The IEEE 802.11 subcommittee is constantly working to improve and expand upon the 802.11 standard. This month, we’ll bring you up to date on some of the new 802.11 standards that have recently been completed or that are in development. We’ll discuss 802.11e, f, h, d, j, k, p, r, s, t, v, and w.
802.11e defines Quality-of-Service extensions to 802.11.
The 802.11 standard was designed primarily with LAN-style data communications like file transfer or HTTP in mind. These types of applications are relatively tolerant of changes in the latency of the network, as long as the network can offer sufficient bandwidth to support the application’s needs. But applications like VoIP or streaming multimedia need consistent (and usually, low) latency to operate properly. Sudden changes in latency can result in drop-outs in the multimedia stream, which are especially intolerable to users of these services. Devices and applications that support 802.11e will be able to gain priority access to an 802.11 network, allowing them to get the consistent latency and bandwidth that they need. The 802.11e standard has been ratified.
802.11f defines the Inter-Access-Point-Protocol, which facilitates roaming of clients between access points.
The original 802.11 standard defined methods whereby a client station could request to roam between APs, but it didn’t define the method whereby the APs would communicate between each other to handle the roaming. As a result, clients typically could not roam seamlessly between two APs from different vendors. Clients will be able to roam seamlessly between two APs that support 802.11f regardless of whether the two APs are manufactured by the same vendor or not. The 802.11f standard has been ratified.
802.11h defines spectrum management functions that allow 5 GHz transmitters (like 802.11a devices) to avoid interfering with other 5 GHz devices.
This standard was specifically written to meet European regulatory requirements, but it has since become mandatory for 5 GHz devices in the U.S. as well. 802.11h defines Transmit Power Control (TPC) and Dynamic Frequency Selection (DFS). TPC allows an AP and its clients to change their transmit power so that they use the minimum power necessary to communicate with each other. DFS allows an AP to detect other, non-802.11 transmitters in the frequency space, and then move to a frequency that is un-used if any non-802.11 transmitters are detected. The 802.11h standard has been ratified.
802.11d and 802.11j extend the 802.11 standards for use outside the United States.
Not all countries use the same 2.4 GHz and 5 GHz frequency allocations and the 802.11 standards, as originally written, don't always apply to the rules outside the United States. 802.11j specifically applies to Japan, while 802.11d applies to many other countries. The 802.11d and 802.11j standards have been ratified.
802.11k defines methods whereby 802.11 stations and access points can measure and report various aspects of the RF environment.
All 802.11 stations report signal strength (RSSI), but 802.11k extends this capability to allow, for example, a station to report to an AP all other 802.11 devices that the station can see, or for all stations to report to the AP the level of noise around the station. By using this information, stations and APs can improve the performance of the WLAN. The 802.11k standard has not been ratified yet. Work is still in progress on it.
802.11p improves the applicability of the 802.11 standards to a vehicular environment.
802.11 is not ideally suited to cases where one client is moving at high speeds relative to another, such as might be the case when an AP is mounted on a light-post and the client is in a vehicle moving at highway speeds. 802.11p also includes provisions to facilitate ad-hoc communication between vehicles. A key term associated with 802.11p is Wireless Access in a Vehicular Environment, or WAVE. The 802.11p standard has not been ratified yet. Work is still in progress on it.
802.11r provides functionality to enable fast roaming in an 802.11 network.
Although 802.11f defines a protocol to enable standardized roaming, the period of time when a client is “between access points” can be too long for a highly-latency-sensitive application like VoIP. Access points and clients that support 802.11r should be able to roam quickly enough that VoIP and multimedia streams are not interrupted. The 802.11r standard has not been ratified yet. Work is still in progress on it.
802.11s provides a standard way for 802.11 devices to form a mesh network.
Three characteristics of a mesh network are that the devices automatically form a topology that enables connectivity from any point to any other point, that the devices forward packets from one to the other to enable that connectivity, and that the devices can detect failed or sub-optimal links and automatically transition over to a better link. Currently, vendors produce mesh routers that fulfill these functions. but these devices are non-standard and only work with other devices from the same vendor. The 802.11s standard has not been ratified yet. Work is still in progress on it.
802.11t defines a standard set of tests to allow for consistent performance comparisons between different 802.11 devices.
The 802.11t standard has not been ratified yet. Work is still in progress on it.
802.11v defines standard methods for remotely managing and configuring 802.11 stations.
Examples might include setting a station’s output power or other such options. 802.11v is a sub-set of 802.11k. The 802.11v standard has not been ratified yet. Work is still in progress on it.
802.11w defines a method for encrypting and authenticating management packets in an 802.11 WLAN.
Currently, most 802.11 management packets are relatively benign—enabling stations to join or leave a WLAN. But some management packets can be used to effect a denial-of-service attack against a WLAN, and the 802.11k and 802.11v standards will expand the functionality of management packets, increasing the importance of protecting them from unauthorized use. The 802.11w standard has not been ratified yet. Work is still in progress on it.
Connect802’s engineers are experts at Predictive RF CAD Designs, but we also perform on-site services, including calibration surveys, basic channel and noise assessments, and RF spectrum analysis. The first thing to consider when performing an on-site survey is what equipment you plan to take with you. Nothing slows down a site survey like having to spend two hours driving to a hardware or electronics store to pick up something you forgot. Not every site survey will need all of this equipment, but here are some of the items that we have found to be useful:
One or more “test” access points, to create a signal that can then be measured. The APs are placed in potential mounting locations and then their signal strength is measured. These APs should be as close to the actual APs that will be installed as possible—at the very least, their transmit power should be the same.
An uninterruptible power supply (UPS) to power the “test” AP. Even a UPS that is only rated for ten minutes when it is powering a computer can power an access point for several hours because access points draw so little power compared to computers. The UPS will allow you to place the AP in locations that do not yet have power drops. Most UPSs have a beeping alarm that goes off when the UPS is unplugged from the wall. This can be very annoying during a site survey, so make sure to buy a UPS that allows the alarm to be disabled (usually via software). At Connect802, we prefer to buy APC UPSs because our research shows that all APC UPSs have the ability to disable the audible alarm.
A stand or tripod to hold the AP. We have found that tripods intended for photographic lights can be found relatively cheaply ($30 or so) and are perfect for this application. Keep in mind that high-end photographic light stands can be several hundreds of dollars—you don’t need one of those! Use a length of wire or duct tape to attach the AP to the stand and then you can place the AP anywhere you need it while surveying.
A 50-foot extension cord. Of course, the UPS will power the AP in locations where plugs aren’t available, but an extension cord can also be handy.
The image to the right shows a test AP in the configuration described above. The AP is mounted to a tripod with masking tape (useful because it doesn’t leave a sticky residue on the AP) and is powered with an extension cord. In the image below, the AP has been placed on a man-lift and is being raised up to the ceiling of a warehouse to test how the signal would propagate if the AP were mounted in the rafters. The AP is being powered by a UPS which is not visible in the picture.
A method of measuring
signal strength. The
free software program
NetStumbler is one
common choice, but
we have found it to
performance with certain
wireless cards. A commercial
solution like AirMagnet may provide better results. A tool that runs on a laptop will be easier to read and screen-shot (for documentation), but will require you to carry around a larger and heavier computer. A PDA-based device may not provide as comprehensive information, but will be much lighter, and will probably have longer battery life too. A tablet PC may provide an optimal compromise between a laptop and a PDA.
A digital camera. Digital cameras are essential for documenting the site survey. The digital camera will probably get banged around a bit, so it shouldn’t be an expensive, top-of-the-line model. It should be small enough that you can drop it in a pocket when you’re using your hands for other things (like taking measurements). We find that the ability to shoot videos is often better than taking still photographs—e.g. when documenting how moving from one location to another results in a change in signal strength. The video can show the move in real-time, which is hard to show with still pictures.
A spectrum analyzer
can provide an unparalleled
method of measuring
signal strength and,
most importantly, is
the only method of
sources of interference.
These tools can be
had for as low as $3,000-$4,000
dollars, with higher-end
models running as much
as $13,000 dollars.
Floor plans (for an indoor survey) or a site map (for an outdoor survey). These can be used to anticipate obstructions to the RF signal, mark measurement locations and signal strength values, and so forth. Bring several copies so that you can mark them up.
This may be too obvious to mention, but bring a clipboard or notebook in which you can take notes. Taking notes on a computer is high-tech, but can be cumbersome in the field. Connect802’s engineers regularly rely on good old pen-and-paper when they’re bouncing around in a bucket at 60 feet in the air. These scribbled notes are then transcribed into spreadsheets and word documents when the engineer is safely back in the office.
A distance-measuring device such as a surveyor’s wheel or laser range-finder is useful for estimating signal range and for correlating real-world measurements with maps or floor plans. For outdoor surveys, this can be used to calculate free-space path loss and Fresnel zone size between two antennas.
Outdoor surveys often benefit from a GPS receiver. The GPS receiver can be used to accurately map the locations where signal strength measurements were taken. It can also substitute for the previously-mentioned distance-measuring device. A GPS receiver may also be able to calculate altitude, allowing you to determine, for example, how tall of a pole is necessary to get the signal to clear a hill that sits between two antennas. We recommend a hand-held “hiking-style” GPS receiver for site surveying, as opposed to a car-oriented “navigation-style” receiver. The “hiking-style” receivers usually allow you to mark waypoints (measurement locations) and measure altitude, while “navigation-style” ones typically don’t.
A piece of software to measure throughput can provide practical confirmation that your signal strength measurements are actually translating into network performance. Three options for throughput measuring are iperf, wsttcp, and Qcheck. Iperf and wsttcp are free and open-source, but only support command-line usage. Qcheck is also free, but requires registration with Ixia software, its distributor. Qcheck has a graphical front-end. Using any of these three programs will require a second computer to attach to the AP to provide an endpoint for the throughput test.
These are some of the major tools that we take to a site survey and the ways that we use them.
Ask the Expert
How to deal with support walls and I-beams (part 1)
I’m installing a wireless network in a large, open building with a cement retaining wall running through the middle and metal support columns (I-beams) in the open areas on either side. My preliminary site survey indicates that the wall and the I-beams are creating lots of RF shadows. How can I cover this space with signal with the fewest APs?
The issues you’re describing fall into two general categories. The cement retaining wall primarily absorbs the RF signal, resulting in straightforward attenuation—a decrease in signal strength on the other side of the wall. The metal I-beams, on the other hand, primarily reflect the signal. Reflection means that the RF energy is not totally lost—just redirected. This can result in more complex situations than a straightforward shadow behind the beam. Generally, however, the network should be designed to avoid both absorption and reflection.
The image to the left shows a rough outline of a building such as you describe. The building is 300 feet on a side, for a total area of about 90,000 square feet. A cement support wall runs horizontally down the middle of the floor and metal I-beams are spaced in the open areas. This layout is similar to some real buildings that we have worked on, but it’s obviously not up to architectural standards. It’ll be sufficient for illustrating the effects of these obstacles on RF propagation.
Assuming an omnidirectional antenna, one would typically place an AP in the middle of the area to be covered. But when an absorptive obstacle is in the middle of the area, this is not usually the best case. The obstacle will absorb much of the signal energy, wasting it.
In the image to the left, the support beams have been removed so that we can focus just on the wall. The red contour in the drawing above represents a signal strength roughly sufficient to get 54 Mbps data rates; the yellow contour represents a signal strength roughly sufficient to get 24 Mbps data rates, and the green contour represents a signal strength roughly sufficient to get 6 Mbps data rates. For reference, the AP has an output power of only 5 mW, which is less than a real AP would probably use, but we wanted to have relatively small coverage contours to make the effects of the wall more obvious.
Notice that if the AP is mounted on the wall, its signal barely goes through the wall, so stations on the far side of the wall get hardly any coverage. Meanwhile, stations on the AP’s side of the wall are further from the AP and get lower data rates than they might otherwise have.
If, on the other hand, the AP is mounted away from the wall, the area that it can cover with 54 Mbps data rates (the red contour) is larger and, as an additional bonus, its signal starts to wrap around the wall, facilitating roaming.
Maximum coverage on the other side of the support wall occurs when the AP is mounted as far from the wall as possible, but this reduces the area that is covered with 54 Mbps data rates.
If the AP is placed in-line with the wall’s axis, instead of perpendicular to it, coverage is maximized on either side of the wall. Although this solution doesn’t provide the highest data rate to the whole floor, it allows the AP to cover the most area so far. If 24 Mbps data rates were acceptable, this would be the most cost-effective solution so far.
Notice the difference in the picture to the left when the AP is mounted to the support wall itself, instead of on the exterior wall facing the support wall. Now, the wall creates a substantial shadow.
Ignoring the I-beams for the moment, the best solution for this design would probably involve two APs, located as shown here.
This assumes that 24 Mbps data rates are sufficient for the whole floor. If higher data rates are needed, then coverage would have to be filled in with either more APs or by increasing the APs’ power output to a more realistic value. This solution, with APs at roughly 9-o’clock and 3-o’clock allows each AP to cover the maximum number of users. If it were desirable to limit the number of users per AP, then placing the APs at 12-o’clock and 6-o’clock would work just as well, as shown to the left.
This design prevents users on the top side of the wall from seeing the AP on the bottom side, and vice versa, while still allowing each AP’s signal to “wrap” around the wall enough to facilitate roaming. In addition, this design maximizes the area of 54 Mbps coverage because the APs are not placed on a wall. This design might utilize ceiling mounting, for example
To sum up: when there is a highly absorptive obstacle in an environment, you only waste signal strength by mounting an AP to that obstacle. Chances are that the AP’s signal will not be strong enough to push through the obstacle to cover users on the other side. Another option might be to mount the APs to the obstruction, but use directional antennas to limit the amount of signal energy that is absorbed. For example, the image to the left shows the result of replacing the APs’ omnidirectional antennas with 180-degree beamwidth patch antennas. Notice that the area of 54 Mbps coverage has increased, but this solution does not address the issue of roaming between APs. There is still a dead zone along the wall’s axis.
We'll continue this discussion next month to include the impact of the steel I-beams in the space and the effect they have on RF propagation.
Pioneer Predicts that WiMAX Will Complement Cellular Networks
A recent Pioneer study examined the effects of upcoming fixed-wireless technologies like WiMAX (802.16). Some have speculated that WiMAX might act as a competitor to 3G/4G cellular networks, but the Pioneer study suggests that WiMAX will complement, not replace the existing cellular networks. Although WiMAX is an exciting technology, the existing cellular networks have years of investment in their infrastructure and the skills required to support it, so we at Connect802 agree that it’s a little early to call WiMAX, the new kid on the block, a major contender. The report also points out that WiMAX serves two major blocks of users: fixed point-to-point and mobile. The cellular networks are optimized for mobile users, but WiMAX might excel in point-to-point scenarios. At Connect802, we believe that convergence between wireless technologies will be a major market force in the next two to five years. WiMAX, 802.11, cellular technologies, BlueTooth, and other wireless technologies not yet on the map will each find areas where they excel, but because of the specialization of each technology, it’s unlikely that any one technology will become dominant.
Atheros Discusses Optimal 802.11n Configuration
In an article on CommsDesign.com, Atheros engineer Winston Sun argues that a 3x3 MIMO configuration on the AP and a 3x2 MIMO configuration on the client is the optimal balance between cost and performance in 802.11n stations. 802.11n allows for configurations from 2x2 up to 4x4, with higher configurations offering more data rate, but also more size, power consumption, and cost.
One thing to keep in mind is that this analysis focuses on downstream throughput, and so may not apply to cases where upstream throughput is a major factor. That being said, many WLAN applications are focused on downstream throughput, so this analysis is widely applicable. We’ll have to wait until standards-compliant 802.11n equipment is actually on the marketplace to perform our own tests, but until then, articles like these keep our appetites whetted for 802.11n!
Web Searching: The Connect802 Web Presence
At Connect802 we're your PAGE ONE resource for wireless networking!
Connect802 has the experience, expertise, and resources to help you with your wireless network system. Use your favorite search engine and see what Connect802 is doing. Each month we give you some suggested search terms for you to explore. Here's this month's list. As you look down the search engine results you'll find Connect802 either at the top, or on the first page (true for Google and Excite, unknown for the rest).