Number of workstation

[Mike Baroukh]

Hi.

I'm a new hostap user so sorry if this is a dumb question.
I made an access point that works nice but seems to have a limit on 
number of workstations that can connect to it.
When too much users are connected, the next who will try will trigger in 
syslog :

Jul  5 16:01:16 jrmx hostapd: wlan1: STA 90:21:55:c5:f8:f6 IEEE 802.11: 
authenticated
Jul  5 16:01:16 jrmx hostapd: wlan1: STA 90:21:55:c5:f8:f6 IEEE 802.11: 
associated (aid 8)
Jul  5 16:01:17 jrmx hostapd: wlan1: STA 90:21:55:c5:f8:f6 IEEE 802.11: 
Could not add STA to kernel driver
Jul  5 16:01:20 jrmx hostapd: wlan1: STA 90:21:55:c5:f8:f6 IEEE 802.11: 
deauthenticated due to local deauth request

If another workstation disconnect, this one will then success.
My hope is to handle about 50 workstations but I actually, the max I can 
handle is 7 simultaneous users (saw using kill -USR1 <pid>) .

My question is : could this be a built in limit of the usb dongle I use ?
Parameter max_num_sta is set to 500 so the limit is not on hostapd.
In fact, i'm not really sure to understand the limit between what 
hostapd does, the wifi driver does and the wifi card does ...

Infos on my config :
My box is built on a small pc so I had to use a usb wifi dongle : 
TP-LINK  TL-WN721N
(lsusb : Bus 001 Device 002: ID 0cf3:9271 Atheros Communications, Inc. 
AR9271 802.11n)
I used linux-wireless to build the ath9k_htc driver.
The OS is a debian squeeze 32 bits.

I tried with debian provided version (0.6.10) AND with compiled current 
stable version (0.7.3) : both give the same result.

Don't know if it can be helpful, but when using kill -USR1, I have :

hostapd state dump - Wed Jul  6 08:35:01 2011
num_sta=7 num_sta_non_erp=0 num_sta_no_short_slot_time=0
num_sta_no_short_preamble=0

then for each device (max 7) :
STA=90:21:55:c5:f8:f6
   AID=1 flags=0xa3 [AUTH][ASSOC][AUTHORIZED][SHORT_PREAMBLE]
   capability=0x431 listen_interval=10
   supported_rates=82 84 8b 96 24 30 48 6c 0c 12 18 60
   timeout_next=NULLFUNC POLL

Thanks in advance for any advise !...

Mike