Return-Path: Date: Wed, 2 May 2007 10:52:58 +0100 From: Peter Stephenson To: BlueZ users Message-Id: <20070502105258.18bf397b.pws@csr.com> In-Reply-To: References: Mime-Version: 1.0 Subject: Re: [Bluez-users] signal strength, link quality and transmit power Reply-To: BlueZ users List-Id: BlueZ users List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Sender: bluez-users-bounces@lists.sourceforge.net Errors-To: bluez-users-bounces@lists.sourceforge.net "Mark S. Townsley" wrote: > I am trying to interpret the values return from reading a connection's > signal strength, link quality and transmit power. These are all determined in the device, so you may get a different range of results from different manufacturers. Signal strength is output power received locally from a remote device (see note below). Link quality is some estimate (it's allowed to be anything) to tell you how successfully data is getting through. CSR has always used bit error rates for the estimate; this is very common. We don't *guarantee* anything more than a higher number means we think the link is better. However, it turns out that up to now we've always done it the same way and Marcel has a table at http://www.holtmann.org/linux/bluetooth/csr.html to convert to actual bit error rate. I'm not sure where you're getting the information on transmit power from, but in most contexts it refers to the power at which the local station is sending, probably in dBm (see below). Hence it tells you how loudly the device you have thinks it needs to shout to be heard. (In fact, on devices where the transmit power changes, this is signalled from the far end, which measures its own RSSI and requests the originator to increase or decrease power.) > For example, I would sometimes get rssi values in the negative. What > does that mean? Normally, RSSI values are measured in dBm; this is a logarithmic scale such that 1 mW of power is 0 dBm, while 10 mW is 10 dBm and 0.1 mW is -10 dBm, etc. (Try going into a shop and asking for a 50 dBm light bulb.) However, you need to be cautious in interpreting RSSI values if you're getting the usual status read out supplied over HCI: it doesn't tell you the *actual* RSSI, it tells you how much above or below the "golden range", i.e. the preferred range of "good" signals, the value is. So it's not actually in dBm; a negative value indicates it's so many dB's "too quiet" for the chip to be happy, and a positive value so many dB's "too loud". RSSIs from inquiry results (rather than as reported during connections) are in straight dBm, on the other hand. > And in general, are higher values on these parameters indicate a > better connection quality? For the first two, broadly yes. What *exactly* they're telling you is a more complicated matter. -- Peter Stephenson Software Engineer CSR PLC, Churchill House, Cambridge Business Park, Cowley Road Cambridge, CB4 0WZ, UK Tel: +44 (0)1223 692070 To access the latest news from CSR copy this link into a web browser: http://www.csr.com/email_sig.php To get further information regarding CSR, please visit our Investor Relations page at http://ir.csr.com/csr/about/overview ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Bluez-users mailing list Bluez-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bluez-users