Nortel Networks, North America's biggest maker of telephone equipment, has filed for bankruptcy protection as the global economic crisis continues to crush its already fragile business. ┬á The Toronto based company, and key sponsor of the 2012 London Olympic games, filed for US Chapter 11 bankruptcyÔÇö a ┬áprotection law that allows a firm to continue trading while it aims to sort out financesÔÇö a day before the firm was due to make a $107 million interest payment on outstanding loans.