RPi edimax EW-7811Un

Installing the Edimax EW-7811UN wireless adapter on Raspberry Pi

Hardware

This guide is about installing the Edimax EW-7811Un nano wireless adapter on a Raspberry Pi with a wireless router supporting DHCP and configured to use WPA2-PSK. The guide may be useful for installing other wireless adapters as well and for configuring WPA-PSK.

Software

The software used was the Debian squeeze package debian6-19-04-2012.img plus a replacement driver module (8192cu.ko) available from here. The driver is a tar.gz file and needs to be unpacked twice. No other modifications were made to the software.

Power supply

There have been reports that wireless adapters take a lot of power, but the Raspberry Pi with an EW-7811Un adapter runs on a Samsung Micro USB mains charger, a standard Kindle power supply and even the Kindle lead attached to a USB hub on a laptop running on battery power.

Accessing RPi

It is possible to configure the adapter using ssh over Ethernet, but it is a lot easier to see error messages if a display screen is directly attached.

Configuration

All of this configuration needs to be done as a privileged user, so you can prefix all the editing commands with sudo or just run “sudo su” to turn yourself into an administrator.

/etc/udev/rules.d/70-persistent-net.rules

Delete anything in this file about wlan0 or wlan1. This file is used to remember the physical characteristics of USB adapters and may well contain incorrect or misleading information, and should be empty apart from comments unless you have added other USB devices.

/etc/network/interfaces

Update this file to add the wlan0 section, so that it looks like this.


 * 1) Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or
 * 2) /usr/share/doc/ifupdown/examples for more information.

auto lo

iface lo inet loopback iface eth0 inet dhcp

auto wlan0 iface wlan0 inet dhcp pre-up wpa_supplicant -Dwext -i wlan0 -c /etc/wpa_supplicant.conf -B

/etc/wpa_supplicant.conf

Create this file looking like the following.

ctrl_interface=/var/run/wpa_supplicant ctrl_interface_group=0 ap_scan=2 network={ ssid="myssid" scan_ssid=1 proto=RSN key_mgmt=WPA-PSK pairwise=CCMP group=CCMP psk=”this is my secret phrase” psk=58cfe1c8a36ce8a1f3cd938e281ca1222707df58e27df4745148bfebca7c4197 }
 * 1) choose one of the following

This file is critical, and you should note the following points:
 * 1) There must be quotes around the SSID.
 * 2) These parameters relate to WPA2-PSK, even though it says WPA-PSK.
 * 3) The key (psk) can be specified in quotes or as a hexadecimal string obtained from running wpa_passphrase.  If you are concerned about security, you may prefer this, but it’s not necessary.
 * 4) You can make the file unreadable by ordinary users with the command chmod 600 wpa_supplicant.conf.

Installing the driver

To install the new wifi driver in place of the one supplied in the Debian image, you need to add the new driver to the operating system and block the old one.


 * 1) Copy the file 8192cu.ko to the directory /lib/modules/3.1.9+/kernel/net/wireless.
 * 2) Add the line blacklist rtl8192cu to /etc/modprobe.d/blacklist.conf.
 * 3) Add the line 8192cu to /etc/modules so that wifi support is loaded at boot time.
 * 4) Run the command depmod –a.

Testing the system

Reboot the Raspberry Pi and it should all just work! The blue light on the wifi adapter should come on, and should acquire an IP address from the router. Check by running the command ifconfig –a on the Raspberry Pi to find the IP address and connect using ssh (PuTTY). Troubleshooting 1.	If the blue light on the adapter comes on, but the adapter does not acquire an IP address, the problem is in wpa_supplicant.conf. ifconfig will show the adapter, but not an IP address. 2.	If the blue light does not come on, the problem is in the earlier stages. 3.	Use the command line utility dmesg review kernel messages. It is helpful to save the output to file so that it can be edited and examined at leisure (use dmesg >dmesg.log and then edit dmesg.log). 4.	Despite the fact that rtl8192cu.ko has been blocked, dmesg still gives the following message: usbcore: registered new interface driver rtl8192cu. This does not indicate an error. 5.	dmesg does not record error messages relating to errors in wpa_supplicant.conf. The only way to see these is to boot with a directly attached display and watch the messages going by. These are errors such as a failure to put quotes round the SSID. Stability Tomasz Milas in his very helpful posting Raspberry Pi meets Edimax recommends a large number of fixes to improve the stability of the Debian system. These are not required to make the Edimax adapter work, but may solve other problems. The only problems discovered so far using the standard Debian image and the above configuration changes are that the wireless connection does not re-establish itself after the wireless router is rebooted, and /etc/init.d/networking restart brings down the network but does not re-establish it, either for Ethernet or wireless (possibly the same problem). Removing the wifi adapter and reinserting it does not restart the network either. Additional points 1.	The above configuration file is for WPA2-PSK and DHCP. WPA-PSK and static IP addresses have not been tested. 2.	The simplest solution might be to rename the new driver to rtl8192cu.ko and replace the existing driver. This would avoid the need for blacklisting, but probably not for pre-loading using /etc/modules. It might also break updating mechanisms. 3.	The above process is cumbersome if you want to use different wireless access points, so you may want to consider one of the following (not tested). These are described in the Debian wiki here. a.	wicd is an open source wired and wireless network manager that does not require graphical dependencies. The Farnell site also mentions wicd-curses. b.	Network Manager is a network manager that does use a graphical interface. 4.	You may want to configure your wireless router to give a fixed IP address to the Raspberry Pi Ethernet and wireless connections. 5.	You can use the command lsusb to identify the chipset used by your wireless adapter.