SNR and RSSI Values.

[k_sinjish]

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      

[Kayle Miller]

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]

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)

Source:

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

VoWiFi VoWLAN WIPT SNR

[Dean Bickmore]

Great Explanation.....Kayle

[grigione2015]

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]

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]

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]

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]

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

[Kayle Miller]

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]

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]

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]

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]

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]

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