I am not able to understand the ISDN part from the Cisco Press book for ICND exam. Is it necessary to do it in that detail for CCNA exam. If there is a simpler matter then please help.
ISDN is very easy once you practice it on Live equipment. What part do you find difficult? If it's the many terms like ref points, functions, E, I, Q series, that's ok. Everyone finds that boring. But, you should find configuring ISDN easy once you do it.
I have been through what you are facing now when I was doing the CCNA 5 years ago. The problem was, the institute did'nt have ISDN lines to show us. But, when I implemented ISDN where I work, it was very easy.
I think that your problem will be solved once you see ISDN in action. If your institute does'nt have it, ask a friend or anyone if they can show you how it works in organizations. You can also practice ISDN using online racks. But, the best of all is using real gear. Works for me 100%.
The ICND book gives the best explanation on ISDN. It may be a lot technical. You can try Todd's book on CCNA. It is very simple & easy to understand. Also, try cisco docs on their website. If all else fails to appease you, just do a search in google for ISDN explanation. There you will find info very easy to learn.
For the exam, I think that you should know the ref points, functions, signalling (Q931), basic config & DDR at least.