Ad-hoc Sensor Networks (ASN) is the most demanded and happening research area now a day. Here mainly the security is important issue in ASN. The following paper describes process of communication in MANET, Security issues, Black hole attack, and Intrusion detection. In the mobile ad-hoc network the nodes keep travelling in its surrounded environment. In the process of data communication from source to destination the intermediate nodes plays important role. We know that MANET is most demanded and useful because of its decentralized and wireless infrastructre.Due to this nature it also faces so many security issues such as black hole attack, Denial of Service, Grey hole attck,Sink Hole Attack, Intrusion detection system. In the following discussion we have described DS architectures and different intrusion detection mechanisms.