This sentence is technically not correct, but the error is one which I think most people wouldn't really notice that much, and the meaning is still quite clear. This sort of thing often happens in technical contexts because there is so much jargon and some words have different meanings than their usual ones that things can sometimes seem correct even when they technically aren't (or vice-versa).
You do not need to have a "by" clause to use passive voice, so that actually is not the problem. To clarify things, let's break this sentence down a bit:
Here, "This is because" is a leading conjunction which ties this sentence to what came before it, but is not really important for analyzing the structure of this sentence itself, so we'll start by dropping that. "which means that" is also being used as a conjunction to connect two independent sentences into one, so we can break that apart into two sentences and look at each one separately:
The data packets are transmitted electronically.
These messages can be exchanged at a much higher rate than any physical medium.
The first sentence here is passive voice, but it's absolutely fine. No extra work needed.
The problem, however, is that in the second part, you do need some sort of preposition in front of "any physical medium", because using "than" without a preposition makes (the noun) "any physical medium" the implicit subject of the verb "can be exchanged", which means the sentence becomes a comparison between "messages can be exchanged" and "any physical medium can be exchanged", which is clearly not what the author actually meant to say.
If we add an appropriate preposition to "any physical medium", then the phrase becomes an adverb instead, which is what was actually intended. One preposition that does work for this is "by any physical medium", but you could also use "in", "using", or even "via any physical medium", etc, all of which would also work in this case:
These messages can be exchanged at a much higher rate than via any physical medium.
The "than" here is now comparing "messages can be exchanged (electronically)" with "messages can be exchanged via any physical medium", which is pretty clearly the comparison the author actually intended to make.