From Halopedia, the Halo wiki
Communications deck is a generic United Nations Space Command term for the part of a space station, such as an Orbital Defense Platform, that handles the station's communication systems. Reach's Station Gamma also possessed one or more communications decks.
Cairo Station communications deck
During the Battle of Earth, Covenant boarders invaded Cairo Station's communications deck. In response, several quick-response teams of Marines were sent to repel them. The actual deck is never visited during the course of the level Cairo Station, and is only mentioned over the loudspeaker.
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