This is the first Tunneling Device that Aperture Produced for public use. It's portals are smaller than portals from newer models, but are still capable of transporting humans. It does not feature the ability to carry objects, unlike newer modles.
Quantum Tunneling Device MK II
This is the second Tunneling Device ever produced by Aperture. It is the first Tunneling device to feature Human-sized portals. It is also the first one to feature the ability to carry small objects, but only ones under 10 Lbs.
Quantum Tunneling Device MK III
This is the newest model of Tunneling Devices produced. They feature the ability to sync with other Tunneling Devices, which allows up to four portals to be placed and connect to each other. It can also carry objects up to 10 Lbs. in weight.