[kragen]

There's a paper from the 1990s (https://gnunet.org/sites/default/files/10.1.1.43.4892.pdf I think, Pfitzmann 1991) on building an ISDN MIX for anonymous real-time voice traffic. ISDN gives you two 56kbps or 64kbps channels in each direction for voice, which you can connect to whoever you want. So you just keep these channels open to somebody all the time and keep them full of data, even if it's nonsense. Using fully isochronous streams in this way, assuming a fully connected directed graph, which is overwhelmingly probable, it's impossible for a global passive attacker to determine anything at all about who is actually talking to whom, or how much; the traffic pattern simply is not affected by the communication graph.

A thing I don't know is whether Pfitzmann et al.'s scheme held up to subsequent analysis. I haven't even read the whole paper, actually.