Connect802 is a nationwide wireless data equipment reseller providing system design consulting, equipment configuration, and installation services.


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Spring 2008

Essential Wi-Fi:
For those who are new to Wi-Fi networking...
 
Technology and Engineering:
For the engineer and Wi-Fi network administrator...
 

Ask the Expert
Questions from our readers...

To Infinity... and Beyond!
News from the wireless marketplace...
 

Essential Wi-Fi

Outdoor Wireless Networks
The majority of wireless installations are for indoor networks, which means that the average wireless network engineer is unprepared when he or she encounters an outdoor wireless network. In this article, we’ll discuss how outdoor wireless networks differ from indoor ones and give you information to assist you in designing and troubleshooting an outdoor wireless network.

Indoor wireless networks seldom have clear line-of-sight between the access point and the clients. There are usually walls or other obstructions in the way. There are some exceptions, such as warehouses, convention halls, and “cube farms”, but it’s rare for an entire indoor installation to consist of these types of areas. When the line-of-sight between the access point and the client is obstructed, the range of the signal is reduced significantly and the number of access points needed to cover an area increases.

In comparison, outdoor wireless networks often allow for clear line-of-sight between the clients and the access points. Imagine a courtyard, stadium, or parking lot that requires RF coverage. In all of these cases, it’s likely that an antenna could be mounted in a location that allowed for clear line-of-sight to the entire coverage area. When clear line-of-sight is available between the access point and the clients, the range of the access point increases dramatically—sometimes by a factor of three to five times, and sometimes even more!
Increased coverage range due to clear line-of-sight is the first major difference between outdoor and indoor RF installations. Connect802’s engineers report that they regularly have to re-calibrate their expectations of coverage distance when they go from an indoor job to an outdoor one. “The customer (who had only done indoor installations) had estimated three access points to cover the parking lot, but after doing the survey, it turned out that one AP was more than enough,” said one engineer. If you’re not accustomed to doing outdoor installations, get an access point and a client device and take some measurements. You might be surprised just how far you can go before signal strength drops to an unacceptable level.

This increased coverage range is only present when line-of-sight is clear. It doesn’t take very much obstruction to dramatically reduce range, so you have to be much more aware of obstructions in outdoor installations. Obstructions have fundamentally the same effect in indoor installations, but since they’re pretty much always present, indoor wireless networks are pretty much always designed to deal with them. With an outdoor network, you’re usually designing based on the assumption of clear line-of-sight and greatly increased range. If that assumption fails, even a little bit, you can end up with big coverage gaps.

Trees are the main type of object that reduces the range of outdoor wireless networks. The exact amount of reduction depends on many factors, including the type of leaf on the tree, the season (and amount of leaves), and the number of trees (a thick stand obstructs more than a single decorative plant). The topic is too complex to fully explore in a short article, but suffice it to say that whenever there are trees in your coverage area, you should pay close attention to areas that are shadowed by the trees in your survey. Connect802 has often found that a thick band of trees essentially completely blocks RF coverage. Additional access points had to be added to compensate.

The way around this problem is to place access points such that they have clear line-of-sight to as much of the coverage area as possible. For example, if you’re installing access points to cover a street, place the antennas out on the end of a lamp-post such that they overhang the middle of the street, as opposed to on the poles, where the signal might be blocked by trees planted near the sidewalk.
A second major difference between indoor and outdoor wireless networks is the effect of interference. Indoor networks are somewhat protected from outside interference by the concrete, brick, or sheet metal walls of the building. Outdoor networks, on the other hand, are totally susceptible to whatever interference might be present. It is sometimes the case that there are not any interfering devices near the outdoor coverage area, but especially in heavily populated urban areas, there can be so much interference that network performance can be significantly compromised.

The only way to be sure is by surveying the area, which is why Connect802 recommends surveying before installing any outdoor network. A spectrum analyzer can identify sources of interference, but we also recommend performing actual data-transfer tests to determine the practical effect of interference on the wireless network. It can be difficult to determine just from looking at a spectrum analyzer trace whether a wireless network’s performance will be compromised—after all, 802.11 networks are designed to work around interference to some degree.

A data transfer test can be performed with an access point and two laptops. One laptop plugs into the AP’s Ethernet port and the other laptop connects wirelessly. Then an FTP file transfer or an iperf test is run between the two devices and the overall throughput is measured. For an 802.11b network, throughput should max out around 6 Mbps, while 802.11a/g networks should max out around 18-22 Mbps. Because interference can be intermittent, you should especially pay attention to sudden drops in throughput. You might have an average throughput of 15 Mbps, but if there are occasional five-second periods where the actual throughput drops to 250 kbps, something is wrong! You might be tempted to ignore the intermittent dropouts as anomalies, but don’t! Ask yourself whether a user’s experience of the network would be compromised by those dropouts, and if the answer is yes, design the network to compensate for the interference.

What are the options for mitigating the problem if an outdoor network is experiencing interference? Blocking the interference is probably not feasible, since that would require building a wall around the coverage area. One option is to increase signal strength (and therefore, signal-to-noise ratio) by installing more access points, closer to the users. This will only work if the interference itself is sufficiently weak that your signal can overpower it. A second option is to use higher-gain antennas with narrower, more focused coverage patterns. This will have the dual effect of increasing signal strength in the coverage area and of reducing the degree to which the antenna can “hear” the interference. Finally, the option of last resort is to move to a different frequency range. In some areas, the 2.4 GHz spectrum is swamped with interference, and the only feasible option is to move to the 5 GHz band with an 802.11a network. Networks that are used for public safety also have the option of moving to the 4.9 GHz frequency band.


 
Technology and Engineering

Is Diffraction Helping Your Network?
RF Engineers usually think of solid objects as creating “shadows” in RF coverage. This analogy works well for rough estimates of RF coverage, but environmental effects like reflection and diffraction mean that the signal doesn’t always go where a straight-line approximation of its path would predict. In this article, we’ll explore how diffraction can actually improve coverage in outdoor installations.


An example of how a signal might diffract around the edges of a building.

Put simply, diffraction means that when the RF signal encounters a sharp-edged object, the signal seems to “bend” around the edge of the object. This is illustrated in the graphic above, which depicts a top-down view of a building with an 802.11 antenna on the left side of the building. The RF signal will diffract around each corner of the building. In the zone marked “1”, the receiver is in direct line-of-sight to the antenna, and no diffraction will affect the signal. In the zone marked “2”, the signal has diffracted around the lower-left-hand corner of the building. The signal is weaker than in line-of-sight, but is still present. In the zone marked “3”, the signal that made it into zone 2 has further diffracted around the lower-right-hand corner of the building. This signal is still weaker, but might still be detectable by a receiver. In the zone marked “4”, the signal is too weak to be received; this zone is considered to be in shadow.

This example demonstrates that RF shadows are not as simple as they might at first seem. If you were to consider only line-of-sight, you would conclude that zone 1 would get signal and zones 2, 3, and 4 would be in shadow and would receive no signal. In reality, this is not the case. Zones 2 and 3 do receive some signal, albeit at much decreased signal strength.


Diffraction means that the actual shadow is much smaller than line-of-sight would suggest.

Diffraction is usually beneficial to RF networks, allowing signal to propagate to locations where it otherwise wouldn’t. For example, Connect802 once installed an antenna on the south-most wall of a long rectangular building. The building ran for approximately 300 feet to the north of the antenna. The antenna was mounted approximately six feet above the roof-line. If you were to go just by line-of-sight, you would conclude that the roof would create a large shadow on the north side of the building, but in reality, this shadow was not very pronounced. Effective throughput of about 2 Mbps could be achieved just 50 to 75 feet north of the north wall of the building because diffraction “bent” the signal down around the northern edge of the roof, causing it to hit the ground much closer to the building than life-of-sight would suggest.

RF networks should not be designed to rely heavily on diffraction to achieve RF coverage. Connect802 has found that the difference between “in the shadow” (zone 4 in the graphic above) and “in the diffracted signal” (zone 3 in the graphic above) can be as small as five or ten feet. A client can be achieving 24 or 36 Mbps data rates in the diffracted signal area and then, after moving a relatively short distance, suddenly lose connection altogether because they’re in the shadow. When you have line-of-sight and you move away from the access point, a graceful, slow decrease in signal strength and data rates occurs. With diffraction, the drop-off can be much more sudden and unexpected.
Diffraction can be most helpful when clients are roaming between two access points. For example, imagine a wireless network covering a grid of city blocks. Through diffraction, an access point on a north/south street will have a little bit of coverage around the corner onto each cross street, and vice versa. This means that there will be a small zone of overlap between the APs on the north/south streets and the APs on the east/west streets, assisting in roaming.

Finally, what about indoor networks? Yes, diffraction does occur in indoor networks too, but it is less of a factor than in outdoor networks, because indoor networks have so many objects in the signal path between the access point and the client device. This means that there are many edges around which the signal diffracts, and the net effect is much less discrete than it is in an outdoor network where there are likely to be only a few objects in the signal path. In indoor networks, the entire signal path is likely to be obstructed by walls or other objects, and so the effect of diffraction is insignificant compared to the absorption of the signal by the obstructions.

Ask the Expert

Two Radios Better Than One?
I was called in to help troubleshoot another engineer’s wireless installation. The network has a dual-radio access point and both radios are set to use 802.11g channel 6. Is this actually helping performance at all? I think not, but I want to be sure.

Dual-radio access points can increase a wireless network’s capacity, but not when they’re configured as you describe. CSMA/CA, the network access method that’s used in 802.11, prohibits two radios from transmitting at the same time on the same channel. If the two radios in your access point were set to different channels, then they could transmit and receive at the same time, basically doubling the total capacity of the network. If both radios are set to the same channel, however, only one of them can operate at a time, and there’s basically no benefit. (Okay, in theory, if one radio failed, the other one could still operate as a backup, but realistically, if both radios are in a single access point, it seems unlikely that one would fail and the other wouldn’t.)


To Infinity... and Beyond!

Wi-Fi Certified Draft-802.11n Devices Might Cause Interference
One of the goals of Wi-Fi certification is to ensure compatibility between devices, but the Wi-Fi Alliance’s Draft-802.11n certification might not achieve that goal. Devices certified as Draft-802.11n compliant will only be tested in 20 MHz channel mode. In this mode, 802.11n channels are the same width as 802.11b/g channels, and so no additional steps need to be taken to avoid interference between 802.11n and 802.11b/g networks. But 802.11n networks can also use a 40 MHz wide channel—and indeed they have to do so in order to get the highest data rates. A 40 MHz wide channel centered on channel 6 of the 802.11b/g channel set will interfere with channels 1 through 11, effectively disrupting communications for all transmitters in the 2.4 GHz range. Of course, the 802.11n standard includes provisions to avoid this interference. For example, an 802.11n device that detects “legacy” 802.11b/g devices will switch to 20 MHz mode in order to avoid interfering. But without the Wi-Fi Alliance’s certification, there’s less certainty that this functionality is working properly in a given device. The Alliance stated that there’s still too much debate over how backwards compatibility between 20 MHz and 40 MHz devices will be handled, and thus they aren’t testing for it. We can hope that, once the final standard is approved, this testing will be added to the Wi-Fi 802.11n certification.

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