Nokia 6210 modem: why so slow ?? (Linux vs. WinXP)

[Wolfram Brenig]

Hi,

I'm not sure if this is the proper group to ask this
question, but I have a problem with the data transfer
rate of the modem of my Nokia 6210 cellular.

1) Both, under Linux (SuSE 8.1 on a ThinkPad A31) and
WinXP I have the modem configured as a standard AT
modem. Under Linux I've done this via Yast2->Network/Basic->
Modem configuration. The serial port in both cases is
the IR port (dev/ircomm0 under Linux) set to a baud rate
of 115200. No special init strings other than 'ATZ' are
sent to the modem. No HSCSD, GPRS or other 'high speed'
connections are sought for, i.e. a plain 9600 baud
connection to the ISP is expected.

2) Both, under Linux and WinXP connecting to the ISP
works well. However, downloading several test-files
shows, that the data transfer rate in the Linux case
is >>> a factor of 6-8 *slower* <<< than under WinXP ?!?

What is going wrong here ?
Anyone had this experience before ?
Is there any way to fix this / any diagnostic strategy
to find the bottleneck / any special ppp option to
speed up things ...? Any help would be most appreciated.

Wolfram

PS: In the 'Datarate Monitor' of the 'KInternet' Tool I
have realized, that the B/s-rx graph on transfer through
the cellular modem consists of equidistant very short,
sharp spikes with rather long 'empty' periods. This looks
very different from that graph for other modems. (maybe
this is a clue?)