The wireless and dynamic nature of mobile ad hoc networks (MANETs) leaves them more vulnerable to security attacks than their wired counterparts. The nodes act both as routers and as communication end points. This makes the physical layer more prone to security attacks. The MANET physical layer is challenging to DoS attack and also some passive attacks. The physical layer protocol in MANETs is responsible for bit-level transmission between network nodes. The proposed model combines spread spectrum technology Direct Sequence Spread Spectrum (DSSS) with key management technique ISAKMP to defend against signal jamming denial-of-service attacks in physical layer of MANET.DSSS with ISAKMP is found to be a good security solution even with its known security problems. The simulation is done using network simulator qualnet 5.0 for different number of mobile nodes. The proposed model has shown improved results in terms of Average throughput, Average end to end delay, Average packet delivery ratio, and Average jitter.