WLAN Design Flaws

Not so long ago, back in Corporate America I worked on a Network Team; my job was to design, engineer, and oversee the implementation of wireless LANs for company warehouses. Now warehouses being what they are NOT radio frequency (RF) friendly places. Some areas have two floors, and with the metal shelving units these areas were in effect RF traps--man made caves. The signal got dispersed, trapped, deflected, and absorbed in these caves and getting consistent end to end coverage in them was an engineering challenge to say the least. In open areas (like the dock) sometimes the signal bounced across great distances, like a pebble skipping over the surface of a pond.

But there were other unfavorable factors as well. There were shelves of products that absorbed RF signal (particular products such as grass seed), and products that bounced RF signal in unpredictable ways (such a plumbing pipe). Very often an area was scoped for a certain kind of product, and with the change of seasons these locals got restocked with a product sometimes having reverse characteristics. Where once a spot that absorbed signal (and consequently required a beefier signal) now reflected RF and was wreaking havoc with its neighbor Access Points.

The end devices that used this WLAN were mobile—ruggedized vehicle mounted touch screen PCs lashed to the grid work of a forklift or “tote tugger”, and their constant roaming in and out of RF zones with varying conditions challenged the design as well. A few of the larger warehouses (one was over one million square feet) had a considerable number of Access Points (APs)--80 or more. With three practical channels to use with 802.11g, getting enough coverage saturation without bleeding into a like channel was a test of both physics and premise.

In the beginning we hired a company to design, engineer, implement, sub-contract, and enable these large complex WLANs. We paid a hefty fee for these services, and generally things went smoothly, but after a while the end users began to notice repeated problems such as latency (delay) in specific areas. Our service vendor did “certify” the installation and coverage, but when one read the certification it said in effect that they were not responsible for malfunctioning equipment, components not working within prescribed specifications, wiring faults, end equipment faults, wireless signal saturation, operating system problems with the Access Points, antenna problems, low signal areas, interference by other systems, and on, and on. Basically they certified the paper that the certification was written on, and nothing else.

We would often sub-contract them (at one thousand dollars per day on multi-day trips, plus zone charges, and consumables) to revisit the site and perform another survey/certification report. Sometimes they were able to identify and resolve a problem, but mostly the end result was merely another report. That’s when I became involved in wireless to a higher degree. I started really knowing wireless visualization, to virtually see how waves move, propagate, bounce, diffuse, and just radiate through that theoretical pond. I accomplished this by reading, studying, and experimenting. If you can visualize how water waves move in a pool, you have a great start on visualizing radio waves. It is a bit more complex than that, but oddly enough there are many similarities to this resemblance.

The more I learned (and visualized) the more I realized that there were countless design flaws in the way these WLANs were designed. For example, the Access Points that we used had two antennas, but the engineer failed to realize that only one of the antennas was send/receive, the other was receive only. He laid out an impressive array of APs and antennas on alternate aisles. It looked really good on paper too…

The problem with this design is of course that a client ion in aisle B can talk to the AP but the AP isn’t set up to talk back to the client (in real life the aisles were more isolated than my quick drawing). After using tools like AirMagnet and Omni Peek I began to see in explicit detail how a WLAN should be designed and implemented. No automated system is perfect; sometimes one has to rely on skill and experience, even creativity to positively create the optimum WLAN.

As time went on, I worked with our vendors and made adjustments on paper before implementation, and in later years designed and implemented the network myself. When I certified, I certified coverage, speed, throughput, noise, and interference. I provided metrics, and graphics, confirming my findings. Most important though, was that the fact that survey was ACCURATE—not just a best guess; a hope for the best. As the quality of the installation, tweaking, tuning, and certification improved, the problems declined radically.

Some of the older WLANs had been plagued by the same problems for years; they got used to it and seldom complained. Once they got used to things being right, the expected it—demanded it, and rightfully so. This is the way it is supposed to work. Wireless LANs, although relatively easy to set up (at least on a small scale), need special handling for large installations. The experts in my example did their very best, even though in this instance fell short of what was demanded by the environment. Everyone lived for years with latency, disconnects, and frustration; not realized that they could hope for more.

Blue Sky Wi-Fi

Blue Sky Wi-Fi Launches

Although we are in what they call an "Economic Downturn", only a true optimist like myself gets excited about the prospect of starting something new.

For years I've been spending all my time and talent pursuing corporate initiatives, helping them chase their dreams for their spare change.

It didn't matter though--I loved the work I did; to me the task was effortless. The pay only sweetened the deal. Then we tumbled into economic downturn; suddenly the world looked, and acted quite differently. I was introduced to a new boss who didn't share the same passion about the network that I did. The room kept getting smaller, and dimmer--every day I began to dread going into work more and more. At one point I was unsure about what I did anymore.

Finally--enough was enough.

I have had this dream for such a long time to start my own wireless business. There doesn't seem to be a lot of competition out there. Maybe this means that there isn't a lot a business out there to pursue, but I believe that this isn't the case.

People need geek service, technical expertise, and the secure feeling they get knowing that their data is safe and that no one is electronically peeking over their shoulder.

I am venturing into a new Service Initiative called: Blue Sky Wi-Fi.
Look for me out there.

Services that I offer:

• Wireless Installation and Setup
• Security Testing and Installation
• Wireless Performance Tuning
• Wireless Site Survey
• Wireless Corporate/Campus Desing and Enineering
• Prevention of you neighbors piggy-backing on your wireless network.
  

How To Conduct A Wireless Site Survey –Part 5

Well I have you roaming around the floor with a laptop quantifying network metrics and on the hunt for rogues. Now is the time to revisit how waves radiate, propagate, disperse, and reflect. You need to know this stuff in order to provide a logical and accurate survey.

Recall my analogy about a pebble tossed into a calm pond. Always go back to this picture when trying to visualize radio waves. The calm pond is open air. In your environment, most likely open air will be those spaces between obstacles. Obstacles change the behavior of dispersing RF radiation. When you think of obstacles, think of how an in-flight tennis ball might react to the obstacle.

You could throw the ball straight down a hallway, but if you bounce it off of a wall, you can visualize the ball bounce back and forth between the two sides as it transverses the hallway. Radio waves do that too, only it looks more like this: Part of the wave goes straight down the hallway, but each edge of the wave bounces back and forth between the two walls. The portion of the wave that heads straight down the hallway arrives at the antenna first, and the bouncing signals arrive milliseconds later. The network card is smart enough to know that although it is receiving that same transmission over again, only one copy of the transmission is utilized.


As a column will cast a shadow in the path of a flashlight beam, obstacles in the path of a wireless transmission will generate RF Shadows. If you have an Access Point antenna mounted on a building column, the air on the backside of the column will be in the RF shadow.

Heavy walls and doors dampen or attenuate signal; metal reflects or bounces the signal backwards. RF signals cannot penetrate metal. Materials that are porous absorb the energy of the signal, materials high in water content also suck up signal. Going back to the pebble on water analogy, RF waves can sometimes skip like a stone across the surface of the water and end up farther than what would be a normal range.

You can see that the more stuff that is in the area to be surveyed, the more complex the survey. As a surveyor, you must make sure that you have ample (but not too much) coverage in any given area.

How to conduct a wireless site survey part 4

Spend some time prepping your survey before you arrive on site. Study your maps and photos. Guess at likely locations for Access Points. Make sure that all your tools and drivers are ready to go. When complete head on into the airspace. This is where all the fun begins. If this is a survey for a new wireless LAN you will need to be VERY observant of many things that will impact the quality of both the air and the survey. This space is an empty pool that you are going to fill with radio signal.

Please understand--if you are not going to be as accurate as you possibly can, it is best not to even bother with a survey as it will be as waste of time, expense, and effort. This is not to say that an accurate survey will be a waste if you don’t spend lots of time and money on it. You will be surprised on what you can accomplish on a “shoe string”. Creative thinking, experience, and innovation go a long way here. You must be diligent, tenacious, and through.

A good place to start a survey is in the center of the air space. Boot your laptop computer and with one of the utilities that come with you wireless NIC (Network Interface Card) take a listen. If you have a combination card that can hear several RF bands at once that’s a good thing. If not shuttle your cards in and out in compliance with your survey requirements.

Record any Access Points, Channels, signal strengths, and Clients you may pick up from your NIC. You will be working with and around these units now.
For your survey you will be capturing and recording several metrics based upon what your equipment can “hear”.

• Signal Strength
• Noise
• Signal to Noise
• Channel (analogous to FM radio station channels)
• Rogues (a rogue is a neighboring Access Point sharing the air space that you are surveying)
• SSIDs (an SSID is basically the Wireless Network Name)

Surveying is both an Art and Science based discipline. It is difficult only in that we are attempting to quantify that which we cannot see or hear. We rely on equipment to tell us if we are on track, how well the system is operating, and what can be done to improve conditions.

There is something else though. Call it experience, call it intuition. An experienced surveyor can look at the environment and know all kinds of details before a laptop key is ever pressed. Remember your visualizations? That only touched the surface of what you need to be able to visualize. Your imagination combined with the right equipment however may be what is needed to do a ACCURATE survey. My definition is accuracy is practicality. If people are satisfied with the outcome the survey is accurate. It does not matter if the numbers are favorable, or not favorable. We survey for usability. We survey to resolve problems.

There are commercial packages on the market that do lots of things for you in a survey. They are like calculators being used in a math test. They get right to the answer. To understand, to really understand a survey though, you should be able to survey manually--pencil and paper.

Begin your survey with your map and notebook. Start in one corner and work yourself around the environment finally ending where you began. Figure on moving at about ten feet every twenty seconds for a detailed survey, and about half of that for a quick summary of the area.

If it is a new survey, hang an Access Point in one of your targeted locations, power it up, and match your SSID and Security data to the AP.

If you have done your preparation, you have a good idea where the trouble spots will be and if it is a new installation, perhaps where the new Access Points will serve best. What does your utility tell you?

• How many Access Points it can hear?
• How many SSIDs?
• Signal Strength?
• Noise?
• Thuput?

Whatever your utility "sees" Use IT! Record it where you are at on the map and move on about ten feet in a detailed survey, and fifty feet in a general survey. Eventually you will have your map populated and you will be able to "see" what is in the air.

As you move from point to point on your map recording what you hear, you will be able to understand what is happening in the environment. Pay particular attention to noise. The relationship between signal strength and noise should be equal to or greater than 20dB. So if there is little or no noise you can have a faint signal, but if there is a lot of noise you will need a signal that is at least 20dB higher than what is referred to as the "Noise Floor".

How to Conduct a Wireless Site Survey Part 3

Meetings

In part 1 I talked about what equipment you will need to do a site survey; in part 2 I helped you visualize what the airwaves would look like while walking through the area to be surveyed. Having the equipment and an idea of what you are looking for is a great start; we need to push on however into specific skill sets.

The next thing that we need to do is to get the Survey Requirements, especially if it is a new installation. We need to meet with the end users of the system and determine what they need, and what they want. They may not necessarily realize that they need security, but they probably know that they want to push graphics through the air which means to us engineers, high speed.

If you are re-surveying a live pre-existing space for specific metrics such as throughput, errors, noise, or rogues, you will need to get detailed information about what the end user is experiencing. Latency, loss off data, and other tangible events are where you should start.

You will need blueprints of the facility (if available), otherwise you will need to make your own maps. Square footage, room size, ceiling heights, and obstructions need to be taken into consideration.

TIP: Take detailed and accurate notes at all of your discussions. As systems evolve sometimes expectations do as well.

You are surveying for the middle links in a network, so logically you will need to know both endpoints:
* Where data is sent from (laptops, hand held scanners, etc.)
* Where data is going to (Internet, across the office, to a printer, mainframe, etc.)

Many professionals fail to scope for capacity. A single access point could probably cover an area the size of a grammar-school gymnasium but if there are a couple of hundred people in seated in the room, the system will choke big time. How many users in each AP's zone is a crucial metric to know. A good rule of thumb may be no more than 20 per AP.

As long as I brought up the topic of a high density of people in a small area, how would you service them? In the 802.11g world you have (realistically) only 3 channels to choose from--we said 20 per AP... that's 60 people running OK. What about the other 140 people?

You would have to have a high density of Access Points, closely mounted, all running minimum power and the most attenuation you could get on their antennas. You would want the coverage of each AP zone to be very small. Very small. Not pretty but doable.

Knowing what the end user needs and wants is the basis of what you will accomplish in your survey. For example if you determine that most of the clients have 802.11g NICs with a few 802.11b units, you may want to explain how converting 802.11g wall to wall is a better investment, especially if they are going to be sending graphics.

To recap this installment provides direction on :
* End user requirements on a new install, or report on pre-existing problems
* Type of equipment they are using
* Accurate detailed notes and Maps.

Survey Views