Tested with a Cisco Aironet 350 card, and the Atheros chipset under GNU/Linux kernel 2.6.20.
/usr/bin/wlanconfig ath0 create wlandev wifi0 wlanmode ap
/usr/sbin/iwconfig ath0 essid GROS_NICOLAS
/usr/sbin/iwconfig ath0 channel 6
/sbin/ip link set ath0 up
You may no be able to change your wlanmode on your vap (here, ath0) since the ath_pci driver will create a new device in client mode as soon as it will be loaded. In order to avoid that, you may add the following option to your /etc/modprobe.conf file :
options ath_pci autocreate=none
And reload the driver :
rmmod ath_pci
modprobe ath_pci
Or, with (almost) a oneliner :
modprobe ath_pci autocreate=none
That's pretty easy, once that you tried to use iwconfig to change the chipset mode to master. It won't work with that chipset (and with none of my test cards, in fact).
Thanks Pierre-Emmanuel for the tip !