The standard communication medium of the day has become sub-space. Sub-space is mathematically defined as an infinite space that exists within our realm inside a finite boundary. It was discovered in the past 300 years that in any given sub-space realm there is a direct correlation between each point in that realm and each point within our own. However, energy behaves differently within any given sub-space realm than it does within normal-space. As an example, radio waves travel at extremely high relativistic speeds within certain sub-space realms, allowing the injection of a radio wave at one point in normal-space where it will cross sub-space at extreme super-luminal speeds (on the order of 10^21c) where the receiving unit will then pull the signal back out of sub-space again. This method allows for nearly instantaneous, extremely low latency communications across nearly limitless spans of real-space.
Sub-space communications technology is still relatively large physically, with transceivers requiring around six cubic feet of space and large power supplies.
Most normal, short distance communications (short distance being under 1,000 miles or so) are still done using digitally encoded, secure radio frequency communications. Normal-space communicators use a variety of frequencies from 25MHz all the way up to 10GHz and an even broader array of digital compression/encryption. Most high quality communicators are capable of using any and all combinations of frequency and compression/encryption but some of the less expensive gear is limited in this regard, however it is not usually a concern as all you need is two similar devices to set up a line of communication. In terms of physical size, standard RF transceivers can be as small as a peanut (including the power supply, the actual transceiver is extremely small at about 1⁄10 that size).