The Syrian government has used warplanes and heavy artillery to indiscriminately bombard residential areas across the country in its crackdown on a 20-month uprising, destroying many homes and killing thousands of civilians. In the town of Al Bab, near Aleppo, rebels have set up their own air defence unit in a desperate attempt to warn civilians ahead of bombing raids. Al Jazeera's Andrew Simmons reports on the devastating impact of the bombings on civilians in Al Bab.