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WLAN Design: Range, Performance, and Roaming Considerations

Chapter Description

This chapter provides insight and addresses important elements you should consider to avoid common problems when designing a wireless LAN.

Range Considerations

To satisfy signal coverage requirements, you will need to install access points in optimum locations based on the results of a wireless site survey, as explained in Chapter 15, "Performing a Wireless Site Survey." This involves completing propagation tests that determine the range of the access points based on specific minimum signal levels sufficient to support required client devices and applications. 802.11n systems provide much better range and performance than legacy 802.11b and 802.11g networks, but there is fine-tuning that you can do. To maximize range and reduce the total number of access points, carefully consider the following design elements:

  • Signal coverage requirements
  • Radio frequency bands
  • Transmit power settings
  • Transmission channel settings
  • Data rate setting
  • Antennas
  • Amplifiers
  • Repeaters
  • Physical obstacles
  • Radio signal interference

Signal Coverage Requirements

Review requirements that define the environment where the WLAN will operate and areas where signal coverage is needed. This will give you a feel for the importance of maximizing the range. If the WLAN must provide signal coverage over a large open area where it is not feasible to install access points, for example, the use of higher-gain antennas and possibly amplifiers may prove worthwhile.

Radio Frequency Bands

As part of the design, you can choose to use 2.4-GHz or 5-GHz (or both) 802.11n bands. Communications theory explains that (with all other things constant), an increase in transmission frequency causes a decrease in range of the signal. As a result, the higher transmit frequencies of the 5-GHz band should provide shorter range than the lower 2.4-GHz band. In practice, the use of 5-GHz 802.11n, though, might or might not provide shorter range. In fact, sometimes a 5-GHz 802.11n system provides the same or even greater range as compared to a 2.4-GHz system. This might occur, for example, if the noise in the 2.4-GHz band is considerably higher than in the 5-GHz band (which is often the case). The resulting signal-to-noise ratio (SNR) values of the 5-GHz system, despite a decrease in signal strength due to higher operating frequencies, might be higher due to much less noise in the 5-GHz band.

Figure 11-1 illustrates a case where the range of a 2.4-GHz and 5-GHz access point are the same. At the client device (laptop) associated with the 2.4-GHz access point, the signal level is –70 dBm, and the noise is –85 dBm. This results in a SNR of 15 dB, which of course indicates a specific level of performance. Because of higher operating frequency, the signal at the client device associate with the 5-GHz access point is significantly lower at –80 dBm (an arbitrary number chosen for illustration purposes only). Because of the much lower noise level (–95 dBm) in the 5 GHz band, the SNR for the 5-GHz system is also 15 dB, which would likely provide similar performance as the 2.4-GHz system. This indicates that the 2.4-GHz and 5-GHz systems have the same range, which is a probable outcome depending on difference in noise levels between the 2.4-GHz and 5-GHz bands. Therefore, be certain to take into account the actual environment where the WLAN will operate choosing 2.4-GHz or 5-GHz bands based on range requirements and expectations.

Figure 11-1

Figure 11-1 A Case Where the Range Is the Same for a 2.4-GHz and 5-GHz System

Transmit Power Settings

For a constant performance levels, increasing the transmit power of an 802.11 radio increases range. As the transmit power increases, communications at a particular data rate, such as 12 Mb/s, will be possible at greater ranges. The reason for this is that increasing transmit power improves the SNR at points farther away from the radio. The range expands to cover areas where increases in the transmit power causes the SNR at points farther away to be at or above the minimum signal values needed to for reliable operation. This higher SNR allows the end radios to receive communications at these farther points where they might not have been able to before.

Figure 11-2 illustrates this point. With the access point tuned to 10 dBm, the SNR at Location A is 15 dB, which for this example we will assume is the signal strength necessary for reliable communications at 12-Mb/s data rates. At Location B, the signal level is –76 dBm due to free space loss, attenuation, and so on, which results in a 9 dB SNR. Therefore, at Location B, the access point tuned to 10 dBm will support something less than 12 Mb/s. If you increase the access point transmit power to 16 dBm (a 6 dB increase), the signal level at Location B will increase by 6 dB to –70 dBm. This makes the SNR at Location B equal to 15 dB, which allows reliable 12-Mb/s operation. As a result, increasing the transmit power has made it possible to extend the range for a specific data rate.

Figure 11-2

Figure 11-2 Transmit Power Increases Provide Greater Range by Increasing Signal Strength

This increase in range, however, only impacts the communications in one direction, which is the outward path relative to the radio with increased transmit power. The increase of transmit power of an access point, for example, only improves the range of the communications path from the access point to the client radios. To improve the overall communications of 802.11 signals, which occur in both directions between the access point and client radios, you will need to increase the transmit power of the client radios as well. In fact, it is usually not useful to increase the transmit power of the access points because the client radios are almost always operating at much lower transmit power. As a result, it will likely only be worthwhile to increase the transmit power of the client radios. The signals going from the access points to the client radios are already relatively strong. The increase in client radio transmit power alone will improve the range of the overall communications in this case.

An advantage of using transmit power changes to improve range is that there are no expenses for additional hardware. A problem, however, is that it might not be possible to increase the transmit power on the client radios (the devices that would likely need a boost in power) because they might already be set by default to the highest power. Also, if you have little control over the client devices operating on the network, such as the case with public networks, you might not have the ability to change the transmit power of some or all the client radios.

Transmission Channel Settings

Within each of the 802.11 frequency bands, specific operating channels span from the lower-frequency end of the band to the higher-frequency end of the band. These channels use different transmission frequencies, but there negligible impact on range from using the lower-frequency channels versus the higher-frequency channels based on the theory that increases in frequency causes shorter range (and vice versa). For example, choosing channel 1 versus channel 11 in the 2.4-GHz band has no significant impact on range. There is not enough spread in the frequencies across the band to make a notable difference in range.

The choice of transmission channel settings does, however, make a difference on range if it is possible to choose a channel to avoid radio signal interference. An access point, for example, may be set to channel 11, but you might notice from a spectrum analyzer that there is significant interference in the upper part of the band (including channel 11). The lower part of the band, channels 1 through 3, may be relatively free from interference. By changing the access point to channel 1, it is possible to improve the SNR throughout the area, which improves range. For example, the noise levels relative to channel 1 might be 6 dB lower than what it had been for channel 11. As a result, the signal level can be 6 dB lower and still constitute acceptable SNR. Figure 11-3 illustrates this concept.

Figure 11-3

Figure 11-3 Transmission Channel Changes Can Provide Greater Range by Lowering Noise Levels

As with transmit power, changing the transmission channel to improve range does not cost anything in terms of new hardware. In addition, with infrastructure WLANs (ones with access points), there is no need to make changes to client radios. As a result, transmission channel changes can be made on networks where you may have little or no control over the client radio settings. Keep in mind, however, that there is a limit of nonoverlapping channels available (especially in the 2.4-GHz band) and radio signal interference may change over time. For example with larger WLANs, you might have significant inter–access point interference if you only set access points to lower-frequency channels to avoid the interference present in the higher-frequency channels.

Data Rate Settings

At first, it may seem that data rate settings only impact the performance of a WLAN. The data rate settings, however, has an indirect impact on range. As a general rule (with everything else constant), an increase in data rate causes a decrease in range. Therefore, data rate and range are indirectly proportional. The reason for this is that higher data rates require higher received signal strength at the radio for the receiver to decode the 802.11 signal. Companies that make access points and client radios publish the received signal strength that supports various data rates.

As a result, be careful when adjusting the data rate in access points and client radios. By default, access points and client radios usually have their data rate setting configured as "auto" so that they will rate shift as needed to maintain associations with users. This is usually the best setting for general usage. Most access points also allow the data rate to be set to specific values, such as 12 Mb/s. If it is desirable to maximize range, consider setting all access points and client radios to low data rates. Just be sure that data rates are set high enough to provide adequate levels of performance.

Something to realize is that the setting in the access point only applies to the data rate that the access point uses, not the wireless clients. For example, setting the access point to 54 Mb/s causes the access point to transmit all data frames at 54 Mb/s. In this situation, wireless clients set to auto still continue to use higher data rates if possible. To extend range by forcing lower data rates, set both access points and user radio cards to the lower data rate configurations. That ensures that the data rates are the same in both directions.

As you lower the data rate settings, the range should increase where the connection is lost between the access point and the client device. The reason for this is that, as explained earlier, 802.11 radios have better receive sensitivity at lower data rates, which allows the radios in the client device and access point to be farther apart.

Antennas

The factory-default antennas that come with an access point usually have low gain (around 2dB). If the access point has removable antennas, replacing the default antennas with higher gain omnidirectional or directional antennas boosts range. For example, replacing a standard 2 dBi antenna with a 6 dBi omnidirectional antenna effectively adds 4 dB to the signal strength throughout the coverage area. As shown in Figure 11-4, the result of adding this gain improves the signal strength at Location B enough to maintain 15 dB SNR as compared to only 9 dB is using the standard 2 dBi antenna. Therefore, the increase in antenna gain has provided greater range for a specific data rate that corresponds to 15 dB SNR.

Figure 11-4

Figure 11-4 Higher-Gain Antennas Boost Range by Increasing Signal Strength

A higher-gain antenna, installed for instance on an access point, improves range from the access point to the client radio and from the client radios to the access point. This is different from increasing transmit power on only the access point, which would only increase range for the communications going from the access point to the client radios. The reason that a higher-gain antenna improves range in both directions is that the higher gain of the antenna improves both transmission and reception of radio waves. Therefore, the installation of higher-gain antennas can provide significant increases in range without making changes to the client radios.

In addition to using higher-gain antennas, antenna diversity can also help extend range in both directions because it minimizes multipath propagation. Diversity is an important part of 802.11n, and various vendors sell 802.11n access points and client radios that have different levels of diversity. If your intent is to maximize range, choose components that have high levels of diversity.

An advantage of using higher-gain antennas or diversity is that it impacts range in both directions. As a result, you may be able to get by with changing the antenna configuration on only the access point, avoiding the need to alter each client radio. The cost of upgrading the antennas, however, might be considerable (possibly $50 to $100 or more per access point). Therefore, the cost might be prohibitive in larger networks.

Be sure to take into account different antenna gain and diversity with actual propagation testing in the target operating environment to determine the lowest overall cost of deploying the network. For example, you might find that the use of standard 2 dBi antennas may require the need for 100 access points, and the use of 6 dBi antennas may reduce the number of access points to 80. The addition costs for 6 dBi antennas in this example would probably be $4000 to $8000 ($50 to $100 each for 80 access points). This additional cost is likely much less than the 20 additional access points that you would need to purchase if going with the standard 2 dBi antennas. As a result, in this example, it would be worth the additional cost for the 6 dBi antennas. This assumes that the goal is to maximize range.

The trouble with increasing antenna gain for purposes of extending range is that you will likely place the access points farther apart (of course to reduce the number of access points and reduce costs). This results in a larger 802.11 collision domain, which limits the capacity of the WLAN. More client devices end up connecting to fewer access points.

Amplifiers

The use of an amplifier is a way of increasing range. Similar to increasing the transmit power on the access point (or client radio) an amplifier boosts the signal strength throughout the coverage area, as illustrated in Figure 11-5. In addition, amplifiers have receive gain, which amplifies the incoming signals coming from the client devices. Therefore, the signal strength increasing behavior of an amplifier is similar to that of an antenna.

Figure 11-5

Figure 11-5 Amplifiers Improve Range by Increasing Signal Strength

Most WLAN amplifiers are rated at a specific transmit power, such as 200 mW, and a specific receive gain, such as 15 dB. Amplifiers with higher transmit power produce greater range for communications going from the access point to the client radios. The receive gain will increase the range for communications going from the client radios to the access point. Therefore, be sure to take both transmit power and receive gain of the amplifier into account when determining which one to use. Of course, as with anything else, it is always a good idea to do some propagation testing with amplifiers to realize their actual impact on range.

Companies such as Hyperlink and RF Linx sell amplifiers for WLANs. These amplifiers are designed to install between the antenna and the access point. As a result, you can use an amplifier only if it is possible to remove the access point antenna.

Repeaters

A WLAN repeater is meant to reside between access points and client radios and regenerate signals it receives. As a result, a repeater increases range between access points and client radios (see Figure 11-6). A repeater might double the range, but it can significantly reduce the capacity of the WLAN because the repeater retransmits data frames it receives. This causes a duplication of data traffic, which reduces the overall capacity by up to 50 percent. This could be a problem if performance is important. Also, a repeater requires electrical power, which might be costly to install. Because a repeater does not connect to the distribution system, Power over Ethernet (PoE) is not an option.

Figure 11-6

Figure 11-6 Repeaters Extend Range Between Access Points and Client Radios

Physical Obstacles

Certainly physical obstacles may be present within the operating environment of the WLAN, and these obstacles offer varying amounts of attenuation. To improve range in some areas, consider installing access points in locations that avoids obstacles. If possible, even consider moving some obstacles if it is advantageous (and feasible) to improve range. This effectively improves the signal strength throughout the applicable areas.

For example, signal coverage may be needed in an office where there are ceiling high filing cabinets along one of the walls in the office. These cabinets will likely highly attenuate RF signals. As a result, it would be wise to position one or more access points to cover the office so that the signals do not travel through the cabinets.

Radio Signal Interference

As discussed earlier in this chapter, a way of avoiding existing interference is to use frequencies where interference is less, such as using the 5-GHz frequency band or turning the access point to channels having lower interference levels. By reducing radio signal interference, you can increase range between access points and client radios, as illustrated in Figure 11-7. Another similar way of improving range is to remove the source of interference. For example, it might be possible to decrease the noise levels by 6 dB in the 2.4-GHz band by stopping the use of 2.4-GHz cordless phones (and possibly switching to 5-GHz or 6-GHz phones instead). Also you could use RF shielding to reduce noise. Special RF shielding paint and wallpaper is available that offers high degrees of attenuation (80 dB). Of course the elimination of interference sources may not be feasible, but it is at least something to consider.

Figure 11-7

Figure 11-7 Lowering Radio Signal Interference Increases Range

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