[Armed French gendarmes stand next to a toll station as they check vehicles and verify the identity of travellers on the A2 motorway between Paris and Brussels]By Gabriela Baczynska BRUSSELS (Reuters) - The European Commission set out a timetable on Friday to restore the fraying 26-nation Schengen open border area by year-end, warning that failure to do so would cost the European economy up to 18 billion euros ($19.7 billion) a year. It proposed no new measures, but told European Union countries to abide by existing EU law and stop the "wave through" approach that has led to a million migrants and refugees marching north through the Balkans from Turkey through Greece. "We cannot have free movement internally if we cannot manage our external borders effectively." The proposal comes ahead of an emergency EU summit with Turkey next Monday to discuss how to halt the influx of migrants that has driven several EU and Western Balkan states to re-impose border controls.