Solved: understanding dBm in reference to SNR

robert.wright · ‎09-08-2006

Good morning...

Days like today i wish i wouldn't have slept through math class

So heres the situation... I have an area of my facility which has an SNR of 25. Roughly 5 yards in another direction i have a SNR of 46. I know what is causing the noise but i want to fully understand the effects, besides noting theres a 21dBm difference. As from my understanding its much more than 'just' 21.

I understand that dBm is a ratio, in this case with miliwatts. However im getting confused and looking to see if someone can help.

dagyb · ‎09-13-2006

Another way to look at it is: for every 3db of change equates to the signal doubling.

for every -3db the signal gets cut in half

View solution in original post

Rob Huffman · ‎09-08-2006

Hi Robert,

I'm with you on that one. I always thought (in math class) am I ever going to use this?? It turns out, I guess the answer was yes!!

I have attached some links that describe SNR in laymans terms rather than the in depth mathmatical versions that are certainly out there. Even reading these today gave me some new insight.

The ratio of the power or volume (amplitude) of a signal to the amount of unwanted interference (the noise) that has mixed in with it. Measured in decibels, signal-to-noise ratio (SNR or S/N) measures the clarity of the signal in a circuit or a wired or wireless transmission channel.

The Larger the Number, the Better

The greater the ratio, evidenced by a larger number, the less noise and the more easily it can be filtered out. The lowest number is an SNR of 0, which means that noise and signal levels are the same. Although signals contain non-random intelligence and can be isolated and separated, with a 0 SNR, it would be extremely difficult to isolate the signal in realtime. It would be more easily accomplished offline.

With wireless networking with 802.11b, your speed is determined through calculations of the signal you are receiving, and the background noise that is on that frequency at that time. When your signal reaches a level that is within 10dBm of the background noise, your data rates shift to 5.5Mbps from 11Mbps. At 8dBm from the noise level, your data rates shift to 2 Mbps. Similarly, at 6dBm, the data rates go down to 1Mbps (practically nothing in comparison). This difference between the signal level, and noise level is called the Signal-to-Noise Ratio (SNR).

http://is.med.ohio-state.edu/Wireless%20FAQ.htm#low

http://www.gwec.org/students/resources/Files/Understanding%20WLAN%20signal%20strength.pdf#search=%22UNDERSTANDING%20WIRELESS%20SIGNAL%20TO%20NOISE%20RATIO%22

Here is a good doc from Airespace;

Simplify WLAN Planning and Deployment

http://www.airespace.com/products/appnote_wlan_planning_design.php

Hope this helps!

Rob

robert.wright · ‎09-08-2006

thanks for the help..

What i am looking towards is that it was my understanding that snr of 21 and 20 were not simply a difference of 1, as dBm is an ratio. Thus im trying to determine what the actually effect of the said interference is. Like a difference of 1 is say 25% more. (purely an example.

Ive been 'stalking' your posts and pulled the other 2 from yours i think. =)

Rob Huffman · ‎09-08-2006

Hi Robert,

Sorry I missed the mark, my personal SNR must be getting pretty low ;)

Here is a really interesting article (and pretty funny!!) that explains the dBm and decibels in general in the basic electrical sense.

http://www.prorec.com/prorec/articles.nsf/articles/EA68A9018C905AFB8625675400514576

In the view of "wireless", here is a pretty good "rule of thumb" that seems quite applicable;

Here is an excerpt-

"I recently ran user-oriented tests to determine the impacts of SNR values on the ability for a user to associate with an 802.11b/g access point and load a particular webpage. For various SNRs, here's what I found for the signal strength (found in the Windows radio status), association status, and performance when loading the http://wireless-nets.com/staff.htm webpage from a wireless laptop. To ensure accurate comparisons, I cleared the laptop's cache before reloading the page:

> 40dB SNR = Excellent signal (5 bars); always associated; lightening fast.

25dB to 40dB SNR = Very good signal (3 - 4 bars); always associated; very fast.

15dB to 25dB SNR = Low signal (2 bars); always associated; usually fast.

10dB - 15dB SNR = very low signal (1 bar); mostly associated; mostly slow.

5dB to 10dB SNR = no signal; not associated; no go.

These values seem consistent with testing I've done in the past, as well as what some of the vendors publish."

From this doc;

http://www.wi-fiplanet.com/tutorials/article.php/3468771

Hope this helps! (or maybe I'm still way off base)

Rob

gpulos · ‎09-11-2006

waaay to much info for a quick and easy answer....

5 points just for the research and dissemination of the quality information!

Rob Huffman · ‎09-12-2006

Hi Greg,

Thanks for taking a few moments out of your busy day for the nice nod!! I have read many great in depth posts from you, so it really means alot. It is really something that so many people contribute such time and effort in these forums to help each other out. When I stumbled upon the NetPro forums I was blown away! I didn't know that such a thing existed and it came at a time when I had really hit the wall in my Cisco studies. Reading books is one thing, but the knowledge that these people were sharing took things to a whole other level. I guess what I'm trying to say to all the participants of these forums is Thank you!! You have made this experience so enjoyable and rewarding, and I look forward to reading your great posts as often as I can. Special thanks to the people who have always gone above and beyond in the quality and sometimes humour of their answers;

ScottMac,Ginger Kavan,Shanky,Travis Dennis,Brandon Buffin,Chris Deren,Jeff Lindborg "the ultimate Unity Guru",Aaron Harrison and all the Cisco staff engineers who add their sage advise in ongoing threads and "Ask the Expert" events. I know I have missed many other great NetPros in many forums that I don't read, so thanks to all of them as well!

Thanks again Greg, and take care!

Rob

dagyb · ‎09-13-2006

Another way to look at it is: for every 3db of change equates to the signal doubling.

for every -3db the signal gets cut in half

robert.wright · ‎09-14-2006

a ha! wooohoo! :) thanks.

I have been attempting to do the math and such from one of the sites Rob gave me earlier. But other items keep getting in the way.