Wireless networks are established on a shared medium which makes it possible for the adversary to easily launch jamming attacks that threaten the availability. Denial of service is a type of attack that threatens Availability (one of the security goals). When a network is vulnerable to DoS attack, it cannot be protected using an encryption algorithm. This attack strikes at the physical and MAC layer. A Denial of Service attack is detected by measuring four network parameters (Packet delivery ratio, Bad packet ratio, Energy consumption amount, Signal to noise ratio) obtained from MAC and physical layer. Other Jamming attacks against a wireless medium caused by the attacker are also discussed in this paper.