Transdimensional portals were the transport used by the Threllips. Each required a projector, tertiary signals and aligned sub-space modifiers to work. It allowed the transport of objects or information.
On 11 July 1982, Vengorr and Thon were sent to Earth to set up a transdimensional portal to allow the Threllips to invade. The Eighth Doctor distracted them and reprogrammed it. When Vengorr and Thon went through it, they were transported to one of the moons of Mordelius Prime. (AUDIO: Living Legend)