Posts Tagged ‘cwna’

Absorption and Reflection in the Microwave and in Wireless LANs

Technical | Posted by Tom Carpenter
Nov 21 2009

I don’t know about you, but I like popcorn with a great movie. Of course, today we can get freshly popped popcorn in under four minutes thanks to the power of the microwave. Microwave ovens use the 2.4 GHz ISM frequency range (some use it all and some use just a portion) to pop that popcorn. The popcorn absorbs the RF energy by converting it to heat. Eventually, the heat builds up pressure and you hear that wonderful poppety pop pop sound coming from the microwave, which means you’ll be enjoying your movie and your popcorn in just a few minutes.

So, what does a microwave oven have to do with wireless LANs? Well, the answer to that question is twofold. First, it can be used as a teaching tool to understand concepts like absorption and reflection. Second, microwave ovens can cause interference with your wireless LAN in many scenarios.

As a teaching tool, the microwave oven can help you understand both absorption and reflection. When you put a glass of cold water in the microwave and turn the microwave on, the water heats up. Why? Because absorption occurs. Absorption, remember, is the conversion of RF energy to heat. Now, you can take out that glass of water and dip a nice tea bag in it to get some soothing hot tea.

Reflection is seen in the fact that very little of the output energy escapes from the microwave. Why? It is being reflected inward by the design of the internal unit. Place your cell phone in a microwave (without turning the microwave oven on, of course) and close the door. After a few seconds, open the door again – you’ll likely see that your phone is looking for service. Why? The design of the microwave keeps as much of the RF energy in as possible and that results in keeping the cell tower’s energy out as well.

Microwave ovens can cause interference simply because they operate in the same frequency space as 802.11, 802.11b and 802.11g devices. Many 802.11n devices may also attempt to communicate in the 2.4 GHz frequency space. While the microwave ovens do a good job of protecting you as a human (by keeping dangerous levels of RF energy inside the microwave), they certainly let plenty of the energy escape from the perspective of a nearby wireless LAN. Always test the microwaves in the area where you are installing a wireless LAN. It may dictate the channel you have to use.

After testing dozens of microwave ovens, I’ve determined that the average microwave oven leaks about as much RF energy as a consumer-grade (linksys) access point. The energy may be a little less focused, but it is spread over a larger range of the 2.4 GHz band. Watch out for this in your wireless LAN designs.

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RF Wavelength Calculations for Wireless Networks

IT Theory | Posted by Tom Carpenter
Nov 21 2009

The wavelength of a RF wave is calculated as the distance between two adjacent identical points on the wave. The wavelength is frequently measured as the distance from one crest of the wave to the next.

The wavelength is an important factor in wireless networking. The wavelength dictates the optimum size of the receiving antenna and it determines how the RF wave will interact with its environment. For example, an RF wave will react differently when it strikes an object that is large in comparison to the wavelength than when it strikes an object that is small in comparison to the wavelength.

The wavelength and the frequency are interrelated. For a given medium, if you know the wavelength, you can calculate the frequency and if you know the frequency, you can calculate the wavelength. The wavelength is directly related to the frequency and the speed of light. If you know the frequency, you can calculate the wavelength. If you know the wavelength, you can calculate the frequency.

One of the great discoveries in the history of electromagnetism is that electromagnetic waves travel at the speed of light. Since we know the speed of light to be 299,792,458 meters per second (or the simple 300,000,00 meters per second, if you prefer), we also know that this is the speed at which electromagnetic waves travel in a vacuum. This was theorized by James Clerk Maxwell and proved through experimentation by Heinrich Hertz.

You are probably familiar with measurements like 100 megahertz and 3.6 gigahertz. These measurements refer to the number of cycles per second. When we say that the access point is using the 2.45 GHz (gigahertz) spectrum, we say it is using the spectrum that uses a wave cycle rate of 2,450,000,000 times per second. This measurement is named for Heinrich Hertz and his research in electricity and magnetism. A kilohertz is 1,000 hertz or cycles per second. A megahertz is 1,000,000 hertz and a gigahertz is 1,000,000,000 hertz. A terahertz is one trillion hertz, but these frequencies are not commonly found in today’s wireless communications.

Since we know that RF waves travel at the speed of light we can calculate the frequency when we know the wavelength or the wavelength when we know the frequency. The following formula can be used to calculate the wavelength in meters when the frequency is known:

w = 299,792,458 / f

Where w is the wavelength in meters and f is the frequency in hertz and the medium is a vacuum. Therefore, the 2.45 GHz spectrum would have a wavelength that is calculated with the following formula:

w = 299,792,458 / 2,450,000,000

The result is .123 meters or approximately 12.3 centimeters in length. This translates to about 4.8 inches. To calculate inches from centimeters, just multiple the number of centimeters times 0.3937. The formal character used to represent a wavelength is the Greek lambda (λ), and the symbol for the speed of light is c. Therefore, the formal representation of the previous formula would be:

λ = c / f

The calculation for frequency is just the opposite. You will divide the speed of light by the wavelength in meters to discover the frequency. Keep in mind that the numbers we’ve been using have been rounded and that impacts the results of the following formula; however, the results are close enough to recognize that a wavelength of .123 meters would indicate a RF wave in the 2.45 GHz spectrum:

f = 299,792,458 / .123
f = 2437337056.91

Due to the complex measurement number that is the speed of light, this number is often rounded to 300 billion meters per second. While this will change formula results, the findings are close enough for understanding the behavior of RF waves; however, engineers developing RF systems must use more precise measurements. Additionally, formulas like the following simplify matters:

wavelength in inches (λ) = 11.811 / f (in GHz)
wavelength in centimeters (λ) = 20 / f (in GHz)

Because wireless networks use such high frequency ranges, formulas like this make the calculations easier.
 

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What does it mean for a certification book to be “official”?

News, Professional Development, Technical | Posted by Tom Carpenter
Oct 01 2009

I’ve had dozens of people email me to ask what it means now that McGraw-Hill is no longer the publisher of the Official CWNP study guides (CWNA, CWSP, CWTS, etc.). I decided to answer this question in a blog post. This way I can just refer the new emails to the post. Here’s the basic benefit of being the official study guide:

  • The CWNP program picks who the author/authors will be. This is a benefit in that the reader can be sure the CWNP program believes in the author.
  • The book gets to be listed as the official book.

 

No special insights into the exam are provided to the authors of the official book beyond that available to anyone, so no real benefit exists for the reader from the perspective of the "official" label. Giving special insight would be unethical (since the author would always claim to have passed the exam and yet he or she would have been exposed to special information about that exam) and the CWNP program is anything but unethical. In other words, the authors of the official book have no more insight into the exam than the authors of any other book. Anyone who writes a book for the exams – official or not – has access to the same resources:

  • Official classes
  • Objectives
  • CWNP.com

 

That’s really it. Now as far as picking the authors goes. The CWNP program picked me for Wireless#, the first book outsourced to be completely authored by a non-CWNP program employee. They picked myself and two co-authors for the CWSP study guide 2nd edition. They then picked me for the 4th Edition of the official CWNA study guide. They also hand-picked Joel Barrett to help guide the series at that time. Joel is an amazing engineer working with Cisco in wireless implementations on a very large scale. I am a consultant and trainer who implements an average of 4 to 5 wireless LANs each year. I teach from 5-6 week long custom CWNA/CWSP classes each year and I am continually researching the newest in wireless technologies. The point is that they would have picked me to write the official books again if I were available. In fact, I was offered the opportunity to write the official CWTS book for Sybex, but could not due to contractual obligations.

Due to their contractual issues with the CWNP program and nothing at all related to the content or writing style, McGraw-Hill lost the official label to Sybex. Sybex is also a great publisher (I write for them as well on non-overlapping topics) and the authors of the official study guides know their stuff well. Their new official CWNA book is exceptional and I expect their CWSP book will be as well.

I am in the process of writing a CWNA/CWSP All-in-One Guide book for McGraw-Hill that will be amazing because of the books format and intent. It will be a study guide, yes, but it will also be a very practical implementation resource. You will be receiving step-by-step instructions for implementing real hardware. I’m very excited about it. In addition, my CWTS book for McGraw-Hill is done and should hit the shelves in the next four weeks.

I hope this clears up any confusion and let me end by saying this: I work with both McGraw-Hill and Sybex and they are both great companies. I know David Coleman, one of the official authors now, and he is an awesome trainer with excellent knowledge. Finally, I absolutely love the CWNP program and the vendor neutral certifications they offer. Any rumors out there which cast a negative light on any of these three companies (McGraw-Hill, Sybex or Planet3/CWNP) will hopefully cease. Sometimes things don’t work out between companies, but I’m thankful that multiple products exist and the user community gets an excellent set of resources.

Happy modulation!

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