The matter transmission is one of Science fictions many facilitating devices for Transportation: a hypothetical machine which is not rationally plausible in terms of known science (at least at any macroscopic scale) but which is very convenient for certain narrative purposes (Imaginary Science). By virtue of an obvious play on words, matter transmitters were sometimes called "transmats" – as in Lan Wright's "Transmat" (November 1960 Science Fiction Adventures UK) – but the contraction never really caught on. Essentially, a matter transmitter is a Teleportation machine whose plausibility is usually secured by analogies with radio. The best illustration of its narrative utility is in the television series Star Trek, in which the "transporter" not only transfers people from the Enterprise to this week's stage-set with a minimum of fuss but serves as an ever-ready deus ex machina to come to the rescue when our heroes are in a tight situation; like so many other Trek devices it is parodied in Galaxy Quest (1999).
Mat-trans (matter-transmission) gateways are typically located in redoubts, underground military complexes built as part of Project Cerberus. All gateways, with a few notable exceptions, are hexagonally shaped, with glowing metal disks on floor and ceiling, opaque (or rarely translucent) armaglass walls, and a single door. The gateways are color-coded - that is, each chamber's walls are a different color or combination of colours, enabling those who are familiar with the system to identify their location. There is no other way to identify a gate's location from inside the redoubt. All mat-trans chambers were (in predark times) accessible only to those with B-12 or higher clearance. Some redoubts have different clearance levels; it is unclear, in most cases, if this is deliberate or merely an editorial mistake.
In almost all cases, a mat-trans chamber is activated simply by closing the door. Older models have a ship-style hatch with a rotating locking wheel; these must be activated via computer. During a "jump", tendrils of mist swirl around the chamber as the system powers up, then the contents vanish and the mist dissipates. Conscious beings often suffer nausea, headaches, and/or vivid nightmares; unconscious beings are seemingly immune to these side effects (or at least suffer lesser effects). In Dark Emblem, Doc was injected with a cocktail of drugs and sent on several time-jumps while conscious; he suffers no ill effects beyond a severe nosebleed.
It is stated in the Outlanders series that the mat-trans system is actually slightly out of sync with normal space-time, which results in the physical problems and nightmares; Lakesh fixes this problem so that he and his allies can use the system without suffering the effects.
There are a number of known characteristics and failsafe's in the mat-trans system:
◾A gate cannot be used unless the door is closed. The door to a gateway on the receiving end of a jump will automatically close just before it receives a transmission. Interfering with the seal (sticking objects in between the door and the jamb) will not inhibit the jump, but can cause problems for living beings that are being transported.
◾Once a mat-trans unit is used, it cannot receive another transmission for 20 minutes - this is probably to ensure that any people/cargo inside the chamber are unloaded safely. It is unclear what happens to living beings in the chamber on the receiving end of a jump that is activated outside the 20-minute window. It also unclear what happens to nonliving objects like water, debris, etc. - some books have the companions arriving in a gateway that's partially flooded, infested with mold, or inhabited with insects, while others have them jumping with water, insects, etc. and the companions arrive alone (and in one case, bone dry).
◾With most gates, the person(s) using it can input a code for a specific gate or use the LD button (see below); closing the door without inputting a code automatically sets the system to "random".
◾There do not appear to be failsafe's in place to prevent teleportation to a destroyed or disabled gate (though see below). It is unknown what happens if the specific code for an inoperative gate is entered (as opposed to a random jump); most likely the jump simply fails. If it's a random jump, the system will search for a new destination, which appears to take a variable amount of time. According to Rick Ginsburg, there's a code that, when entered before a jump, can prevent the system from choosing an inoperative gate.
◾The "LD" button automatically sends anyone inside a given gateway to the last gateway it accessed, provided it is used within 30 minutes. For example, someone jumps to the gate in Dulce from the one in Alaska. If the Dulce gate isn't used again within 30 minutes, that person can hit "LD", get back into the gate, and be sent back to Alaska without having to know or use the proper code. (Note: In Chill Factor, Zimyanin's men apparently violate this rule by using the gates to jump all over Deathlands and capturing slaves (which takes far more than 30 minutes), then bringing them back. It's possible they knew the code for the gateway, however.)
◾The chambers have a security lockout code - 108J - which, when entered, will disable a particular gateway until entered again.
No one knows exactly how many gates there are, or their locations. The computers in the Anthill have the locations of all the gates in the continental US, and Silas Jamaisvous had an encoded CD that supposedly had the locations of all the gates on it, which the companions took but never managed to decode.