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".