A sensor network consists of a large number of tiny, economical, self powered devices that can invigilate or sense, interact and compute with other devices for the motive of collecting local information to make universal resolution about a physical environment. WSN communicate with sensitive data and work in hostile area. The WSN are vulnerable to various threats because they are physically reachable from the outside environment like interruption, interception, modification, fabrication and attacks like passive information gathering, node subversion, false node, node malfunction, node outage, message corruption, traffic analysis, selective forwarding, sink hole attacks and Sybil attacks. The confidentiality, integrity, authentication and availability are the four security goals towards sensor network.

WSN are promptly appearing as an important area in ubiquitous computing because eavesdropper can easily intercept the message and easily monitored the communication between the nodes; hence intrusion detection technique is important for ubiquitous applications that help in identifying the malicious intruder that is occupying the network domain . Black hole attacks is same as DOS attack that occurs when the man in the middle change a set of nodes in the network to block the packets and produce incorrect/ changed messages instead of forwarding authentic information en-route the base station in WSN .  Authentication based intrusion prevention and energy saving intrusion detection are the two methods used to improve the defense of clustering based sensor network. Intrusion detection techniques, cryptographic techniques, encryption and decryption, authentication by using id and password are used for providing security against WSN attacks. Cryptography is most vital for network security because cryptography is a developing technology and research on cryptography is required for authenticated communication.

WLAN and INTER-NETWORKING :- The wireless technologies have different coverage and bandwidth drawback. So, no specific technology is considered as best. For ubiquitous wireless communication, internetworking among heterogeneous wireless network is most vital. 2G systems are not suitable for data communication. Limited capacity, eavesdropping, fraud are the problems in 1G that are successfully forwarded by 2G. These problems have been reduced in 3G systems as compared to 2G system. The integration of two security architectures named as 3G and WLAN produce an insecure outcome. 3G and WLAN faces major security and performance challenges, exclusively for privacy and access control of mobile stations.