EXETER, N.H., Oct. 22 -- Tyco International Ltd. is buying the 11 percent of TyCom Ltd. it doesn't already own in a deal worth approximately $864 million. Under the agreement, TyCom shares would be converted into 0.3133 of a Tyco share, representing an increase over the conglomerate's initial offer earlier this month of 0.2997 for each TyCom share. Tyco already owns 89 percent of TyCom's shares, and the remaining stake represents about 56 million shares. The deal, which values TyCom shares at $15.42 -- a premium of 4.5 percent -- is based on Tyco's closing price Thursday of $49.20. The move came after TyCom, an undersea-fiber-optic cable supplier, laid off 600 workers, representing about half of its work force in New Hampshire. TyCom, a subsidiary of Tyco, blamed the cuts on a slowdown in the telecommunications industry and the sluggish economy.