Wifi USB dongle configuration on rpi :

$sudo xemacs /etc/network/interfaces

auto lo 
 
iface lo inet loopback 
iface eth0 inet dhcp 
 
#allow-hotplug wlan0 
#iface wlan0 inet manual 
#wpa-roam /etc/wpa_supplicant/wpa_supplicant.conf 
#iface default inet dhcp 
 
auto wlan0 
allow-hotplug wlan0 
iface wlan0 inet static 
address 192.168.1.58 
netmask 255.255.255.0 
gateway 192.168.1.1 
wpa-ssid "SSID"
wpa-psk "PASSWORD"

Save file with Ctrl+x, Ctrl+s

Quit emacs: Ctrl+x, Ctrl-c

$sudo reboot