improving the overall throughput capabilities of the wireless LAN) or the device will improve its range and/or efficiency. (Again, MIMO improves EITHER speed or range/consistency, but not both at the same time.



OptiView XG can use AirMagnet WiFi Analyzer to identify whether MIMO is supported, and boy is it simple. When using other protocol analyzers, a capture on the AP's channel would be required and the HT Capability information element of Probe Request frames would have to be manually inspected. It can be a complicated, time-consuming process. An OptiView XG running AirMagnet WiFi Analyzer, on the other hand, allows MIMO capabilities to be viewed in real-time by simply right-clicking the device in the Infrastructure screen and selecting 802.11n/ac Efficiency. After selecting 802.11n/ac Efficiency, WiFi Analyzer will automatically switch to the WiFi Tools screen. The Efficiency tool will automatically have been selected and Uplink information will automatically be shown. The Uplink information indicates how many MIMO streams – if any – are supported by the selected Wi-Fi device.

A Wi-Fi device that supports more MIMO streams supports higher data rates, but the impact of higher rates is sometimes misunderstood. Higher data rates do not necessarily guarantee higher Internet or LAN throughput levels for individual devices. Rather, higher data rates require that less RF channel time is used when a devices transmits or receives data. That means that higher data rates improve Internet/LAN throughput for the entire channel, not just for an individual device. In short, higher rates are always a good thing for Wi-Fi. Even if an enterprise has strict restrictions on guest bandwidth (say, a 2 Mbps cap on Internet speeds), data rates should be kept as high as possible for all devices so that LAN and Internet throughput is maximized for internal users.


MIMO App Screen-2

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Many Wi-Fi devices lack MIMO support, but MIMO support is increasing. Starting with the iPad Air, Apple tablets began supporting two-stream MIMO. Same for Apple smartphones beginning with the iPhone 6s. Laptops and desktops have long supported MIMO, though support varies between two-stream, three-stream. Lack of MIMO support is most commonly seen in single-function devices (barcode scanners, smart watches, etc.), older tablets, some "netbook" laptops and less expensive smartphones.



Diversity antennas

Diversity antennas improve the consistency and reliability of Wi-Fi by using multiple antennas to receive transmitted radio waves.

MIMO support and diversity antenna support often go hand-in-hand, but not always. Devices may support diversity antennas without supporting MIMO. Because MIMO uses multiple radio chains to transmit data simultaneously over a single RF channel, MIMO causes reduced battery life. Some larger devices, like tablets and “netbook” laptops, may support diversity antennas, but not MIMO.

Smartphones, due to their relatively small size, typically do not support MIMO or diversity antennas. Both MIMO and diversity antennas require that antennas be placed a significant distance apart from each other within a device, and smartphones simply lack the space to make either antenna technology work. (Though it should be noted that both antenna technology and smartphone technology often improve, so diversity antennas could become available in smartphones some day.)

Unfortunately, there is no requirement that Wi-Fi devices indicate whether diversity antennas are supported when transmitting frames. If MIMO is supported (see previous section), then diversity antennas will definitely be supported. If MIMO is not supported, then the Wi-Fi device vendor documentation would have to be consulted in order to verify support for diversity antennas.

Transmit power

Wi-Fi usually works best when APs use the same transmit power as the devices that connect to them. The problem is that different models of smartphones, tablets and laptops often have differing transmit power levels.

Matching the transmit power of an AP with its associated Wi-Fi device works best because Wi-Fi is a two-way method of communication. For example, before a web page is downloaded (a transmission from the AP), a Wi-Fi device must send an HTTP request (a transmission from the device). While there are many applications that create more downlink traffic than uplink traffic, Wi-Fi still tends to work best when transmit power matches. This is because matching AP and device transmit power tends to minimize the number of retransmitted data frames. Retransmitted data frames can be very damaging to a Wi-Fi channel because every retransmitted frame creates wasted channel time. Wasted channel time ultimately limits the speed and consistency of the Wi-Fi channel for all APs and stations.



The AirMagnet WiFi Analyzer application running on OptiView XG can be used to identify whether AP transmit power is approximately equal to the transmit power of a Wi-Fi device. This makes OptiView XG especially useful when users need to access the network via a variety of Wi-Fi devices. Instead of having to procure and analyze data sheets for numerous devices, a relatively quick and simple analysis of Wi-Fi retransmission patterns may reveal whether AP transmit power is set to an appropriate level.

Setting up an OptiView XG for testing transmit power matches requires quite a few steps, but is still far less time-consuming than using a traditional protocol analyzer. To set up an OptiView XG for testing, open AirMagnet WiFi Analyzer and navigate to the Infrastructure screen. Once there, click on the AP that needs to be tested. At this point the 3-stream, 802.11ac radio within OptiView XG will begin capturing on the channel that the AP is using. To the left of the AP, there will be a [+] icon that can be used to display a list of devices that are associated to the AP. The final setup step is to look in the lower right corner of the Infrastructure screen and change the dropbox labeled "Rx Total/Tx Total" to "Tx Total/% Total". That allows transmitted traffic statistics to be viewed by percentages.

The power of AirMagnet WiFi Analyzer running on an OptiView XG is really noticeable once testing begins. To test whether AP transmit power matches device transmit power, simply click on one of the Wi-Fi devices that is listed under the AP in the Infrastructure screen of WiFi Analyzer. At this point, Wi-Fi Analyzer will be showing only statistics that are related to the Wi-Fi device that has been selected. With the Wi-Fi device selected, open the Frames category in the lower right area of the Infrastructure screen to view the percentage of Retry frames. Then record that percentage. The same steps can be repeated for every other device that is associated to the AP. Just keep clicking different devices and keep recording the Retry percentages that are displayed in the lower right area.

The ability to quickly view Wi-Fi traffic statistics for different devices is a big time-saver and a big reason why having an OptiView XG running AirMagnet WiFi Analyzer can be such an effective tool for optimizing wireless LAN performance in a BYOD environment. Other protocol analysis products can capture the same traffic that OptiView XG captures, but AirMagnet WiFi Analyzer software is the only wireless protocol analyzer that allows for switching between different devices' statistics in such a fast, simple way.

Once Retry percentages for device transmissions have been recorded, then AP transmissions must be analyzed. When in the Infrastructure screen of AirMagnet WiFi Analyzer, simply change the dropbox in the lower right area to "Rx Total/% Total" and to view the percentage of received Retry frames for a device. This number indicates the percentage of frames that failed when the AP was doing the transmitting.



The final step when testing whether AP transmit power is appropriate is to compare Retry percentages. If the transmitted and received Retry percentages for a given Wi-Fi device match, then the transmit power of the AP is approximately equal to the transmit power of that device. If the received Retry percentage is higher, then the AP transmit power is too low and if the transmitted Retry percentage is higher, then the AP transmit power is higher than the Wi-Fi device's transmit power.

Of course, the situation may be that AP transmit power is appropriate for some devices, but inappropriate for others. In those situations, choosing an AP transmit power between 12 dBm and 15 dBm is a good place to start.

One last note on checking for AP transmit power is that, as of this writing, Apple devices use a protocol that tends to lead to artificially low Retry percentages when transmitting. The protocol; called request-to-send/clear-to-send (RTS/CTS), often improves stability in high-density BYOD environments. It can also, however, create low Retry percentages that make OptiView XG testing more complex than the way it is described above. The testing steps remain the same, but the person doing the testing will likely need some experience analyzing Apple devices to know which Retry percentages indicate a transmit power mismatch.

If AP transmit power does need to be adjusted, OptiView XG can help with that, too. OptiView XG has a built-in web browser and command line interface. Having web and CLI capbilities may seem like a little thing, but making a configuration change and then being able to test that change immediately; all in the same tool, is a great capability to have.

XG Device Tools

XG Device Tools Screen-2

Probing behavior

When devices are unconnected to a Wi-Fi network, they send Probe Request frames as a way to search for nearby APs.

The problem with probing is that some devices send so many probe request frames when unconnected to Wi-Fi, that the device ends up using MORE Wi-Fi channel bandwidth when its unconnected than when it’s connected.

Roaming behavior

There is good news and bad news about roaming behavior.

The bad news about roaming behavior is that it is one of the most difficult parts of supporting a BYOD environment. Not only do different devices roam differently, but sometimes a single Wi-Fi device will change its roaming behavior because a different application is running, because there are too many APs nearby or because the motion sensor within the device detects movement. Roaming behavior is really hard to predict.

The good news about roaming behavior is that testing for it can be done by looking for Wi-Fi devices engaging in Probing. Probing is the process by which Wi-Fi devices actively scan channel(s) for APs. When a device begins Probing, that is an indication that the device has started the roaming process. An OptiView XG running AirMagnet WiFi Analyzer can be used to test for probing behavior, which thus gives insight into a device's roaming behavior.

In modern Wi-Fi devices, received signal strength indicator (RSSI) triggers the roaming process. Pay attention to the RSSI reading on the Wi-Fi device (usually found in the device's Wi-Fi settings, or possibly in a Scanner utility running on the device) while moving it as part of a roaming test.

OptiView XG shows Probing behavior in the Infrastructure screen of AirMagnet WiFi Analyzer. Once in the Infrastructure screen, the Wi- Fi device must be selected on the left side of the screen. After the Wi-Fi device has been selected, the number of Probe Request frames can be viewed in the Stats area in the lower right area of the Infrastructure screen. Probe Request stats are shown under Frames, and then under Mgmt Frames.

Once the RSSI at which users' devices begin Probing has been determined, then the infrastructure may be designed or adjusted to support device roaming. Most Wi-Fi devices roam well with an 8 dB cell overlap. That means that if a device initiates Probing at -72 dBm, the device is likely to roam smoothly if the device sees a different AP giving an RSSI of at least -64 dBm.

One last note about supporting roaming in a BYOD environment is to expect the unexpected. A Wi-Fi device that starts probing at -72 dBm in a lab may start probing at a different RSSI once it is being used in the field. Supporting roaming is one of the more difficult parts of managing a BYOD environment because device behavior can be unpredictable. So, plan on using WiFi Analyzer more than once to understand how different devices handle roaming.



Remember the LAN

Creating a Wi-Fi network that supports BYOD requires that attention be paid to wireless infrastructure, but without a great wired infrastructure users will be unsatisfied. OptiView XG includes LAN testing and analysis applications that make it a true all-in-one tool for enterprise networks.

The network testing and analysis features of the OptiView XG are simple and intuitive. A trace tool allows the OptiView XG user to see the network path that wireless data takes through the switching infrastructure. A speed testing tool allows for single-node throughput testing, which can be valuable when requirements and service level agreements need to be satisfied. An overview of the network architecture can also be accessed, which can be especially helpful when befuddling problems arise for a single AP. Whether assessing a broad network overview or dialing into a specific device's issue, the OptiView XG tool's unique ability to blend wireless and wired analysis makes it fit for the job.


Wi-Fi Path Analysis

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Network Mapping

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Conclusion

This paper has covered a number of ways in which Wi-Fi devices vary, but there may be other variations that crop up as wireless LANs continue to evolve. It is hard to predict the future accurately, but one thing seems certain: different Wi-Fi device will continue to be different. It has always been that way, and makers of Wi-Fi devices have shown no signs of gravitating towards uniform behavior. For that reason, it can be useful to take some time and analyze the different behaviors of Wi-Fi devices, so that they can be better supported in a BYOD environment.




NETSCOUT operates in more than 50 countries worldwide. To find your local office contact details, go to www.netscout.com.


 
 
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