The interviewers do a very good job of tailoring their questions to the candidates. The most important thing to do is to be honest. When they ask difficult questions they are trying to assess your level of knowledge and they understand if you don't know the answer. Just be upfront and honest. Say out right that you don't know but that you will get back to them with the right answer. Then, actually follow up with them.

One of the onsite questions was to whiteboard a wireless network