From MEPIS Documentation Wiki
Wired Equivalent Privacy (WEP) is an older, less secure (compared to WPA) encryption standard defined by the IEEE 802.11 committee and implemented in most WLAN products. To gain WiFi (Wireless Fidelity) certification by the Wireless Ethernet Compatibility Alliance (WECA), products must support 40-bit WEP. Most vendors also support 128-bit WEP. WEP was designed to provide the security equivalent to a wired LAN and was not originally envisioned as bullet-proof security architecture. WEP’s architecture has been shown to be flawed, and tools are available that can effectively break WEP encryption through passive hacking. WEP was superseded by Wi-Fi Protected Access (WPA) in 2003.
WEP can be enabled in MEPIS 6.5, 7.0 and 8.0 through the Network Assistant. Click on the Wireless tab and make sure the SSID is filled in. Then click the radio button for WEP encryption and enter your key. You can also select the WEP key security mode: restricted or open. The general advice is to go with the default open mode and only experiment with the restricted mode if all else fails.
At times the Network Assistant may have trouble with a particular configuration (CPU, wireless chipset, router). If you are unable to get WEP working with it, try another wireless frontend such as Wlassistant, Wifi-radar, or Wicd.