"谁能解释一下BELL CANADA 和 AT@T 的业务区别" I suggest you'd better to check their website by yourself.

LEC stands for Local Exchange Carriers and CLEC is Competitive Local Exchange Carriers. basically the employer would be a systems company that has been develpoing a telecommunication infrastructure that provides fully convereged voice and broadband data services for the "last mile", usually focus on voice or wireless. good luck.