The habitat ring was the inner of the two rings that made up the structure of Deep Space 9. It was connected to the out docking ring by three crossover bridges which continued through to the mid core. An additional three smaller crossover bridges connected the habitat ring to the mid core section only.
Under the original Cardassian arrangement, of the then Terok Nor, the habitat ring contained 452 large residential quarters, typically spanning four or five window bays, for high ranking officers and 231 smaller quarters, typically two or three window bays, for lower ranking officer and Bajoran nationals who worked for the Cardassians. Under the Federation the quarters were used by Starfleet and Bajoran crew members, visiting dignitaries, commercial operators and some transient ship crews. An addition 343 quarters were also available in the mid core.
Following the handover to Federation control many of the non-load bearing walls were moved or removed to create custom spaces, including duplicate personal services to those available on the Promenade, such as emergency medical services and additional laboratory areas.
Typically residential spaces in the habitat ring faced out into space with views through transparent aluminum windows, the opacity of which could be controlled by the residents to allow more or less sunlight into their quarters. Quarters typically included a replicator and wash facilities. Not all wash facilities in the original Cardassian configuration included sonic showers so many were rotated out from other starbases and salvage when the Federation took control of the station.
Twenty-five percent of the quarters were fitted with the necessary life support equipment to sustain class H, K and L conditions and an addition three percent were capably of supporting class B, N and C environments, in the later case featuring forcefields and materials which could withstand the environments available. (DS9 reference: Technical Manual)
When the Starfleet took control of the station the habitat ring was also fitted with six launch and maintenance bays for runabouts and shuttlecraft. The bays were positioned at sixty degree intervals and featured a retractable launch pad which raised up to allow easy launch of the small-craft. The pad retracted into a bay capable of supporting a single runabout and at least two shuttlecraft and provided an area for maintenance and loading of different runabout modules. All quarters in the vicinity of the bays were either converted into engineering support areas or shielded against potential emissions and accidents. Quarters more than two levels away received minor soundproofing. (DS9 reference: Technical Manual)
In 2370 Suite 959 in the habit ring became the temporary home of a clutch of Horta eggs. The eggs were protected by a stasis field to prevent them prematurely hatching while still on the station. Unfortunately Nog and Jake Sisko decided to borrow one of the eggs and in doing so deactivated the stasis field. Shortly after the Horta hatched and burrowed out of the suite and quickly made there way into the rest of the station. Later Odo, in a Pied Piper like move, took on the form of an adult Horta and managed to lure the babies back into the habitat ring into a single large residential suite. Unfortunately once they were all in one of the Hortas burrowed out into space. Odo was able to seal the hole but the rest of the Horta's soon began to spread out into the station once more. (DS9 novel: Devil in the Sky)