The Starfleet Perimeter Defense Network was a defensive sensor grid for Starfleet made up of a series of long range scanners.

By 2379, the Xarantine Stellar Array and the Poloma Observatory were part of the Defense Network. Both USS Enterprise-E officer Data and Section 31 used the Defense Network to verify the route of the freighter Caedera and the freighters that it transferred contraband cargo to headed for Tezwa. (TNG novel: A Time to Heal)