AT&T Corp, the largest United States telephone firm, is to separate its wireless business and sell shares in the fast-growing unit to the public next year in what may be the biggest initial public offering. The firm yesterday said the tracking stock would reflect the performance of AT&T's voice and data wireless business, interests in international wireless companies and its fixed-wireless operations that provided domestic telephone services. Analysts expect the sale to raise as much as US$10 billion for AT&T.