Turn down that NOISE!!!

Here is a fun experiment if you have a 2 laptops (or 1 laptop and some other WiFi enabled device), a WLAN analyzer like AirMagnet WiFi Analyzer (or something similar such as wireshark) and a microwave oven.

We all know that microwave ovens function in the 2.4Ghz range but how much "noise" is it generating.

Go to your kitchen. Get the device without an analyzer on it and start a really long file transfer via WiFi. Make sure your AP is already on either channel 11. Now Fire up your analyzer and measure the signal, noise and the signal to noise ratio (SnR is signal minus noise) on channel 11. Get an average reading and write it down. Do it several times so you are sure you have a good sample set. Now, restart your file transfer, put a pyrex (or other microwave safe) measuring cup full of water in your microwave and set it for a few minutes on high. Re-measure your signal, noise and SnR. What did you get? Try a few different WiFi adapters with the analyzer. any difference?

Here is what I saw when I did it with the AirMagnet WiFi Analyzer. First is the image of the channel screen on channel 11 with no microwave turned on. Notice the stats.

Signal:      -51dBm
Throughput:  ~34Mb/s
Utilization: 68%
Noise:       -88dBm
SnR:         37 (-51) - (-88) = 37


Here is the important part. Notice that the noise level is completely flat at -88dBm.

Now lets turn the microwave on and do it again, here is what I saw.

Signal:      -47dBm
Throughput:  ~16Mb/s
Utilization: 39%
Noise:       -88dBm
SnR:         41 (-47) - (-88) = 41

Notice anything? Well first off throughput went down and I checked the capture later and packet loss went up. Seems appropriate with a 1200watt device slamming my connection from 2 feet away. Secondly, the signal strength and the SnR INCREASED? Why is that? Well after looking deeper into the frames and looking at the AP configuration, it looks like 802.11N beamforming in action! The AP senses packet loss and pushes harder (no-tech speak for adjusts the phase shift of the signal to focus the waves on top of the client so it gets the benefit of wave addition). Next thing you notice is the increase in SnR from 37 to 41. This is due to two issues. The first we just mentioned. Increased signal strength. The second is the trouble spot. The noise level stayed the same.

What?!!

You heard right, the noise level did not change. I can hear you yelling at me through the intertubes from here. But I thought microwave oven caused noise? Well. they do and they do not. Here I am about to get pretty picky.

You see we at AirMagnet call noise one thing and interference another and WiFi interference a third. Confusing but true. And with good reason. You see, noise readings are fiction. They are falsehood, fabrication, fib. It is a lie. There. I said it. Noise readings (except from a very small number of cards - namely only one kind that I know of) are made up on the spot by each WiFi adapter. They cannot give you a noise reading, they can only give you bits (e.i. 1s and 0s) about things they see and WiFi adpaters only see WiFi modulation. Period. Almost all wifi adapters throw away all other non-WiFi signal long before it ever gets to the the driver. Non-wifi modulation, such as the modulation you get from cordless phone, wireless headsets, video camers etc are thrown away by the radio long before they can be converted to bits. The radios are only tuned for the type of modulation and coding they understand. So a normal 802.11a adapter will only ever pass OFDM modulation. Never QPSK, or FHSS or any other signal that is not OFDM. Thus how can the driver tell us about a microwave oven, or a cordless phone, wireless camera etc? They can't. So we call interference from other modulation types in the same frequency band by a different name, we call it non-802.11 interference.

Then there is WLAN interference, which Tom's Hardware calls "congestion", but could also be thought of as WLAN contention and is sometimes referred to as co-channel interference or even adjacent channel interference. Tom's has a very good article on it here. It is where too many WLAN devices are all trying to use the same medium simutaneously. In 802.11 terms, only one device can talk in an area at one time. Too many devices means collisions and contention in a collision avoidance network that wishes to remain contention free.

So that still leaves us with noise. What is going here? When I open a WLAN analyzer I see a noise reading. You showed me one right there (see images above). Where does that come from?

To understand what the adapter manufacturers are doing requires that we first understand what noise is. Noise can be thought of as the level at which all radio signal become indistinguishable from one another. Wikipedia describes the noise floor as, "the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system." You can create an analogy of this as follows: Imagine five people in a room, each yelling in a different language. You could probably figure out what each language was even if you couldn't have a conversation in there, you might even be able to work out what they are talking about. That example would be considered interference to you if you were trying to yell in English. Now imagine a room with 500 people in it all talking at the same time. in that instance you proabaly could not tell what any of the languages were or what anyone was talking about at all. That is noise. Again, the Tom's Hardware article is a great reference for this, especially slide 9.

 Adapter manufacturers were asked many times for a noise reading so the person conducting the WLAN site survey could determine the signal to noise ratio, or SnR. This helps determine if there is enough signal to be heard above the RF noise floor. So under this presure they created algorythims that can fake the noise reading. I have no idea what the algorythims are but I can assume from tests we have done at AirMagnet that they include measurements of retry rates, CRC errors, channel utilization, average throughput and, mostimportantly, number of devices active in range (as determined by RSSI). The reason I can know this is because at AIrMagnet we have an RF isolation chamber - or Faraday Cage. If I take a noise reading in that chamber with one AP and no STAs (WLAN client adapters) I get a zero reading with many adapters. But if I add STAs the noise floor rises. even if the STAs are mostly dormant.

Here is another catch, aside from the fact that different manufactureres create this value differently, some adapters only give noise readings on a channel by channel basis, others frame by frame and still others give no reading at all. Symbol devices fit this last group and when asked about why they gave no noise reading they replied, "that would be lying". Does that mean that you cannot use that value to create the much sought after SnR? No, I would still use it, but I would be wary if tehre were very few STAs nearby. Or many for that matter.

I hope this helps understand what noise is, what interference is and what co-channel interference or WLAN contention is.

How to (really) find a Rogue AP

I want to tackle a problem that most people think of having been solved a long time ago. Rogue Access Points (I can almost hear the groans). Seriously, it deserves another look. Why? Well, to be truthful, I think when trying to track down and physically remove the AP, most people do it wrong.

While almost every infrastructure vendor has a method for the determination and mitigation of an unauthorized access point, what they cannot do is physically remove the device from your premises. No WLAN WIPS robot has yet been created to go out to a facility and track down exactly where a rogue is and then pick it up and deposit it in a dumpster or ask the owner to please take it home or whatever it is your security policy says you should do when one of these is found.

Instead, we rely on both our infrastructure and a variety of laptop or handheld based software tools to assist us in this task and then we send a real flesh and blood person out to do this. This is where the major mistakes are usually made.

Every person I have met that has this task as one their responsibilities have told me the following, “I want to use a wifi adapter with a directional antenna to find the device. Just point it and it will tell me where to go.” This is wrong. Let me say that again, WRONG!

Let me explain why.

Let’s take one of the “flag” antennas that are fairly popular now (fig. 1).

Fig. 1

 Inside the plastic shell of this device is a Yagi-Uda antenna, commonly called a “yagi”. This is a fancy name for the type of antenna that televisions used to use before cable and satellite. Here is an example of what a yagi looks like without a shell (Fig. 2).

 

Fig. 2

Antennas professionals use a pattern diagram to describe the sensitivity pattern for the antenna. The diagram shows a pattern view from above and from the side (Fig. 3).

Fig. 3

The left view is looking down on the antenna from above known as the azimuth view and the right view is from the side and is known as the elevation view. This illustrates the amount of gain this antenna gets in any particular direction.  The problem here is when you start to use a high gain (in this case 8dBi) antenna indoors you get some strange effects due to the lobes described by the diagram above.

Here is an example. You are determined to remove a Rogue AP physically from your building. You stand in the building and aim your antenna. As you sweep left and right you watch the signal strength of your rogue finding tool. In this case we will use the find tool in the AirMagnet Wi-Fi Analyzer (Fig. 4).

Fig. 4

You monitor the signal strength and figure that when you are aiming right at the AP it will be the strongest. The graph will be at a peak and the meter will show the highest signal strength. However the signal is herky-jerky and very difficult to know when you are aiming right at the Rogue. Sometimes it is high when you are way over to the right and sometimes it is just as high when you are pointing to the left. Why is that? Shouldn’t it just be obvious when you have the device in your sights? No. Let me show you why.

As you aim your directional antenna the gain field of the antenna sweeps out ahead of you and you get a high reading as expected when pointing right at it (Fig. 5).

Fig. 5

Then as you sweep away to the right the signal drops when you hit the gap between the primary and secondary lobe (Fig. 6).

Fig. 6

Then the signal jumps up again as you enter the second lobe (Fig. 7).

Fig. 7

Also it may jump again as the signal bounces off of the walls and reflect back towards you (Fig. 8) since when you aim directly at the Rogue AP the signal is attenuated by the internal walls but the signal reflected back to you is clear.

Fig. 8

You see the directional antenna with a very high gain can be fooled. A user may spend 20 minutes trying to track down this device sweeping left and right trying to find the correct direction.

There is a better way. I will show you how to find a Rogue AP in just a minute or two and as a bonus you get some great exercise as well.

The key here, counter-intuitively, is to use an omnidirectional antenna and walk very fast. The technique is quick and accurate. You will find the rogue in less than a minute, whereas with the previous method it may take many minutes or more.

The first step is to stand at the edge of the building and walk perpendicular to the wall. It is very important that you walk fast as the graph you will be watching will be jiggling around and you will need to be able to observe a distinct change to the graph which will be easier the faster you walk. Keep your eye on that graph and when it peaks and starts to decline, stop and back up to the area where it peaked (Fig. 9).

Fig. 9

Now turn 90° to either the right or left and start walking that direction quickly as well. If your signal immediately declines, reverse your course and walk the other direction (Fig. 10).

Fig. 10

Once again, when the signal peaks and starts to decline, turn 90° and walk quickly. In this way, you will box in on your objective and be able to walk right up to it and remove it (Fig. 11).

Fig. 11

I think once you have tried this method you will be extremely pleased with the result. If you have tried to use a directional antenna to find a Rogue AP in the past, you will know that you can spend quite a great deal of time aiming your antenna before you even try to move towards the device. Then when do start to move around the results are hard to determine and you tend to move slowly and carefully. This method that I have described is fast and efficient and enables you to quickly find your problem Rogue AP.

Our AirMagnet class instructor has taught over 5000 students how to use this method and it really is more efficient.  Try it out and let me know what you think.