Maser

There is more information available on this subject at Maser on the English Wikipedia.

MASERs (Microwave Amplification by Simulated Emission of Radiation) are the primary method of communication on UNSC starships. Factually, they are devices which emit microwave radiation. They were first referred to in Halo: Contact Harvest, where the DCS cargo freighters, the Tiara and Mack's data center use the devices. They are effective short-range devices (planet to planet), but are too slow to be effectively used over longer distances. They are almost identical to the cellphone technology today.