May/June 2016Volume 37, Number 3

THE OFFICIAL TRADE JOURNAL OF BICSIICT TODAY

+ Today’s DCIM Solutions + Optical Fiber to the Classroom + Raising Data Center Temperatures

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Why is it that Wi-Fi often works very well at home but fails in many enterprises and public venues? The main reason is the demand for increased capacity combined with poor Wi-Fi network design. Because only a few people are using the network at home, the Wi-Fi network does not need specific design considerations. But when it comes to public places, corporate offices and warehouses, Wi-Fi networks get more crowded as more devices join. There is also more variation between mobile devices including laptops, tablets, smartphones, Wi-Fi enabled hospital equipment, forklift computers, Wi-Fi phones and Wi-Fi-based location tracking tags. The network needs to be designed to support all of these devices. For an IT administrator, Wi-Fi can be implemented in two ways: u Either more attention is paid to designing the network; oruThe network is built quickly, but constant troubleshooting and adjustments are required.

Coverage Is King A high-capacity Wi-Fi network is one that allows more data to be transmitted wirelessly than a Wi-Fi network built just for coverage. Simply adding Wi-Fi access points (APs) does not necessarily mean high capacity— but it can, when combined with smart frequency reuse and other tips presented in this article. Whether or not it is designed specifically for high capacity, the Wi-Fi network must be designed for sufficient coverage. The level of coverage is a factor of the strength of the signal trans-mitted between the AP and the mobile device. If the signal is too weak, the connection will be unreliable or will break down completely. Oversimplified, the closer the mobile device is to the AP, the higher the signal strength. But it is not just distance that weakens the signal; everything that comes between the AP and the mobile device—building walls, people, furniture—attenuates Wi-Fi signals.

By Jussi Kiviniemi

We have all experienced Wi-Fi networks that failed to meet our expectations, causing us minor inconveniences. A Skype call drops at the airport. A Facebook update fails at a crowded stadium. But the consequences can be serious, too; imagine hospital equipment failing to communicate wirelessly.

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It is also imperative to note that signal strength is a two-way street. It is not enough that the mobile device can hear the AP; the AP needs to hear the mobile device, as well. Therefore, even if the AP radio power levels are increased to the maximum and the mobile device can hear the AP very well, the connection still might not work due to a failure from the mobile device back to the AP. In Wi-Fi networks, distributed antenna system (DAS) solutions are not commonly used. Instead, the antenna is located close to or, more typically, built into the AP (Figure 1). That, however, does not mean the coverage pattern is circular, as it is affected by walls, elevator shafts, and other objects. Because of this, specific Wi-Fi planning tools are used to design coverage that meets the user requirements. In areas that are challenging for radio network design, such as warehouses, the APs may be mounted 15 meters (m [50 feet (ft)]) up in the ceiling, while the client devices are on the floor level. In cases like these, directional

antennas are required to direct the signal toward the client devices. Antennas must be tilted horizontally, as well as vertically. 3D planning tools help in figuring out how to align the antennas. In the end, what is sufficient coverage? For a data-only network, it may suffice to provide a signal strength of -75 decibel milliwatts (dBm) or better. However, -75 dBm signal strength may not be enough to achieve crystal-clear voice call quality. Values ranging between -65 to -67 dBm are often referred to as industry standards for deploying high-quality voice networks. Coverage refers to the signal strength received from the strongest of the audible Wi-Fi APs. However, to ensure Wi-Fi connectivity when roaming from one AP to another, sufficient signal strength from the second strongest AP is also required. A typical design guideline would be -67 dBm signal strength everywhere for the strongest AP, and -75 dBm signal strength everywhere for the second strongest AP.

FIGURE 1: Example of simple Wi-Fi coverage planning.

FIGURE 2: 5 GHz channels in the U.S.

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Reprint with permission from ICT Today-May/June 2016 Issue

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FIGURE 3: APs placed in hallways and all 2.4 GHz radios enabled, causing co-channel interference (gray).

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Minimizing Channel Overlap Wi-Fi networks operate on two frequencies: 2.4 gigahertz (GHz) and 5 GHz. Both frequencies have a number of available, interference-free channels. 2.4 GHz has three interference-free channels, and 5 GHz has more than 20 in the U.S.—but all may not be usable (e.g., because of interference with radar systems [Figure 2]). Most Wi-Fi APs have two radios that operate simultaneously: one operating on 2.4 GHz frequency and one on 5 GHz (some of the newer APs are capable of configuration for dual 5 GHz operation). Mobile Wi-Fi devices typically support both 2.4 GHz and 5 GHz frequencies but operate on just one of them at a time. Many older devices, as well as Internet of Things (IoT) devices, support just 2.4 GHz, preventing many networks from disabling the 2.4 GHz frequency space altogether. Because 2.4 GHz frequency space needs to be supported, and it only has three non-overlapping channels to work with, channel planning becomes a bit of a challenge to achieve high network performance. In 5 GHz frequency space, the 20+ channels are often reduced down to a few due to radar, other networks, non-Wi-Fi interference, supporting legacy devices and especially the use of wider channels (one AP may take the channel space of two or even four APs).

The network should be designed to minimize the number of overlapping channels.

Ideally, this means that in every location, only one AP should be operating on channel number

1, for example. This sounds easy, but when there are just three channels to work with, plus

neighboring networks, plus interference from non-Wi-Fi devices, it becomes challenging, especially in

a multi-floor environment with thick walls and open spaces close to one another.

The key to mitigating co-channel interference is careful channel and transmit power allocation in

combination with coverage design. Designing for coverage—which is relatively easy using modern tools—

will be followed by fine-tuning the channels, as well as the locations of the AP and the transmission powers to minimize channel overlap. A rule of thumb for avoiding channel overlap in many indoor environments is the placement of APs in rooms instead of hallways. This way, the wall absorption reduces the AP signal fingerprint, leaving less chance for channel overlap (Figure 3). On the 5 GHz frequency, the IEEE® 802.11n standard introduced 40 megahertz (MHz)-wide channels as an option for the traditional 20 MHz-wide channels. 802.11ac introduced 80 MHz-wide and even 160 MHz-wide channels. The idea is simple: double the frequency, double the network throughput. However, if not used carefully, wider channels may lead to significant channel overlap, even on 5 GHz, and thus reduced, rather than increased, throughput. Especially in high-density network layouts, 20 MHz is still often recommended. 40 MHz is becoming more of a common choice, while 80 MHz is still being used selectively. With 2.4 GHz on enterprise wireless LANs (WLANs), 40 MHz wide channels should never be used. There are two limiting factors with regards to available Wi-Fi channels on the 5GHz frequency space: client device support and radar. Enter DFS (Dynamic Frequency Selection), a mechanism implemented for sharing the spectrum between Wi-Fi and radar. When an AP is operating on any channel between 52 and 1404, it is mandatory for the AP to monitor radar activity. If radar is detected, the AP must cease from transmitting data and switch to a different channel. When it comes to client devices, the so-called DFS channels can be completely

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unsupported by the device manufacturer, especially with older 5 GHz client devices. DFS support on the APs, as well as the client device channel limitations, calls for extra caution with Wi-Fi channel assignment, especially with outdoor Wi-Fi deployments. Since one AP typically includes both 2.4 GHz and 5 GHz radios, and the 5 GHz frequency space has multiple times more interference-free channels, it often becomes impossible to find a satisfactory channel plan for 2.4 GHz without turning off some of the 2.4 GHz radios. It may sound counterintuitive, but disabling some of the 2.4 GHz radios to minimize overlap actually increases capacity.

Penalty for Interference A question often heard is “Why does our wireless network become unusable at lunchtime?” Wi-Fi radios operate on license-free frequencies. Thisallows anyone to set up a Wi-Fi network, but it also means Wi-Fi needs to compete with other devices using these frequencies. Such devices include microwave ovens, wireless video cameras, Bluetooth®, baby monitors, home automation systems, radar and more.

Microwave ovens, for example, are wide-band inter-ferers from the Wi-Fi point of view. A microwave oven leaks radiation on all 2.4 GHz Wi-Fi channels. One oven does not kill the frequency entirely, but it has a significant impact on capacity for the nearby devices. Many wireless video cameras, on the other hand, utilize only a few of the 2.4 GHz channels, leaving room for a smart Wi-Fi engineer (or a smart Wi-Fi infrastructure) to cope with the situation by adjusting the channel separation of the Wi-Fi network. Interfering devices can easily be detected by a spec-trum analyzer (Figure 4). There are two types of spectrum analyzers, and a wireless engineer would ideally have both tools available:uA portable spectrum analyzer that can be connected to a laptop via USB. It is easily portable for performing site surveys and troubleshooting in desired locations. The downside is the spectrum analysis is periodic in nature at best, not constant.uA spectrum analyzer built into APs allows for constant monitoring of the spectrum once the network is up and running. However, the measurements are measured at the ceiling level, not where the users are.

FIGURE 4: Wi-Fi signals appear as curvy shapes in a spectrum analyzer.

TABLE 1: Properties of selected IEEE 802.11 standards.

STANDARD

802.11 (legacy)

802.11b

802.11a

802.11g

802.11n

802.11ac

FREQUENCY

2.4 GHz

2.4 GHz

5 GHz

2.4 GHz

2.4/5 GHz

5 GHz

TYPICAL AP MAX DATA RATE

2 Mb/s

11 Mb/s

54 Mb/s

54 Mb/s

450 Mb/s

1.3 Gb/s

SIGNIFICANCE

The first WLAN standard

Made WLAN popular

The first 5 GHz standard

Faster speeds on 2.4 GHz

Wider channels, more spatial streams = more speed.

Even wider channels, even better modulation = even more speed.

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On and Off the Network Quickly When it comes to Wi-Fi, one device talks at a time per channel. That means when the AP is sending data to one of the client devices, everyone else is waiting. Then, a single client device might send data to the AP, while everybody else waits for their turn. This is why it is ideal to get the devices on and off the airwaves very quickly. The better this “data rate” of transmission, the faster the job is done. High data rates require high signal-to-noise ratio (SNR), i.e., high signal strengths and low noise combined. In addition, the network simply needs to have a fairly modern wireless network infrastructure that supports high data rates to begin with. The same is true for client devices. The newer IEEE 802.11n and 802.11ac standards have introduced multiple input multiple output (MIMO, which sends multiple data streams simultaneously between the client and the AP) and enhanced modulation schemes that allow multiple times the data rates compared to older 802.11a/b/g standards. At times, the client devices may be connected to APs that are farther out, for whatever reason, and operating at low data rates. Similarly, old client devices (802.11a/b/g) only support low data rates. One possible solution is to disallow low data rates from the APs. This means high data rates, at the cost of losing support for very old client devices, and client devices will not be able to connect far away from the APs. It is important to note that data rate is not the same as throughput. Rather, it is the theoretical maximum at which the client and APs are able to communicate. Throughput is the amount of actual data being transmit-ted. For example, high data rates between a single client and an AP may be achievable at times even on a badly designed network, but the total simultaneous throughput of all the client devices will suffer dramatically if proper coverage and channel planning has not been done.

High Capacity Considerations How should the demand for high wireless capacity be addressed? Following network design best practices is a good start. Here is an example of a process for designing high-capacity Wi-Fi networks:1. Determine the business needs: What are the problems to be solved by utilizing Wi-Fi?2. Specify technical requirements: How many users are there? What kind of devices, and how many, do they have? What kind of applications are they using, and how often?

3. Calculate the need for infrastructure: Based on the number or client devices, their bandwidth require- ments and capabilities, calculate the number of required APs to support the usage. 4. Predictive design: Map out the exact locations, configurations and antennas for the APs. Get simulated heat maps to ensure sufficient network coverage and SNR, and minimize channel overlap based on the walls of the building.5. Network deployment: Install and configure the wireless infrastructure and the wired infrastructure to support it.6. Verification site survey: Walk around the facility with a site survey tool to map out the network and ensure sufficient Wi-Fi coverage and perform- ance. A passive survey is always required; preferably, this is complemented by an active survey, a throughput survey, and a spectrum analysis survey. 7. Maintenance: Every network needs maintenance. If it is well designed, it needs very little maintenance. A badly designed network requires constant trouble- shooting and fine-tuning. Conclusion To design a high-quality Wi-Fi network that supports the increasing demand for capacity: uPay attention to designing the Wi-Fi network to avoid constant troubleshooting and network modifications. A predictive network design (simulation) accounting for wall materials is a good start. uCalculate the need for capacity based on the number of users, their devices and their applications.uAchieve high data rates by ensuring high signal strength, utilizing fairly modern network gear, minimizing noise and verifying the network with a site survey.uMaximize airtime by minimizing channel interference and the number of service set identifiers, or SSIDs, as well as by preferring the 5 GHz frequency. t

AUTHOR BIOGRAPHY: Jussi Kiviniemi oversees the Wi-Fi Design Tools business at Ekahau. An industry pioneer and a familiar face in the Wi-Fi (WLAN) space, he has been featured in publications from IEEE papers to TheStreet.com, and presented at dozens of conferences, including BICSI’s largest events, SCTE, Wireless LAN Professional Conferences, and CWNP (Certified Wireless Networking Professional) conferences. Jussi holds an M.S. in industrial engineering and management. He can be reached at jussi.kiviniemi@ekahau.com.

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