As Assistant Attorney General, he settled a seven-year-old case against AT&T with by far the largest breakup in the history of the Sherman Antitrust Act, splitting AT&T up into seven regional phone companies in 1982.
Each industry is constructed as a large network for transporting its fundamental product (information, telecommunication) between any two feasible points of origin and destination.
This is because in a multi-level market, customers only gain a single final product, and are willing to pay only one "monopoly price" for this good.
To make it clearer, the total profits a monopolist can earn if it seeks to leverage its monopoly in one market by dominating a complementary market are theoretically no greater than the extra profits it could earn in an unregulated environment simple by charging more for the monopoly product itself.
Comparing to rivals in these market, monopolies could offer lower, cost-based rates for the same service because of vertical integration.