[jacquesm]

That's something else, that's known plaintext analysis using cribs. If you know that a certain text may be present in the messages then you can try a bunch of different keys and stop searching if your decrypt contains that text to inspect for a 'hit'.

Network analysis or traffic analysis is this:

https://en.wikipedia.org/wiki/Traffic_analysis

It allows for instance for the de-anonymization of network nodes based on known nodes and it allows for determining which nodes in the network are more important than others.