13

When using mobile 3G routers, you see in the output of the "show cellullar 0 all" command. What is the significance of this value? Is this interpretation the same when talking about 2G or 3G?

Ex:

  • 2G connection, RSSI -75
  • 3G connection, RSSI -75
  • 2G connection, RSSI -100
  • 3G connection, RSSI -100
  • I understood what is RSSI, but I wonder how many times per minutes the RSSI is checked with phone? If someone knows it could be nice! I know that for the RxLev or the RxQual (which occur during the communication) it is around every 20ms, but I think the RSSI could not be like that, otherwise the mobile will be out of battery in a few minutes. – Max Taylor Apr 3 '15 at 13:36
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RSSI is a Radio-Frequency (RF) term and stands for Received Signal Strength Indicator. It is a measure of the power level that a RF device, such as WiFi or 3G client, is receiving from the radio infrastructure at a given location and time. For instance, the power level a laptop is detecting from a nearby AP.

Usually, the higher the RSSI the better the quality and speed of the communication through the radio segment.

14

The closer the figure is to zero, the better. So in the figures you've given above, -75 is better than -100.

In my experience -75 provide a reasonable service. -85 provides an OK service. -100 provides a barely usable service. -110 provides no service at all.

This figure represents your signal strength to the cellular tower. The closer you are to a tower, the closer to zero the number will be.

Note that although I mentioned above that -75 provides a reasonable service, keep in mind that if the tower you're connected to is heavily congested the service won't be all that great. It is similar to the way in which ADSL works - even if you sync at 24mb/1mb, if the DSLAM is heavily congested, you'll get poor throughput.

Is this interpretation the same when talking about 2G or 3G?

Yes.

By the way, if you're using 3G and Telstra is your ISP, take a look at this page. You can force the modem to connect to the 850mhz frequency which performs much better than the 2100mhz frequency.

4

AFAIK RSSI is arbitrary (and of questionable use) unit, but at least my 3G cisco actually outputs dBm:

bu.ip.fi#show cellular 0 radio 
Radio power mode = ON
Current Band = WCDMA 2100, Channel Number = 10737
Current RSSI = -64 dBm
Band Selected = Auto
Number of nearby cells = 1
Cell 1
    Primary Scrambling Code = 0x14A
    RSCP = -64 dBm, ECIO = -4 dBm

dBm is relative change to 1mW of power, i.e. 0dBm is 1mW by definition. And 3dBm change is about half or double of the original signal strength (10^(-3/10) is 0.5mW and 10^(3/10) is 2mW).

So in the example above, my -64 dBm is 10^(-64/10) or 0.4nW (yes nanowatt, it's cray cray how these things work)

If we want to translate arbitrary mW to dBm we use log10, i.e. 2mW is 10*Math.log10(2), which is 3.

  • -64dbm = 0.4uW (yes microwatt, not nanowatt). – user13591 Jan 28 '15 at 0:31
  • RSSI is useful, but it is FAR more useful with EC/IO, which is your signal to noise ratio. You can have a very strong signal... but if the noise is as loud as the signal, you're not going to have a useful service. Likewise, you can have a very weak signal, but if there's no noise, you could have pretty darn good service. Sadly a lot of devices refuse to show/calculate EC/IO. – Nanban Jim May 31 '16 at 18:42

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