large latency with rfcomm serial?

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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