270 Copyright © 2017, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 12 DOI: 10.4018/978-1-5225-2113-6.ch012 ABSTRACT This chapter introduces two diferent algorithms to detect intrusions in mission critical communica- tion systems to guarantee their security. The frst algorithm is a classifcation algorithm which applies the concept of supervised learning. The second algorithm is a clustering algorithm which applies the concept of unsupervised learning. The algorithms detect intrusions using a set of detection rules that are structured in the form of decision trees. The algorithms are described in details and their results on well-known dataset are introduced. An enhancement for the J48algorithm is also introduced, where the decision tree for the algorithm is changed to a binary tree. The change enhances the complexity to reach a decision. The chapter includes a brief introduction about the security in Mission critical systems and the reason behind securing such systems. It introduces diferent methodologies that were introduced to detect intrusions in wireless communications. INTRODUCTION A mission critical system is essential to the survival of a business or organization. When a mission criti- cal system is attacked or failed, business operations and organizations are significantly impacted. For some governmental organizations and some IT sectors, databases are considered as Mission Critical systems. For the internet applications, servers are considered as Mission Critical systems. For public safety organizations, the systems must be reliable and available around the clock to guarantee instant Security in Mission Critical Communication Systems: Approach for Intrusion Detection Karen Medhat Cairo University, Egypt Rabie A. Ramadan Cairo University, Egypt Ihab Talkhan Cairo University, Egypt