Re: large latency with rfcomm serial?

i sent this several days ago -- are there really no opinions
on whether the linux BT stack could introduce 2-3 second
character latencies for serial data?

i'll start debugging it soon, i guess.

paul

paul wrote:
 > hi --
 > 
 > i'm new to bluetooth.
 > 
 > i have a dbt-120 bluetooth receiver dongle that i'm using to
 > connect to a gps unit (for the record, an i-blue 737).  i'm
 > running fc7.
 > 
 > i'm seeing large latency values (2 to 3 seconds) on character
 > delivery.  are there any well-known gotcha's i should be aware
 > of that might cause this kind of thing?
 > 
 > here's the scenario:
 > 
 > i have two GPS devices.  one's a direct USB plug-in, the other is
 > the bluetooth device mentioned above.
 > 
 > the direct plugin unit works fine.  when tracking motion, it
 > tends to lag about a second behind "real-time", which is normal
 > for gps.
 > 
 > the BT device, however, lags real-time by a full 3 or 4 seconds --
 > long enough that it can cause real confusion while navigating.
 > (i.e., it says you're coming up to a turn 4 seconds after you've
 > passed it.)
 > 
 > if i connect both devices at once, and simply watch the output
 > from each (by connecting directly to their respective tty device
 > nodes), it's clear that the output from the BT unit is way
 > behind.  the devices are stationary, so the positions aren't
 > changing, but the GPS timestamps that come across in the BT data
 > are 3 or 4 seconds behind those from the non-BT device.
 > 
 > since they clearly both get the same time from the satellites,
 > the time skew is happening either withing the GPS receiver, or
 > within the bluetooth stack.  to try and determine which, i
 > figured out how use a windows box (same dbt-120 dongle, same gps
 > unit), and did the comparison again.  the BT device was on windows,
 > the direct-connect device still on linux.  the timestamps in the
 > messages coming from the two seemed pretty much in sync.
 > 
 > so my current feeling is that i'm losing 2 to 3 seconds in the
 > bluetooth stack.  any ideas?  any config values to check?
 > 
 > paul
 > 
 > p.s. vital statistics:
 >     machine is an OLPC XO laptop.  
 >     kernel is OLPC-built 2.6.22
 >     modules were built later, from a matching kernel tree.
 >     yum bluez packages:
 > 	bluez-gnome.i386  0.8-2.fc7
 > 	bluez-libs.i386   3.9-1.fc7
 > 	bluez-utils.i386  3.9-2.fc7
 > 
 > =---------------------
 >  paul fox, [email protected] (arlington, ma, where it's 66.6 degrees)
 > 
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=---------------------
 paul fox, [email protected] (arlington, ma, where it's 53.6 degrees)

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