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On my MacBook Air, MacBook Pro and Mac mini, the status bar includes the standard "radiating sector" wifi icon which you can click on to select a wifi network, or Option-click on to see additional info such as BSSID, Channel, RSSI, transmit rate. The icon itself show 1, 2, 3 or 4 lines in black to indicate how good the wifi connection is. My question is this - what do the different numbers of lines indicate? Is it various thresholds in signal-to-noise ratio? Transmit speed? RSSI? Bit-error rate?

The built-in WIFI Diagnostics app (I'm running Mountain Lion 10.8.4) tells me that RSSI is -65 to -70 dBm, noise is between -85 and -90 dBm, SNR is between 10 and 25, Quality is either Good or Excellent, and Tx Rate is between 1.0 and 11 Mbps.

The reason for asking is that I'm at the end of a pretty poor wifi connection right now, but the wifi icon is constantly showing all 4 lines in solid black! Why?

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You may have an excellent wireless reception but still have a slow Internet connection, or what makes you think you are "at the end of a pretty poor wifi connection"? –  jaume Jul 15 '13 at 11:16
    
I think I have 2 problems - firstly the wifi signal from the AP to me isn't brilliant; secondly the bandwidth from the AP to the "internet" isn't brilliant either. But with all this against me, why does the wifi icon still show 4 "bars" implying that everything is as good as it possibly can be? –  kbro Jul 15 '13 at 14:43
    
Apple hints in support.apple.com/kb/ht3821 that the bars display signal strength, and on iOS it indeed does (support.apple.com/kb/HT1695), but it also talks (support.apple.com/kb/PH10681) of "signal quality." Is Apple using both as synonyms? Maybe, and that's wrong, but would perfectly explain why you get 4 black bars, as the RSSI, at least for Apple standards, is high (my Mac displays 4 black bars with -71 dBm), and slow speed, as a SNR ratio between 15 and 25 isn't particularly good (see wireless-nets.com/resources/tutorials/define_SNR_values.html). –  jaume Jul 16 '13 at 7:48
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3 Answers

As Apple is not very explicit on what it actually shows with the bars, I can only tell you my observations. First some Wi-Fi basics:

  • The absolute signal strength, a power level measured in dBm or mWatt. It varies with the distance to the access point. In reality, the value is somewhere between -40 dBm (close to the AP) to -110 dBm (far away).

  • The RSSI (received signal strength indicator) value without unit. This value correlates loosely the signal strength. The mapping of RSSI value to absolute signal strength is defined by the HW vendor of the WiFi chip. (e.g. Atheros uses raw RSSI values 0..127, Broadcom 0..60) The larger the RSSI, the stronger the signal. Being able to handle different chip vendors with varying max raw RSSI, the OS might normalizes the raw RSSI to a normalized RSSI range e.g. 0..100 or 0..1000. Unfortunately, a lot of people (including Apple, see class CWInterface) mix the RSSI and the signal strength.

  • The noise level or noise floor measured in dBm or mWatt. It varies with the interference of other access points and other disturbers. The value is somewhere between -70 dBm (very noisy) and -100 dBm (almost no disturbers).

  • The SNR (signal to noise ratio) measured in dB or percent. It's the "distance" between the two power levels (signal - noise). The larger this value, the better the link quality; SNR larger than 25 dB is good to excellent. lower than 10 dB it starts to get very slow.

From my experience in various environments (calm or noisy, close or far) it seems Apple shows the signal strength. Displaying SNR would correlate more to link "quality".

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RSSI or Received signal strength indication is what is used for the display of the bars.

The higher the number ( closer to 0 ) the better your signal strength.

For Apple devices they used a scale of -100 to 0. But you'll never see 0, or even close. Most people will see a number between about -50 and -80, with around -50 being excellent.

-50 will show all the bars and -90 will show gray bars. Not sure if it will go to -100 since you don't have a signal then ;)

So if you're asking for a scale I think its around 10 to 12.5 RSSI per bar.

While you have a good signal strength, your throughput speed ( Tx Rate ) isn't that fast.

One method to calculate signal quality is to compute the Signal-to-Noise Ratio (SNR). SNR is the signal level (in dBm) minus the noise level (in dBm). Both of these values are typically represented as negative numbers. For example, a signal level of -53dBm and a noise value of -90dBm would yield an SNR of 37dB (i.e., SNR = Signal - Noise = -53 -(-90) = 37)

The calculated SNR value, as measured from a wireless client, would decrease as the range to the base station increases due to applicable free space loss. Also an increase in RF intereference from microwave ovens, cordless phones, walls, ceilings, etc, which would increase the noise level, would also decrease the overall SNR value.

SNR Guideline

40+dB = Excellent signal

25dB to 40dB = Very good signal

15dB to 25dB = Low signal

10dB to 15dB = Very low signal

5dB to 10dB = Little or no signal

In your case -65 - -90 = 25 or -70 - -85 = 15

So you have a 15db to 25db signal OR as diagnostics calls it SNR which isn't that good but isn't really bad either.

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Thanks, but this doesn't really answer my question "what does 4 lines on the wifi icon actually mean?". I suppose my fundamental question is what is the scale for the wifi "bars". –  kbro Jul 15 '13 at 14:41
    
I added more scale information but I'm not sure if it is 100% accurate so if someone can confirm this that would be great! –  FLY Jul 15 '13 at 15:08
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Your scaling doesn't agree with what I'm seeing - right now I have all 4 bars (as usual) but Option-click tells me the RSSI is -68, and WIFI Diagnostics says noise is -88, but Quality is Good to Excellent, when it really isn't! –  kbro Jul 15 '13 at 22:49
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What is shows is signal reception strength, which is the first must have, but does not guaranties fast connection, only that you could receive it.

I would agree with you been on the very low end.

Here is mine, no special (high speed wifi)

enter image description here

So here is a more detailed comparison, that shows why yours is not so good.

enter image description here

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Thanks, but this doesn't really answer my question "what does 4 lines on the wifi icon actually mean?" I understand that -70dBm RSSI isn't that good, and that -90dBm noise isn't so bad, and that they combine to give an SNR around 15-25 which isn't so great either, but my fundamental question is what is the scale for the wifi "bars". –  kbro Jul 15 '13 at 14:39
    
That is not a precise scale, it is a approximate of the signal strength. –  Buscar 웃 Jul 15 '13 at 16:42
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