sporadic link failure

configuration:
AP:    TEW211BRP fw 5.01 (prism 2.5 PRI 1.3.4 inside) WEP128 & client MAC
accesslist enabled (15dBm) + 6dBi omni + .5m pigtail (-1dB)
client:  TEW-USB (w/o flash on 4.5m USB cable) + 10dBi directional + 9m H155
cable (-6dB) -> +4dB
distance: ~500m (-94dB) -> RSS ~= -70dBm
PC: single P3/800 on SMP intel GX m/b with IOAPIC enabled
OS: slack 9.1/2.4.23/JE uhci

initialisation:
#! /bin/sh
echo "usb wlan install..."
# Atmel berlios driver
modprobe -v at76c503-i3861
sleep 4
# mED AP
iwconfig wlan0 mode managed rate 11M rts 2347 frag 2346 essid "mED-AP1"
iwconfig wlan0 key restricted 1234-5678-90ab-cdef-0123-45
iwpriv wlan0 short_preamble 0 auth_mode 1 scan_times 10 500
ifconfig wlan0 192.168.227.254 netmask 255.255.255.0 broadcast
192.168.227.255

iwconfig indicates:

wlan0     IEEE 802.11-DS  ESSID:"mED-AP1"  Nickname:"okuwlan"
          Mode:Managed  Channel:6  Access Point: 00:90:4B:0B:9F:7E
          Bit Rate=11Mb/s
          RTS thr:off   Fragment thr:off
          Encryption key:1234-5678-90ab-cdef-0123-45   Security
mode:restricted
          Power Management:off
          Link Quality:10  Signal level:35  Noise level:0
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

After some time (1/4-2hours) the link is lost (see kernel log) and the
client gets in peer mode???

wlan0     IEEE 802.11-DS  ESSID:"okuwlan"  Nickname:"okuwlan"
          Mode:Ad-Hoc  Channel:10  Cell: 00:00:00:00:00:00
          Bit Rate:11Mb/s
          RTS thr=1536 B   Fragment thr=1536 B
          Encryption key:off
          Power Management:off
          Link Quality:0  Signal level:0  Noise level:0
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

There are some failures in the kernel log (regarding usb bulk transfers).
May the link failure be connected with the JE driver or the lenght of the
USB cable?

Any suggestions welcome.

Best regards,
Gabi

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