Last month’s articles on 802.11n were some of the most-read articles we’ve ever published. In March, more news about 802.11n was released, including some controversial claims about whether 802.11n networks will hurt the performance of nearby 802.11a/b/g networks and how close the final standard will be to the draft proposal that was accepted in January. Although we don’t intend to let 802.11n totally take over this newsletter, we’ve decided to dedicate another month to updating you on breaking news in 802.11n’s development.
The first major news in 802.11n this month was that version 1.0 of the draft 802.11n standard was approved on March 9th. Last month, we reported that a proposal had been confirmed. The approval of a draft standard is the next natural step towards a final standard. This draft comes relatively quickly, which is a good indicator that the final 802.11n standard will meet its target date of late 2006 or early 2007.
Very few changes were made between the proposal and the draft standard—mostly editorial changes to bring the document into line with the IEEE’s stylistic guidelines. This suggests that the final standard will be similar to the proposal and draft standard, although it’s highly unlikely that the draft standard will be approved completely unchanged. This has implications for whether products made today, based on the draft standard, will be able to be made compatible with future products based on the final standard. The fewer changes that are made between the drafts and the final standard, the more likely that pre-standard products will be forwards-compatible. Wi-Fi Networking News reports that Bill McFarland, Atheros’s chief technology officer said, “We expect that you’ll be able to see products on the store shelves certainly by the middle of this year implementing this 1.0 draft.”
For all the good news about 802.11n’s development, some questions remain unanswered. A controversy emerged this month regarding how 802.11n would handle co-existing 802.11a/b/g networks. Some “flavors” of 802.11n use 20 MHz wide channels, similar to 802.11a/b/g networks. Other “flavors” of 802.11n increase throughput by using 40 MHz wide channels. The wider channels give approximately double the throughput but are likely to interfere unacceptably with nearby 802.11a/b/g networks. Manufacturers are still discussing how this feature should be handled, with some arguing that preventing interference is more important, and others arguing that high performance is more important.
In another interesting development, Paul Callahan reports in his blog that not all 802.11n manufacturers are equal. Although Airgo, Broadcom, and Marvell have all announced reference design chipsets based on the version 1.0 draft standard, Broadcom is reported to be much further along in their development and to have much better performance than Marvell. He reports that, “Marvell’s solution barely transmits two meters before failing. There are also major concerns about whether the Marvell solution is really [draft standard] compliant. Marvell claims it is, and everyone else claims it isn’t.”
Although the progress of 802.11n towards a final standard is exciting, these stories confirm our advice that business customers continue to rely on 802.11a and 802.11g equipment and hold off on purchasing 802.11n equipment until a final standard is available. All of these vendors have promised future compatibility via firmware upgrades, but this month’s news shows that many questions still remain. An enterprise buyer should think twice before he or she commits the success of his or her network to a vendor’s promise of future compatibility.
In past articles, we have discussed how signal strength and receive sensitivity interact to influence overall signal range. In short, a signal’s range is reached when the attenuation that occurs between transmission of the signal and reception of the signal is greater than the difference between the transmit power and the receiver’s sensitivity. The factors influencing environmental attenuation are different in outdoor vs. indoor installations, and so are the metrics that we use to estimate signal range.
Because outdoor installations are often designed with just enough signal strength to achieve the desired range, they are often designed so as to avoid obstructions. Specifically, the antennas are positioned so that the first Fresnel Zone—an oval-shaped volume of space, centered around the visual line of sight between the antennas—is clear of obstructions. The radius of the first Fresnel zone is given by the formula below, where d is the distance between the antennas in miles, f is the frequency of the signal in GHz, and r is the radius of the first Fresnel Zone in feet. Connect802’s antenna system designer can calculate this value for you automatically.
In installations with a clear first Fresnel Zone, the primary limiting factor in the range of the system is the free space path loss (FSPL) between the two antennas. FSPL is the weakening of the signal that occurs due to the spreading of the signal’s wavefront as it propagates further from the transmitting antenna. As the signal propagates further from the transmitting antenna, its energy is spread over a larger and larger surface area. The energy is spread “thinner,” resulting in less energy at any given point on the sphere. It’s important to understand that FSPL is not caused by anything in the environment. FSPL would occur exactly the same even if the signal were transmitted in empty space.
FSPL can be calculated according to the formula above, where f is the frequency of the signal in MHz and d is the distance between the transmitter and the receiver in miles. The output of the formula is the loss in decibels. Instead of calculating FSPL by hand, you can use Connect802’s antenna system designer.
FSPL can be used to estimate the range of an outdoor system. First, subtract the receive sensitivity of the receiver from the transmit power of the transmitter, then add in the gain of the transmitting and receiving antenna. Finally, subtract 10 dB to allow for unexpected environmental conditions. The resulting number is the link budget for the link. Next, calculate the distance that gives an FSPL value that is equivalent to that budget. In order to do this, we can rearrange the FSPL calculation to give distance instead of dB loss.
In the formula above, d is the maximum distance between the transmitter and receiver, lambda is the wavelength of the signal, in the same units as the distance (in other words, whatever units you use for distance, use the same units for wavelength), and dB is the link budget that you calculated above.
Because the first Fresnel Zone is usually un-obstructed in outdoor installations, FSPL is the major factor in signal loss. Other factors, such as rain fade, typically cause negligible loss at the distances and frequencies typically encountered in 802.11n networks. Connect802 has found that the distance given by the formula above is close enough to reality for planning most outdoor links.
By comparison, in indoor environments, the line of sight between the transmitter and the receiver is often obstructed by walls, floors, and so on. In these situations, the dominant factor in calculating signal range is the absorption of the obstacles, not the FSPL. Fortunately, most buildings are made with common materials, and so the absorption of the materials can be estimated with sufficient accuracy to make reasonable predictions. This is the basis of Connect802’s Connect EZ Predictive RF CAD Design. For example, a gypsum board wall might typically absorb 6 dB of signal, while a cement block wall might absorb 15 dB, and a poured cement floor might absorb 30 dB.
In lieu of a program that takes into account the proper absorption of many different types of building materials, we suggest the following guidelines: a typical 802.11 access point talking to a typical 802.11 client will be able to penetrate between three and five gypsum board over wooden or metal stud walls; the same AP and client will be able to penetrate one or two cement block walls; assume that metal obstacles like sheet metal walls and elevators are completely opaque to the RF signal; assume that no usable signal can penetrate between the poured cement that typically separates the floors of commercial buildings. These guidelines can be used in most residential and commercial spaces, but shouldn’t be applied to industrial or otherwise exceptional spaces.
Ask The Expert
Your question are important. Last month…
How do you determine how many Access Points you will need for the number of concurrent users? Is there a formula? Thanks.
The first thing that I try to determine is whether the number of APs in my design is going to be limited by coverage area or number of users per AP. For example:
A certain client expects to have 200 users active simultaneously in a 20,000 square foot floor of a building. Users are assumed to be roughly evenly spaced throughout the area because the floor is classrooms and study areas. That works out to 1 user per 100 square feet. A quick estimate based on an examination of the floor plan suggests that eight APs could cover the floor, so each AP would cover roughly 2500 square feet. That means that each AP would have to support 25 users.
How many users does the vendor say that the AP can support for the applications that the users will be running? If it's even close to 25, then I consider that the dominant factor in the design is the number of users per AP and plan to use lower power output with closer spacing between the APs. On the other hand, if the APs can easily support that number, then I design for best coverage and don't worry about the number of users per AP any more.
To answer the question more directly: I start with the vendor's specifications for maximum number of simultaneous users.
Engadget (http://www.engadget.com) reports: “Intel's General Manager of their mobility group, Sean Maloney, has announced that the company is planning on shipping WiMAX cards sometime during the second half of this year. WiMAX, the long-range wireless broadband technology, looked to be further away from deployment than Intel's announcement now indicates, but Maloney says that "we are now at the point where there is a clear coalescing around the 2.3GHz to 2.5GHz, 3.5 GHz, and 5GHz bands" which should cover most of the planet. During his keynote, Maloney also gave the first public demonstration of a chip that is both Wi-Fi and WiMAX-capable, designed to give consumers an option during the predicted-three-year transition period.”
Intel has always had strong support for WiMAX. If Intel can drive WiMAX deployment like it drove 802.11 deployment with Centrino (when was the last time you saw a laptop without an 802.11 adapter built in), WiMAX’s future is bright. Whereas the reporter seems to view WiMAX as a competitor to 802.11—referring to a “transition period”—Connect802 feels that, although there is some overlap between WiMAX and 802.11’s target markets, WiMAX and 802.11 meet sufficiently different demands that one won’t eclipse the other. In general, WiMAX seems to be tailored more towards metro-scale networking while 802.11 excels at building-scale networking.
Airspan announces 16eUSB mobile WiMAX adapter
This month, Airspan announced their 16e USB mobile WiMAX adapter. WiMAX operates at many different frequencies using several different transmission “profiles”. Different frequencies are used in different countries, and devices from different vendors or devices that are tailored towards different applications use different profiles. This makes ensuring compatibility between your WiMAX equipment challenging. The 16e USB card stands out because it will be compatible with every flavor and frequency that WiMAX will use, anywhere in the world. Cost and expected availability, however, has not been announced.
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 the April 2006 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).