AR-558 was a Dominion subspace relay. This relay was located on a small planet in the Chin'toka system, which was located in the Alpha Quadrant. The terrain of the planet consisted primarily of rocky surfaces and caverns, and was generally inhospitable to humanoid species for any extended period of time.
During the Dominion War, AR-558 was established on the planet. Following a victory in the Chin'toka system, (See: First Battle of Chin'toka), the Dominion were driven from the array by Starfleet troops who hoped to use the captured array to tap into Dominion communications. The array was held by these troops for five months without replacements.
The planet proved very difficult to defend. The Chin'toka system was the site of repeated confrontation, making re-supply to the planet impractical for extended periods. The Federation troops guarding the array selected a suitably defensible position at the closed end of a subterranean cavern, but were besieged almost from the start. Repeated Dominion attacks sapped both force strength and morale. Even though the location of their strongpoint forced the Dominion to advance through a narrow canyon, the casualties suffered by the Federation rendered it impractical to take full advantage of the terrain and forced them to wait for each successive assault while hoping for re-supply.
Although there were initially one hundred and fifty personnel posted there, by the time the USS Defiant made a supply run to the planet in mid-2375 that number had been reduced to only forty three surviving troops by repeated Jem'Hadar attacks and the prevalence of subspace anti-personnel mines, nicknamed "Houdinis". It was only when the crew of the Defiant, working with the troops, were able to disarm the mines and re-purpose them to attack advancing Jem'Hadar that the Federation gained some semblance of advantage in the situation. The casualties were heavy, however, and included Captain Loomis, Commander Parker, Lieutenant Larkin, Crewman Kellin, McGreevey and Vargas.
After suffering a defeat at the Second Battle of Chin'toka, the location of AR-558 was labeled on a tactical map that was on display in the wardroom of Deep Space 9. AR-558 was located approximately twenty-five light years from Ferenginar, thirty-five light years from Starbase 375, forty light years from the Badlands, forty-five light years from Bajor, and fifty lights years from Cardassia. (DS9: "The Changing Face of Evil"; DS9: "When It Rains...", okudagram)
Background information Edit
The name AR-558 was an in-joke, as the production number of "The Siege of AR-558" was 558. The script of the episode suggests the planet was also named AR-558 as the scene depicting the planet from orbit is named "EXT. SPACE - AR-558 (OPTICAL)"
According to the Star Trek: Star Charts, on the star chart "United Federation of Planets I", AR-558 was treated by the author, Geoffrey Mandel, as two separate locations. Both locations were in the Alpha Quadrant. The first was an antenna/communications relay. The second was a planet in a binary star system of two F-type stars. This planet was the site of a battle.