ISDN on the CCNA

I'm using the Cisco Press book "Cisco CCNA Exam #640-607 Certification Guide" by Wendell Odom, as well as practical experience to study for my CCNA. The book covers ISDN rather extensively. Are there a lot of ISDN related questions on the CCNA test? At my current job I will never have to touch an ISDN line or router, so, if I can avoid it, I would like to spend a minimum amount of time reading about ISDN and more time studying things that I know are covered heavily on the test like switching and the OSI model.

-Jeff