Jumpgates (also referred to as vortex generators) are large spaceborne structures made up of three or four separate but aligned struts that generate a spatial vortex, or Jump point into Hyperspace allowing any ship to travel interstellar distances.
Jumpgates work by manipulating four stable, energetic forces of opposing polarity generated by each strut. These forces work against each other to create a vacuum effect, forming an unstable vortex which rips the spacetime continuum. This opens a pathway to hyperspace via a singularity. Some jumpgates use three struts instead of four, though the principle is essentially the same.
The struts themselves are each comprised of seven field phasing modules and twelve paired spatial constriction arrays and are assembled in modular sections with Quantium 40 acting as a vital construction material.
Though relatively stable devices, jumpgates, especially ancient ones have been known to be faulty on occasion. Usually this only means a bumpier than usual transition across the event horizon but anything that disturbs the opposing gravitational forces between the struts can cause a reverberating shock-wave that collapses the jump point, essentially forcing the gate closed. If a gate closes prematurely or incorrectly, the resulting explosion and release of energy rivals that of an exploding planetary body, with a blast wave that only the very fastest ships have a hope of escaping in one piece.
Jumpgates require an enormous amount of power and cannot simply be shut down to prevent intruders. In fact it takes at least two days to safely power down a gate and another four or five to power up again. To attempt otherwise would result in a massive explosion of energy, destroying the gate and anything else in range.
It is unknown exactly which race invented the technology as nothing of the original jumpgate builders has ever been found and all that can be determined by examining the gates themselves is that the aliens had been highly advanced, and extraordinary engineers. The first gates were built circa 4800 B.C. (seven thousand years before the Third Age), and had apparently spread across interstellar space as a flourishing civilization for some four to five thousand years hence. By 200 CE they seem to have completely vanished, leaving behind no trace of their civilization, other than the gates themselves.
Some of the younger alien races such as the Minbari and the Centauri discovered the gates while exploring the outer edges of their own solar systems in their early, primitive sub-light ships. Once they had unlocked the complex codes required to activate the jump gate sequence, they began to explore hyperspace in an attempt to map the jumpgate network. However, as later generations know all too well, hyperspace travel is extremely hazardous and many ships were destroyed or simply lost forever. The ones that made it back into normal space had found out the hard way just how perilous hyperspace navigation was and as a result a series of tachyon beacons were established to help ships locate and activate the gates within the chaotic nightmare of hyperspace.
Exactly how the original alien builders had navigated through their own network of gates isn't known, but they didn’t seem to use any recognizable beacons. For this reason, the discovery of an original gate is rare as they have so far only been stumbled upon through sheer blind luck and without any dependable means of detecting one at long range, finding them all could take millennia, if at all possible.
After it was determined how to open a gate, the next step was to determine how they worked so the technology could be duplicated and new gates be built. This required large amounts of Quantium 40, a rare radioactive material that due to its essential nature in jumpgate construction has become the most valuable substance in the galaxy.  Since not all inhabited system had an ancient gate of their own, some races such as the Centauri saw an opportunity for business and profit by seeking out isolated worlds and selling or renting access to their own gate network to trade with other worlds, or if the area was of sufficient value to annex the territory and conquer any inhabited world, as they did Narn. Earth was one of the former, having no military or strategic value the Centauri eagerly sold the Earth Alliance time on their own jumpgates to allow trade with other worlds, though before long humanity figured out how to construct their own gates and ships with jump-point generators, eventually joining the interstellar community as equal participants. The Sol System's gate was originally built in Earth orbit but was later moved to what became known as the Transfer point off Io, amid security concerns that an invading force could arrive practically on Earth's doorstep with little warning. In its new location it was several days journey from humanity's population centres on Earth and the then small colony on Mars. In addition, the natural radiation belt around Jupiter offered the gate sufficient protection from long range targeting.
Jumpgates are at the center of interstellar commerce and trade. Even vessels with built-in jump engines rely on the jumpgate network for navigation. Recording the signature of every ship that passes through, they allow the race that currently controls said gate to toll the travelers for its use and services, which are usually something arranged between governments by treaty. These treaties set tolling rates and rights to use certain trade routes, in exchange for the security codes necessary for opening the gates.
The gates also feature beacons that guide ships in hyperspace to a safe exit point. There are two types of beacons used by the jump gates: an omnidirectional one and a unidirectional one. The omnidirectional beacon indicates the nearby presence of a jumpgate, but it only works on relatively short distances of just over 1,000 km. The unidirectional beacon is what guides ships from one jumpgate to another. It is a tight-beam tachyon signal that crosses the distance between two jumpgates close to each other in the network. A ship can follow the echoing signal down a path that leads directly to the destination. However, going "off-the-beacon" causes this signal to fade and the ship can become lost in hyperspace. Certain races experienced with hyperspace travel, such as the Minbari, have shown the ability to travel quite far off the beacon without becoming lost.
Even ships equipped with jump engines often use jumpgates in order to save energy, and are often required to do so as a matter of book-keeping to log their travels. They use the network beacons to plot their course through charted space, then use a nearby gate as a basis point for journeying out of the network. Only larger, self-sufficient ships - such as the Earthforce Explorer class ship - tend to do this, as well as other military ships in the course of maneuvers. It is also possible to enlarge the size of a jump point simply by moving the struts further apart, though it's not known what the upper limit of this capability is as it most likely requires an increasing amount of energy the larger the vortex becomes.
The destruction of jumpgates in interstellar combat is considered a very grievous violation of the accepted rules of civilized warfare, as it jeopardizes the entire beacon system and is ultimately self-defeating as all races use and need the jumpgate network. Whoever may win or lose a conflict, the network must go on. 
Although destroying jumpgates is generally frowned upon during a war, it is common practice in wartime for belligerent powers to program their jump gates to deny access to certain frequencies, thus preventing an enemy from using friendly gates and forcing them to open their own jump points. This was done by Earth during the Minbari War, though given the degree of Minbari technical superiority, it offered little in the way of a tactical advantage.
Some gates, particularly those built by surveyor corporations, utilize encrypted beacons that make it impossible for someone without the correct cipher to determine the gate's location in normal space.
New gates are being constructed all the time by various worlds, depending on the availability of Quantium-40. Opening up access to new, previously inaccessible areas of space, most are constructed for the purpose of finding yet more sources of Quantium-40 to build or repair even more gates.  Occasionally long- range probes discover "new" ancient gates, and most still function despite being many thousands of years old. However, some of the older ones have proven to be a little bit "quirky", which can result in a somewhat rough transition in and out of normal space for the ship using them.
Such is the nature of jump point physics and the energy involved with creating them that if one attempts to use a jump-drive to open a jump point within an already active jumpgate vortex, then the massive release of energy would destroy the gate and most likely the ship that opened the point as well. Earth experimented with this tactic during the Earth-Minbari War, though it quickly became known as the "Bonehead Maneuver" as no Earthforce ship was fast enough to open the point and clear the blast radius before being destroyed. In 2260 the White Star 1 became the first and only known ship to survive this maneuver when it was used to destroy a pursuing Shadow Vessel at the Septis jumpgate in Sector 47.
- The Vorlons have been known to have constructed at least one type of gate, that being the Thirdspace Gate, built and jettisoned into hyperspace some one million years ago.
- The Official Babylon 5 Magazine: Volume 2, Issue #1 (July 1998) - Page 34 (The Great Machine)
- Babylon 5 Security Manual
- To Dream in the City of Sorrows
- A View from the Gallery
- Thirdspace (movie)
- JMS post on rec.arts.sf.tv.babylon5.moderated - 6/20/1995 2:59:00 PM
- A Distant Star
- Meditations on the Abyss
- Movements of Fire and Shadow
- JMS post on rec.arts.sf.tv.babylon5.moderated - 12/28/1993 7:43:00 AM
- A Distant Star
- Matters of Honor