After a while the raspberry loses the network connection via WLAN0

Reason:
Powermanagement turns your USB devices off after a while

Fix:
First: Use the newest firmware → German Guide
Second: Disable suspend

vi /etc/modprobe.d/8192cu.conf

insert

# Disable power saving
options 8192cu rtw_power_mgnt=0 rtw_enusbss=1 rtw_ips_mode=1

Third (optional): Create a cronjob with a script to check the connection and restart if wlan0 is turned off → Github WiFi_Check

#!/bin/bash
##################################################################
# A Project of TNET Services, Inc
#
# Title: WiFi_Check
# Author: Kevin Reed (Dweeber)
# [email protected]
# Project: Raspberry Pi Stuff
#
# Copyright: Copyright (c) 2012 Kevin Reed <[email protected]>
# https://github.com/dweeber/WiFi_Check
#
# Purpose:
#
# Script checks to see if WiFi has a network IP and if not
# restart WiFi
#
# Uses a lock file which prevents the script from running more
# than one at a time. If lockfile is old, it removes it
#
# Instructions:
#
# o Install where you want to run it from like /usr/local/bin
# o chmod 0755 /usr/local/bin/WiFi_Check
# o Add to crontab
#
# Run Every 5 mins - Seems like ever min is over kill unless
# this is a very common problem. If once a min change */5 to *
# once every 2 mins */5 to */2 ...
#
# */5 * * * * /usr/local/bin/WiFi_Check
#
##################################################################
# Settings
# Where and what you want to call the Lockfile
lockfile='/var/run/WiFi_Check.pid'
# Which Interface do you want to check/fix
wlan='wlan0'
pingip='192.168.178.1'
##################################################################
echo
echo "Starting WiFi check for $wlan"
date
echo
# Check to see if there is a lock file
if [ -e $lockfile ]; then
# A lockfile exists... Lets check to see if it is still valid
pid=`cat $lockfile`
if kill -0 &>1 > /dev/null $pid; then
# Still Valid... lets let it be...
#echo "Process still running, Lockfile valid"
exit 1
else
# Old Lockfile, Remove it
#echo "Old lockfile, Removing Lockfile"
rm $lockfile
fi
fi
# If we get here, set a lock file using our current PID#
#echo "Setting Lockfile"
echo $$ > $lockfile
# We can perform check
echo "Performing Network check for $wlan"
/bin/ping -c 2 -I $wlan $pingip > /dev/null 2> /dev/null
if [ $? -ge 1 ] ; then
echo "Network connection down! Attempting reconnection."
/sbin/ifdown $wlan
/bin/sleep 5
/sbin/ifup --force $wlan
else
echo "Network is Okay"
fi
echo
echo "Current Setting:"
/sbin/ifconfig $wlan | grep "inet addr:"
echo
# Check is complete, Remove Lock file and exit
#echo "process is complete, removing lockfile"
rm $lockfile
exit 0
##################################################################
# End of Script
##################################################################