WordNet sense 1 (a workplace that serves as a telecommunications facility where lines from telephones can be connected together to permit communication):

exchange, telephone exchange

WordNet sense 2 (serving as an essential component):

cardinal, fundamental, key, primal

WordNet sense 3 (of great significance or value):

important, of import

WordNet sense 4 (located or occurring within or closer to a center):

inner

WordNet sense 5 (constituting or like a nucleus):

nuclear

WordNet sense 6 (having or localized centrally at a focus):

focal

WordNet sense 7 (equally distant from the extremes):

center, middle