Superluminal communications

Different civilizations have developed various technologies which enable communications at superluminal (faster-than-light) speeds over interstellar or even cross-galactic distances.

UNSC
While reliant on starships to carry information across interstellar distances for a significant portion of their spacefaring history, humanity had successfully developed superluminal communications technology which allows interstellar communication in virtual real-time by July 2552. How this technology functions, and whether it uses slipstream space or some other mechanism as a carrier, has not been specified. UNSC communications technology received further improvements following the discovery of Forerunner technology on Trevelyan in early 2553, enabling even smoother instant information transfer and the capability for ships to maintain communications while in slipspace.

This system may be related to a form of communication known as "slipstream packets", which are, in essence, recorded audio messages, rather like letters. Human civilians were able to use slipstream packets by September 2552. These messages are sent through Waypoint, a UNSC communications network similar to the 21st-century Internet.

Before the advent of superluminal communications, most human long-range communiques were carried on the shipboard memory of starships such as automated freighters. On the other end of the ship's slipspace journey, the message would be relayed to the intended recipient. At the time of the establishment of the Cole Protocol in 2531, while unable to send messages through slipspace itself, the UNSC was able to use a form of slipspace manipulation to obfuscate radio transmissions sent from Earth centuries earlier in order to prevent the Covenant from using the signals to triangulate Earth's location.

The UNSC also developed a piece of technology known as the Slipspace COM launcher, which sent independently guided probes through slipspace as fast as any UNSC starship. The strength of this system was the probes' ability to precisely home in on the designated target and securely deliver the message to the recipient. However, this technology was highly expensive and as a result there were only three such launchers in existence by 2552: one on Reach, one on Onyx and one on Earth. As of the conclusion of the Human-Covenant War, both the devices on Reach and Onyx have been destroyed, with the fate of the Earth device unknown.

UNSC emergency locator beacons, judging by their nickname of "slipbeacons", can apparently send signals through slipspace.

Forerunner
The Forerunners had virtually abandoned the electromagnetic spectrum as a means to carry their communications, instead relying on more secure quantum entanglement. These communications were routed over proprietary encryption protocols, which could be used to track the source or destination of the communication. This allowed instant data transfer over vast distances: Offensive Bias, for example, was capable of simultaneously coordinating the deployment of the Halo Array at Installation 00 outside the galaxy and its fleet of war at the Maginot Line.

The Domain, the Forerunners' collective information repository, was not usually employed for the purpose of communications due to its unreliability: it had a known propensity to alter information without explanation. However, it was still known to be used for communications on certain occasions.

The Forerunners also used a form of superluminal communication involving wormholes; however, these communications were significantly slower than communication via the Domain.

During the Battle of Installation 05, Cortana was able to utilize Forerunner technology to send an emergency signal directly through slipspace, without a physical carrier, declaring codes Bandersnatch and Hydra. According to Endless Summer, the energy required to send such a transmission would have required more energy than the combined output of all UNSC assets, demonstrating the Forerunners' superior grasp of energy manipulation. However, this appears to have been an ad hoc or emergency communication system given the Forerunners abandonment of wave-based communication, likely carried within the anomalous wake of the Forerunner Dreadnought's passage to the Sol system.

Forerunner starship sensors were capable of instantaneous scanning and detection across interstellar distances. As a particularly impressive example of their available sensor ranges, the Forerunners' core authority routinely tracked individual slipspace jumps across the galaxy. Forerunners shipboard sensors were also capable of providing a detailed analysis of a single planet's geological and ecological composition from lightyears away through the use of long-range entanglements.

Covenant
The Covenant use slipspace as the basis of their virtually instantaneous interstellar communications. When installed on a planet, for example, Covenant communications relays enable communications to corresponding devices across many lightyears. At the Covenant hegemony's height, their starships possessed an automated messaging system which transmitted new orders and communiques from ship to ship upon arrival at or departure from a given star system, enabling them to rapidly relay information across their vast empire. However, as noted by Cortana, these messages were not properly encrypted by UNSC standards, perhaps due to the Covenant's arrogance or ignorance of the workings of their own technology.

Flood
The Flood's collective intelligence, the Gravemind, employs an esoteric mechanism which can quite aptly be described as telepathy, controlling Flood forms across vast distances. In certain circumstances, the Gravemind has also demonstrated to be capable of using this ability to telepathically communicate with non-Flood beings, namely John-117.