Before the offensive the Allies were not aware of German troop movement. During the liberation of France, the French resistance had provided information about German movements. Once they reached the German border, this information was not available. In Germany such orders were typically transmitted using telephone and teleprinter, and a special radio silence order was made on all communications about the attack. The major crackdown in the "Wehrmacht" after the 20 July plot to kill Hitler resulted in much tighter security and fewer information leaks. The foggy autumn weather also prevented Allied reconnaissance aircraft from seeing the Germans on the ground. German units in the area were given charcoal instead of wood for cooking fires to cut down on smoke and reduce chances of Allied observers realizing a troop build up was underway. 