bluetooth link quality and rssi ?

[Iain Hibbert]

On Wed, 23 Jul 2008, awnish upadhyay wrote:

> In BlueZ, what does 'hcitool rssi <address>' return? does it return the
> actual RSSI value or is the output similar to the result of the
> HCI_Read_RSSI command as mentioned in the BT spec?
>
> The BT spec says that HCI_Read_RSSI will read the value for the difference
> between the measured Received Signal Strength Indication (RSSI) and the
> limits of the Golden Receive Power Range for a connection handle to another
> Bluetooth device.
>
> Put in other words, will 'hcitool rssi <address>' return this difference or
> does it give the exact RSSI value which is compared with the GRPR?

(FreeBSD is not BlueZ btw)

It returns the value that the controller supplies (from Read_RSSI command)

> I noticed that the RSSI values were highly variable.

go figure

> is how accurate is the RSSI value? I know the BT spec says that there can
> be a 6dB +/- variation. Is the result value in dB?

it is dB above or below the Golden Receive Power Range (whatever that
might be :)

> Can I use RSSI to quantify the distance between two BT devices? Meaning
> lower values of RSSI -> higher distance?

Not with any guarantee of precision. Radio waves bounce off walls and and
are absorbed by furniture or human bodies. I think the reasoning for
providing this value is that you can use it to compare different links and
choose the strongest ones (eg for a mesh network) rather than interpreting
it on its own.

> And one last question.. do any of these values depend on the manufacturer -
> i.e. for the same distance and identical environmental conditions.. will a
> 3COM device, Belkin device and say a cisco device give different values?

nothing is absolute

regards,
iain