Re: 3G Signal CSQ

[Dan Williams <dcbw redhat com>]

On Thu, 2012-03-22 at 11:14 -0500, Dan Williams wrote:
> On Sun, 2012-03-18 at 16:51 +0100, Aleksander Morgado wrote:
> > Hoola,
> > 
> > > I see that NM request signal quality from 3G modem. I need dbm or
> > > similar. Any command line option while modem it is locked by NM.
> > > I need obtain dbm or result of at+csq
> > >
> > 
> > You can query the signal quality retrieved by NM/MM directly using the 
> > DBus interface and the 'dbus-send' command line utility (assuming your 
> > modem is at index 0 here):
> > 
> > dbus-send --system --dest=org.freedesktop.ModemManager --print-reply 
> > /org/freedesktop/ModemManager/Modems/0 
> > org.freedesktop.ModemManager.Modem.Gsm.Network.GetSignalQuality
> > 
> > That won't give you neither the raw dBm value nor the rssi, though. It 
> > will give you a computed percentage value [0,100] representing the 
> > signal quality.
> 
> Right; most modems don't support reporting dBm or RSSI, including almost

That's 1/2 a lie, they support RSSI, which is the value they return from
AT+CSQ.  RSSI is an arbitrary unit that's assumed to be linear in scale
(unlike dBm).  CSQ is defined to be 0 - 31, and internally in NM that's
normalized to a 0...100 scale.  Technically GetSignalQuality *does*
return RSSI, but may not be reported in the modem's native RSSI scale.
On some devices (Ericsson MBM) the scale is 0...5 even.

If you're looking for a much deeper RSSI, like radio diagnostic data,
then that's actually impossible to get for most devices.

Dan

> all that only support AT commands.  Proprietary interfaces like CnS,
> QMI, WMC, or DIAG etc may support reporting dBm though.  However,
> remember that dBm is really only useful for 2G and 4G technologies.  For
> 3G, whether EVDO or UMTS, the unit you're really looking for is EC/IO
> (energy-per-chip over interference) which is how signal quality is
> calculated for [W]CDMA-based air interfaces.
> 
> Dan
> 
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