What is signal / noise?

Unanswered Question
Feb 25th, 2009

Hi everyone - hope someone knows the answer to my following question. I simply need to know what is signal and what exactely is noise.

Signal:

I guess the "signal" value, is the signal strength of the 802.11 traffic. right?

Noise:

From my understanding, noise is ONLY non-802.11 traffic. So stuff in the 2,4 or 5 GHz band - that is not 802.11 traffic, right?

So, if there are two neighbor APs, that operate in the same channel, they don't see each other as noise - is that right?

They share a common CSMA/CA domain which means throughput reduction.

So, are my thesis correct? :-) Could someone please enlighten me?

I have this problem too.
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George Stefanick Wed, 02/25/2009 - 11:53

Signal:

I guess the "signal" value, is the signal strength of the 802.11 traffic. right?

Signal is how loud a client in this example hears the AP. The lower the dB the louder it is. For example -35 is close to the AP. -85 is much further from the AP.

Noise:

From my understanding, noise is ONLY non-802.11 traffic. So stuff in the 2,4 or 5 GHz band - that is not 802.11 traffic, right?

Noise, like signal has a measurement as well. You dont want noise to be loud. So noise should be on the other end, for example -100 to -90.

Noise CAN be either 802.11 OR NON-802.11. If you have 2 access points on the same channel for exmaple. One access points traffic is another access points noise. Think back to the early cordless phones days. What happen when you picked up the phone and you can hear your neighbor. Now imagine for a moment you made a call while your neigbor was on a call. It would be hard to have dual conversations going on, right ... Same holds true with access points.

So, if there are two neighbor APs, that operate in the same channel, they don't see each other as noise - is that right?

They share a common CSMA/CA domain which means throughput reduction.

yes, they will see each other as noise.

one aps tarffic is another aps noise if very close.

So, are my thesis correct? :-) Could someone please enlighten me?

jeff.kish Wed, 02/25/2009 - 14:39

Something to add to gstefanick's great post (+5 for that) is that the signal/noise ratio (SNR) is what determines the quality of a received signal. In other words, the signal itself could be very good, but if the noise is high then it might not matter.

This is perhaps common sense, but it matters because when surveying for wireless signal you can't just look at the signal strength itself. You must make sure that the SNR is of a certain quality in order to truly gauge wireless signal strength. You want to see SNR values of 25 or more, generally. 35 or more for phone surveys.

(Signal and Noise are both measured logarithmically, so taking the ratio of the two results in the two numbers being subtracted. In other words, a SNR of 25 doesn't mean to expect a noise that's in the -1200 range when your signal is -50. A good situation would be a signal of -55 and a noise of -90, so -55 minus -90 is +35).

robinjellum Wed, 02/25/2009 - 15:01

Slight correction. If two APs on the same channel can decode each others 802.11 signal, it is not noise. If the signal is degraded such that it cannot be understood, it is noise.

dennischolmes Wed, 02/25/2009 - 20:05

Noise as a rule is usually caused by native APs in your own network outfitted with omni directional antennae and outside sources such as microwave ovens etc. You see WLAN transmissions are actually the transmission of electrons into an environment. The way we transmit those electrons and the patterns in which they are dispersed result in signal strength and noise.

Signal strength can be measured in several ways but the most common is relative signal strength indicated or RSSI. RSSI is measured dbm and the lower the number the stronger the signal. RSSI is calculated by the formula: tansmit power + antenna gain - cable loss = EIRP or radiated power. Radiated power - path loss = RSSI. The excessive electrons in the area also affect the reciever of an AP. The earth has an actual background noise that normally is around -94dbm. The use of omni directional antennae results in a lot of free flowing elctrons in the environment. The higher the power, the more electrons.

Signal to noise is the relationship of signal to noise. For instance, RSSI of -65 and noise floor of -90 is a SNR of 30. One of the quick fixes to maintain a good SNR is the use of directional or patch antennae in 85% of your deployment. This focuses and places the electrons where you want them and doesnt spray them all over the floor. Downtilting the antenna also helps as downtilted antennae spray the electrons in a more predictable manner. So, in closing, control noise by removing outside noise sources like microwave ovens, baby monitors, etc., and using directional patch antennae where you can. Use omnis sparingly and always remember the lower the power the cleaner the signal. A good rule of thumb is 50mw of power, -65 cell edges with a 10-20% cell over lap for 2.4 ghz. Use 85% directional and you should see a 5-10dbm reduction in noise on the floor. Also reduce the data rates broadcasted by turning off lower data rates when not needed. 1 and 2mb data rates for sure need to go. This lowers the number of broadcasts and as a result lowers the amount of electrons put into the air.

johannes.luther Wed, 02/25/2009 - 22:10

Thank you all for you great post. That helps. The one open point is still the co-channel interference (2 APs on the same channel can "hear" each other).

Is the received signal from the other AP noise or not? We have two opionions here.

I also guess, that foreign 802.11 packets cannot be noise (except they got damaged on the way).

dennischolmes Thu, 02/26/2009 - 04:30

Yes. Other APs can be noise. If the level of interference is recognizable by the AP you are using as RF packets then it is either cochannel or adjacent channel interference. If the electrons are so dispersed that they no longer contain readable data packets and as a result only raise the ambient noise floor, then they are called noise generators. Does that make sense to you?

Scott Pickles Fri, 02/27/2009 - 12:15

It is often difficult to understand these concepts when we're just talking about frequencies, channels, dBm, etc. So let me try an alternate viewpoint. Think of two guys rocking it out on stage with guitars. The guitars are both representative of APs on the same channel. If the two players are far apart from eachother, the people in the audience right in front of them will be able to distinguish the particular guitar solo being played. But now if one guy brings his guitar over next to the other one and they are both jamming at the same volume level (dBm) it will be difficult for the audience members to distinguish the individual solos being played. Therefore, the one guitar solo is considered noise from the perspective of the other soloist. The separation between the two players and the difference between the volume levels of their solos is the SNR value. SNR takes into account how much louder (or softer), better/worse, etc. the intended signal is from competing signals in the air. To summarize, if two APs are near enough to one another on the same channel, they will mutually cause an increase in the noise value for the other AP. If the noise is low enough such that the signal from the desired AP can be heard above all other signals in the area, then you will have reliable communications. How much better the signal you want is as compared to the noise will indicate to you what data rates are possible. This is why a high SNR value is necessary for voice. We need a value of ideally 20-25 so that we can reliably use the higher data rates (which require a cleaner signal to successfully utilize the more complex encoding schemes to cram in more data). Without this superior throughput, real-time applications like voice suffer. As a general rule, when deploying 2.4 GHz for voice, -65 to -67 dBm signal strength (depending on the receive sensitivity of the client) with 20-25 of SNR. Once you have separated two APs such that the signal strength of one as it relates to the other reaches a value as low as approximately -90 dBm, you can safely reuse that channel. The power settings and antenna types on the APs will go a long way to allowing you to successfully deploy such a scenario.

Regards,

Scott

johannes.luther Fri, 02/27/2009 - 23:28

Thanks for your explanation. Allow me one question :-)

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Do you really need both values when doing a site-survey? The SNR value directly depends on the signal strength. The APs (and clients) decision which data-rate to use is based on the SNR value, right? So I thought, that the SNR checking is enough.

Scott Pickles Sat, 02/28/2009 - 11:21

While signal strength is important, and it is the one variable you have complete control over, let's not forget that SNR is equal to the absolute value of the difference between your signal strength and the noise. So while you can control your signal strength, if you neglect to consider the noise component you will not have effective QoS (particularly for voice). Voice needs to maintain a maximum delay of less than 150 ms. You aren't going to get that without fast transport. For wireless, the only way to get faster transport is high data rates. What do you need to achieve a higher data rate? A clean signal. What is a clean signal? A high SNR value. You need that clean signal to take advantage of the more complex encoding scheme that allows you to cram more data into the same space provided by the CF (carrier frequency). Therefore, without considering noise as well, you will not have reliable voice grade communications by focusing solely on signal strength. There are tools available to measure noise, and my company (VPN Systems, Inc.) uses spectrum analysis along with other monitoring software.

Regards,

Scott

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Posted February 25, 2009 at 11:17 AM
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