In the Dune books, I couldn't understand how interplanetary communications work. Is there a real-time way of communications, or do they depend on Guild ships to make the trip, and thats how they get messages across?
Repeated throughout the Dune novels a reader will find "communications room".
Example from Heretics of Dune:
A tall Reverend Mother came from the adjoining communications room. "Mother Commander," she said, "the messages have been sent to Guild, Ix, and Tleilaxu."
And about 30 lines later a reader can find this:
The Reverend Mother from the communications room returned, glanced once at Sheeana and spoke to Odrade. "Chapter House acknowledges receipt of your report. They approve your response
Which makes me think they had those working as real time communications in place.
Maybe they were using something like ansible for this?
But then I was just reading Dune Encyclopedia and found this:
The centrality crucial to Imperial administration administration was impossible to maintain when there was no quick method of communication from one end of the Empire to the other — and until the discovery of Holtzman Waves, the only way to send a message was to send a ship, which made for impossibly expensive administrative costs, and diffused the power of the emperor irreparably.
In 7562 B.G., I.V. Holtzman discovered the Holtzman Wave; in 7556 E.G. he passed on information necessary for its use to six planetary systems
Also in 7556, his research was interrupted and he was unable to resume it until his orbit brought him back within panel range of his sun. In his second return (5694 E.G.), Holtzman kept silent; however, he evidently used this period to discover the curious "existence" of the pointsource node. On his third pass (3832 B.G.), Holtzman was able to communicate freely after some initial difficulties were overcome, and on that pass he published the theory that led to the development of the twodimensional Holtzman Effect, the famed Defensive Shield.
Theoretically, the range of a Holtzman Wave is infinite; in practice, its effective range is limited by the ability of the sender to aim the wave so that it will strike a large (planetary or larger) mass at or near the receiver. This requirement limits the useful range of Holtzman Wave communications to approximately 250 light-years without re-broadcasting.
Improvement came with the Holtzman Wave, nationally a mathematical phenomenon causing microcatastrophic folds in real spacetime along a selectable vector, when the vector impacts with matter denser than interstellar hydrogen, it excites that matter to emit long radio waves, which can then be received on normal radio antennae. With interstellar communication now possible, navigation could be enormously improved.
This makes me believe that they were using HE with Communication Rooms to the extent that HE allowed and then the message would reach the target planet via another planet.
In The Dune Encyclopedia there was a concept called "Holtzmann Waves" which were a 1 dimensional incarnation of the 2 dimensional defensive shield and the three dimensional suspensor nullification effect. This could be used for instantaneous communication across interstellar distances.
I don't believe this is considered canon though.
Also, there is the Tachion net for the post-Dune novels especially, although Daniel and Marty appear in the last book of FH.
The Net, or Tachyon Net as it came to be called, was a galaxies wide instrument of communication, particle manipulation, and entity envelopment developed by the Outside Enemy. The elderly couple Daniel and Marty tried to ensnare the no-ship manned by Duncan Idaho, with their tachyon net.
Also Omnius communicates through update ships (a system that proves flawed), probably:
apt-get upgrade omnius
Ships with Holtzmann engines. (There's nothing to indicate that no-ships used a different type of engine; they just had the no-tech cloaking.)