SNR and RSSI Values.

Unanswered Question
Jul 28th, 2010

HI ,

What is the  tha average ,minimum and maximum  values of  SNR(Signal to noice ratio ) and RSSI (Received Signal Strength Indicator ) in  cisco  access points   .

And how it is depends on the client connectivity with the Access point.

Thanks & Regards,

Sinjish.K      

I have this problem too.
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k_sinjish Wed, 07/28/2010 - 07:15

Hi Kayle ,

It is highly explanatory and really helpful.

Thanks a lot

Thanks & Regards,

Sinjish.K

Kayle Miller Wed, 07/28/2010 - 06:36

I am not really sure what you asking here, but My guess is your trying to get a grasp on what SNR and RSSI

values are how they relate to clients, and how they relate to connectivity. If this is the case read on. otherwise well ingore this post..

SNR (Signal-to-Noise Ratio) is a ratio based value that evaluates your signal based on the noise being seen. So let's look at the components of the SNR and they see how to determine it.  SNR is comprised of 2 values and is measured as a positive value between 0db and 120db and the closer it is to 120db the better: Signal Value and Noise Value typically these are expressed in decibels (db).

     So we will look at the Signal (Also known as RSSI) first this value is measured in decibels from 0 (zero) to -120 (minus 120) now when looking at this value the closer to 0 (zero) the stronger the signal is which means it's better, typically voice networks require a -65db or better signal level while a data network needs -80db or better.  Normal range in a network would be -45db to -87db depending on power levels and design; since the Signal is affected by the APs transmit power & antenna aswell as the clients antenna (I'll explain this later.)

     Now to the Noise side of this equation, noise is any signal (interference) that is not WiFi traffic such as cordless phones, microwaves, radar, etc. This value is measured in decibels from 0 (zero) to -120 (minus 120) now when looking at this value the closer to -120 (minus 120) is better because that means there is little to no interference. Typical environments range between -90db and -98db.

     So to calculate your SNR value you add the Signal Value to the Noise Value and it generates (or should) a positive number that is expressed in decibels (db); for example lets say your Signal value is -55db and your Noise value is -95db.

                    -55db + -95db = 40db this means you have an SNR of 40, my general rule of thumb is that any SNR above 20 is good.

RSSI (Recieved Signal Strength Indicator) is a more common name for the Signal value; meaning it is the strength that the device is hearing a specific device or signal.RSSI is most common used in bridge links where on client laptops they just call it Signal.

As for your question about the maximum and minimum supported by cisco access points that is hard to answer because the antennas attached to the access points also factor into it. In general the Cisco APs have the following maximum transmit powers: 802.11a/n is 40mw or 14dbm, 802.11b is 100mw or 20dbm, 802.11g is 50mw or 17dbm; although in some cases countries and specific channels can alter these limits.

The last three things I will kind of explain is the EIRP, Free Space Path Loss, and Client side.

EIRP (Effective Isotropic Radiated Power) is the actual amount of signal leaving the antenna and is a value measured in db that is based on 3 things:Transmit Power (db), Cable Loss (db), & Antenna Gain (dbi). To determine EIRP follow this equation: - Cable Loss + Antenna Gain = EIRP. For example we have a Cisco 1242AG access points running at full power with a 6dbi antenna on the 802.11a radio and a 2.5dbi antenna on the 802.11bg radio.

802.11a EIRP = 17db (40mw) - 0db + 6dbi = 23db = 200mw of actual output power

802.11bg EIRP = 20db (100mw) - 0db + 2.5dbi = 22.5db = 150mw (approx) of actual output power

based on the example above in theory if you were to measure it right at the antenna you could get an RSSI of -23 or -22.5 respectively.

Free Space Path Loss is a measure of how much signal power you lose over a given distance typically you lose about 0.020 db per foot in an outdoor or wide open office; doors, walls, glass, and etc. affect this. This is why as you walk away from an AP your signal gets weaker.

All this relates to the client because it determines the signal the client recieves, also keep in mind that when looking at the client you have to account for it's antenna as well.  much like the EIRP...  So if a client card has a 2 dbi antenna (although they are typically either 0dbi or 2.2dbi) that boosts the incoming signal. as shown here assuming the actual raw RSSI signal being seen is -68db (not the value displayed in the client window).

Actual RSSI + Antenna Gain = Displayed RSSI

-68db + 2db = -66db

I hope all this helps with your question...

Please rate useful posts.

Thanks,

Kayle

johnruffing@sun... Thu, 12/02/2010 - 13:00

The following post has the "short version" published by Cisco:

https://supportforums.cisco.com/thread/2055709

Please note that there are differences between data only and Wireless IP telephony:

(Unfortunately, 54Mbps is the max. published data rate.  I am hoping that someone may know of an updated table with additional 802.11n data rates that goes up to 300Mbps)

http://www.cisco.com/image/gif/paws/68666/wireless-site-survey-faq-1.gif

Source:

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_q_and_a_item09186a00805e9a96.shtml#q26

VoWiFi VoWLAN WIPT SNR

grigione2015 Fri, 02/13/2015 - 23:22

Please,can you explain me why with an EIRP of +22.5 dBm/+23 dBm the RSSI is  -22.5 dBm or 23 dBm near antenna?Why there is a negative number?thanks

Kayle Miller Sat, 02/14/2015 - 16:04

Grigione2015,

There is a very long a detailed explanation that could be put forth, but the from a more simple perspective is that industry wide and IEEE Specifications dictate that transmit power is generally expressed as a Positive Number greater than 0 thus the EIRP is measured in Positive dBm, but the same Industry and IEES Specifications dicate that the RSSI or Received Signal Strength as well as Signal be expressed as a Negative Number.. Why they chose to do this is not a simple answer; I admit that as a result it makes it very confusing for some people.

grigione2015 Sat, 02/14/2015 - 23:49

If I convert RSSI level in positive values I get good results for measurements near antenna router but If I go far away from router I get measurement values over 20dBm!!In my situation with RSSI positive I get 20/25 dBm near router but 60/70 dBm far away from router!!It's no physically possible that!!I presume that level (in dBm negative) over some definite threshold  is not a valid measurement in reason of ADC conversion limits.What are you thinking about it?
 

George Stefanick Sun, 02/15/2015 - 08:35

In yiur client the Rssi is a negative bud .. Why Re you saying positive ? 

 

Kayle you did a great job explaining this 5 years ago .. Man we are getting old ..

grigione2015 Sun, 02/15/2015 - 13:10

The problem was following : if eirp of router is +20dBm then why RSSI is -20dBm very very close to the router antenna?

Kayle Miller Mon, 02/16/2015 - 08:40

I think what you are seeing in your client is either the SNR (which should be a Positive Number) or your client software is mis-representing the number as a Positive when it's really a negative.

grigione2015 Mon, 02/16/2015 - 22:55

Any wifi analyzer application on my Android show me RSSI negative values always.Near antenna router value is about -20dBm and far away vaue is about -60dBm.If I convert them in positive value it's phisycally impossible.

Kayle Miller Tue, 02/17/2015 - 04:01

Grigione,

 

 so if your application is showing you a -20 dBm at the antenna you say that your output is 20dBm then that is potentially accurate, but the true factor is if you then look at your SNR which is -20 - -95 (guessing) = 75 that would be correct. or -60 - -95 = 35, again which is valid correct and completely possible.

grigione2015 Tue, 02/17/2015 - 05:34

Ok if my application show me a -20dBm value near to antenna and I consider it as +20dBm then ok ,but  if my application show me a -60dBm value far away from antenna router I can't consider it as  +60dBm value  because it's physically impossible!!!

Scott Olsen Wed, 02/18/2015 - 05:28

I think the crux of the issue here is not fully understanding or appreciating the *significant* loss of RF power as it actually transitions from the transmitting antenna element, through the air, and induced into the receiving antenna element.

I get the impression that Grigione2015 is trying to rationalize why there is an apparent loss of over 40dB with the transmitter and receiver physically right beside each other.  The answer, my friend, is physics.  The power driven into the transmitting element will be no where near what you'll actually be able to receive/induce into the receiving element.

http://en.wikipedia.org/wiki/Antenna_%28radio%29

Grigione, if you are new to wireless and/or RF measurements; I can understand why this is perplexing.  Sometimes a chart can help give a little perspective:

http://www.rfcafe.com/references/electrical/mw2dbm.htm

Best point to remember; values greater than 1mW are expressed as positive dBm values, and energies *less than* 1mW are measure in negative values.  Every change of 3dB is a doubling or halving of power, depending on whether its +3dB or -3dB.

I hope this was helpful...  if it was, please rate! 

Cheers!

 

grigione2015 Wed, 02/18/2015 - 06:10

Please,explain me theoretically with physics why there are 40db attenuation from tx antenna (eirp +20dBm) to rx receiver( -20dBm) when the distance each other is very very near (near field) and frequency is about  2500MHz.Thanks

Scott Olsen Wed, 02/18/2015 - 07:33

Well, to be honest, this unfortunately exceeds my current knowledge of the physics involved.  At least my comfort level in being able to succinctly and accurately attempt to explain it is exceeded.

A good portion of the explanation lies right in the Free Space Loss equations and models.  *Initial* free space loss is considerable due to the logarithmic nature of the radiated power, even at _very_ close ranges.

I've dug up the following in hopes of explaining things a bit better (and refreshing the knowledge myself);


http://www.antenna-theory.com/basics/friis.php

http://en.wikipedia.org/wiki/Friis_transmission_equation

https://www.youtube.com/watch?v=3d_SZ-kE7o0

http://ict.siit.tu.ac.th/~sgordon/its323y14s1/reports/its323y14s1h11-path-loss-model.pdf

 

If anyone else has some additional 'physics' style explanation as to why initial free space losses are so extreme, I myself would like to hear it as well.  I just know that it is.

Cheers.

grigione2015 Wed, 02/18/2015 - 12:01

In the link budget for Ptx=20dBm ,Transmitter Gain 1dBi, Receiver Gain 1dBi,Frequency =2.5Ghz,Distance =1Cm  then recived power is equal to +19.60dBm!!!!

http://www.pasternack.com/t-calculator-friis.aspx

 

 

 

 

Scott Olsen Wed, 02/18/2015 - 13:16

I'm not sure what to tell you.  I can assure you however, that you will be very unlikely to register an RSSI value greater than -20dB in real life.

In fact, I wouldn't be surprised if most radio hardware out there now have some form of protection mechanisms to limit received radio power to 0, -10, or -20dBm.  They really aren't designed for power levels that high in typical operation.  Sensitivity at much smaller received power levels is of much greater importance and practical use.

grigione2015 Wed, 02/18/2015 - 23:03

"I'm not sure what to tell you" ??? Please ,control yourself by http://www.pasternack.com/t-calculator-friis.aspx

Second my opinion smartphone frontend receiver is automatically protect for overload and adc conversion is made for a precise range power values.When the receiver go near transmitter the received power increase and for a specific threshold (-30 dBmfor example) the receiver limites input power to prevent saturation problems.

 

patstew00 Mon, 07/13/2015 - 04:28

The Friis transmission equation is only valid in certain conditions, one is which is that the distance between antennas must be much greater than the wavelength of the signal (see wikipedia for more info). For Wifi it's inacccurate for distances below a meter or so.

You will never get anywhere near the amount of power you put into one antenna out of another antenna nearby, unless the antennas are specifically designed for maximum efficiency in the near field (NFC, wireless chargers). In reality you will be lucky to catch a ten-thousandth of the power (-40dB). The rough explanation is that the power from the antenna is radiating in all directions (actually this is impossible due to the 'hairy ball theorem', but it's close enough) and only the part that passes through the receiving antenna is 'caught' by that antenna. Also having another antenna in the near field will reduce the efficiency/gain of the antenna, and the EM field in the near field is not the right shape to be picked up efficiently (this is difficult to explain, the maths gets pretty complicated...).

Looking at it another way, if what you're expecting to happen actually happened, when you put your receiving antenna close to the transmitting one it would prevent any other device from accessing your wifi by sucking up all the power.

Someone above suggested that the positive/negative thing is just a random choice or that the sign might be wrong. It's not, it's a measurement of power. dB means a ratio measurement relative to *something* (ratio = 10^(dB/10)). +10 dB means 10 times bigger and -10 dB means 10 times smaller, so -40 dB means 10x10x10x10 = 10000 times smaller. dBmeans dB relative to 1 mW, so 20dBm is 100mW, and -20dBm is 10uW. Wifi transmitters are usually ~100mW, and you will never get better than -30 to -40 dB of path loss with normal wifi antennas, so the received power is always less than 1mW, so is always negative when measured in dBm.

Kayle Miller Wed, 02/18/2015 - 11:01

Grigione,

 

If you goto this site http://www.pasternack.com/t-calculator-fspl.aspx?gclid=CLGX46qW7MMCFW4Q7AoduH4AVg

 

this will allow you to calculate the free space path loss,

if you assume a frequency of 2437 (channel 6), a distance of 30ft., the transmitter antenna has a gain of 2.2 dBi, and the receive antenna is a typical laptop device which has a 0 dBi antenna then the free space path loss is equal to 57.20 dB

 

You can read more about free space path loss here

http://en.wikipedia.org/wiki/Free-space_path_loss

 

George Stefanick Sat, 02/14/2015 - 16:47

The easiest way to explain this .. A radio that is transmitting is a positive like 20dbm . A radio receiving this transmission isn't putting anything on the medium he is listing so he is a negative .. 

Abhishek Abhishek Tue, 02/17/2015 - 17:51

The received signal strength indicator (RSSI) represents the value in dBm of the signal strength at which a wireless packet is received. Figure 4 shows the scatter plot of approximately 200 RSSI values received at the WSM and primary radio collected from different points. The primary radio is the internal radio of Cisco Aironet 3600i.

Figure 4.      RSSI Comparison between WSM and Primary Radio

Figure 4 shows the high degree of correlation between the RSSI for the two radios. On average, the RSSIs received by the primary radio are about 2 dB stronger than the corresponding RSSIs from the WSM. This clearly indicates that the WSM has similar coverage to that of the Cisco 3600i access point.

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