Sunday, February 15, 2015

Understanding SNR and RSSI values

Hi Guys

I saw this post on a forum while investigating other issues and had to note it down for you and for my own reference! Finally a definitive answer easily understood to a question I have had for ages regarding determining wireless performance


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.

Great stuff, found the post here:
https://supportforums.cisco.com/discussion/10954591/snr-and-rssi-values


 Also worth pointing out as per his post that the 7925g handsets can actually be used to perform site surveys! Another handy trick!
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7925g/7_0/english/administration/guide/7925wrnt.html#wp1117827

 For more information

3 comments:

  1. RSSI is the received signal strength indicator. It's a relative value. It cannot be easily compared between different devices as parameters like the antenna gain are important too. This is different from 802.11's RCPI value. However, regardless of RCPI or RSSI both express a power value. Instead of indicating tiny little values in muliples of Watt [W], signal strength (RSSI) is often measured in dBm. There is a convention that dBm indicates levels relative to 1 mW. RSSI and RCPI are never expressed as dB as the latter indicates relative values. If you have five eggs and somebody else has ten eggs, the latter has 3 dB more (in terms of comparing eggs with power). But 3 dB does not tell you an absolute value. -62 dBm, however, tells you that the received signal strength is -62 dB below 1 mW of received power. So, RSSI and RCPI are commonly measured in dBm, just dB is wrong.

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