Although I have only a very superficial understanding of modems, I'd say demux() should be a function library, not an instantiated object. But even if it is an object for various reasons such as caching, that's quite fine with me; a Demultiplexer can be thought of as an conceptual component that a Modem "has" inside it.

What I don't like is when things like options, parameters, and intermediate representations of data that aren't actually intuitive are represented as instantiated objects of their own. In some sense I expect every instantiated object to "do" something useful of its own, and represent a functional block in a flowchart. Objects should, to the greatest extent possible, represent "is" and "has" relationships in a system, IMO.

Also, class hierarchy should be well designed. For example, an Exception called ConnectionError (that inherits from Exception) is great. It can be parametrized from there. What sucks is if there is SomeFooConnectionError, SomeBarConnectionError, SomeBlahConnectionError that all inherit directly from Exception and don't inherit from a common ConnectionError, because that makes the try/catch block super verbose and easy to miss a possibility.