The sub-space transmitter is an example of Tollan technology. Based on principles of advanced physics and the true nature of space, it uses a beam of energy to instantly communicate information across vast distances. The Tollans used the device to contact the Nox from Earth. The SGC acquired a sub-space transmitter from Martouf and has used it to contact the Tok'ra and the Tollans.
Cross Reference: Tollans
Episode Reference: Enigma, Serpent's Song, Show and Tell