Date: Wed, 23 Jul 2008 15:36:55 +0100 From: "awnish upadhyay" <awnishupadhyay@gmail.com> To: freebsd-bluetooth@freebsd.org Subject: bluetooth link quality and rssi ? Message-ID: <c162203e0807230736n7750db22u832303c20ab8fde9@mail.gmail.com>
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Hi , 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? I ran some test between an Nokia, Windows MObile and a Belkin BT USB device. I was measuring the RSSI value as seen by the belkin BT device (connected to a PC) while the mobiles weres moved to different locations (I was not doing any data transfer.. but I had the two devices connected and collected the data in a file). I noticed that the RSSI values were highly variable. Even two sets of observations with identical conditions were giving very different values for RSSI. So, my next question 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? Can I use RSSI to quantify the distance between two BT devices? Meaning lower values of RSSI -> higher distance? I was looking at other parameters (link quality, transmit power) and found that the link quality seemed to be a better indicator of the distance.. with increasing separation between the devices, the command 'hcitool lq <address>' was giving lower values.. meaning there was a degradation in the link quality. Any comments on this? i wanted to know the unit of link quality.and what is the relation between rssi and link quality ? also can we predict the distance depending upon the link quality and rssi.. In Bluetooth, the transmitted power for a link is adjustable and the LM can change the power of a link depending on the conditions. So again, can the transmit power level (hcitool tpl <address>) be used as an indicator of the distance? In my tests, I did see a variation in the tpl value for different positions. 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? -- Awnish Upadhyay Msc. Mobile and Radio Communication, 07809682838.
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