what is RSSI , SNR and technical specification value in dbm

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Oct 12th, 2010

Hi,

I am little bit confused about RSSI concept and SNR , Signal strength.

One more thing that , what is technical specification of signal strength and SNR and RSSI for 802.11 a,b,g and n

Because as far as application concern , this values get changed eq. voice , video

I need to get the values according to application.

I have this problem too.
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Nicolas Darchis Tue, 10/12/2010 - 10:17

Hi,

RSSI means Receiver Signal Strength Indicator. It's the strength of the signal as perceived as received by the client. It does not have a precise unit of measurement. On client softwares, it's usually a percentage (that has no real meaning). WLC and APs usually show in dbm.

SNR is Signal/Noise ratio. The noise level is usually at -90dbm let's say. This means that if you have a signal at -90dbm, it's completely lost in the noise, it has a SNR of 0.

A signal at -80dbm has a SNR of 10 in the same conditions, it is barely decodable.

However if the noise level is -95 then a signal of -80 will give a SNR of 15 and will be much more decodable.

Noise is usually between -100 and -90 in normal circumstances.

Dbm is another way of expressing power (rather than milliwatts). The advantage of dbm is that you can add or substract antenna gains very easily since those are in db.

20dbm=100mw

The rule is "if you substract 3dbm, divide the mw by 2" and "if you substract 10dbm, divide the milliwats by 10"

So 17dbm = 50mw

13dbm=25mw

10dbm=10mw

The received signal strength which are usually between -40 and -80dbm are then fractions of fractions of microwatts :-)

vinodjad1234 Tue, 10/12/2010 - 23:06

Thanks for your information. I am really thankful to you

I need to have information about technical specification about wireless data,voice and video dbm value.

what are the standard value , it should be for voice and video as well as data ............

I am looking for specification ............

Nicolas Darchis Tue, 10/12/2010 - 23:10

This is the document you should be reading for complete specifications :

http://www.cisco.com/en/US/partner/docs/solutions/Enterprise/Mobility/vowlan/41dg/vowlan_ch3.html

In summary, for voice, it's recommended that the "worst" RSSI a phone gets is -67dbm. At that signal level it should be able to roam to an AP having better signal. The coverage cells of APs should also overlap by 20%.

But the document will explain it better.

Video is similar from an RF perspective but demands more bandwidth.

Nicolas

ppokorny25 Fri, 03/04/2011 - 06:54

Hi,

Let me ask you a question (I'm still little confused).

IE :RSSI -91 and SNR -4

What you say (if I understand well), is that signal strenght is OK, but is very fogy (becasuse of low SNR).

So if this one is an anonymous AP, heard by my AP, I don't have to bother with this?

The problem would be, in case : RSSI -91 and SNR 35.

Am I right, or completely wrong?

Thank you very much

BR

Pavel

Nicolas Darchis Fri, 03/04/2011 - 06:58

The idea is there yes.

The permanent air noise level is around -95dbm (this explains why -91 gives a 4 SNR) so -91 is a very bad signal in any case.

Let's say if you have -60dbm which is a very good signal, it could be impossible to decode if the SNR is 4. That would mean that the noise level is -64dbm.

Pretty much like you hearing someone in a nightclub. The person may speak loud (-60dbm), if the global noise is big enough, you won't hear the person anyway.

SNR is "how better than the base noise level is the signal".

ppokorny25 Sat, 03/05/2011 - 05:27

Hi,

So, then. What to do, for best?

Seems to me :

- keep RSSI closer to 0, as possible

- try to have SNR high as possible

Am I right?

Thanks

George Stefanick Sat, 03/05/2011 - 11:13

Yes, your thinking is correct. It really all depends on the design requirements. Lets pick on voice. You would need the following to support voice..

SIGNAL -67 or lower

SNR -25 or higher

ppokorny25 Sun, 03/06/2011 - 00:47

Great,

Thanks for your time.

And one (I hope) last question.

I can see you are working with WCS - how do you recommend to categorize Rogue APs with attributes I have written above (IE RSSI -83, SNR 2, etc.) Because from I have catched, this AP is no harm form my infrastructure.

Thank you

George Stefanick Sun, 03/06/2011 - 09:20

Pavel,

Personally I dont know that (I) would change anything. Only because if you were to have a rogue on the inside powered very very low you could still reg a -85. I would recommend rogue on the wired implementation.

If any of this is helpful please support the rating system. Its a way for others that use 'search' to find their answers quicker. And well frankly, helps me catch up with Leo ! LOL

Stop back if you have any other questions..

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