The intrusion detection system (IDS) for mobile ad hoc networks (MANET) consists in monitoring the nodes' behavior, in order to detect the malicious activity of nodes. Many existing solutions deal with the problem at each layer separately. But new kinds of misbehavior attacks are cross-layer attacks. And such smart misbehaviors cannot be detected at the level of one layer. In this paper, we propose a new cross-layer approach based on physical, MAC, and routing layers for a monitoring mechanism. A new analytical model is proposed to illustrate the parameters' effect on these different layers. The impact of the signal to noise ratio (SNR) and the distance between monitor and monitored nodes are clearly introduced. Moreover, the difference between the carrier sense, the interference range, and the transmission range is taken into account in our model. The proposed model improves the evaluation of the nodes' cooperation and reduces the risk of having any false positive rate. The analytical study and simulation results illustrate our purpose. In addition, with the simulations' results, we illustrate the impact of the distance between monitor and monitored nodes on the monitoring mechanism. Finally, we show that our cross-layer mechanism has a lower false positive rate than the classical Watchdog mechanism in different network's parameters such as the nodes' density, the speed mobility, and the different traffic loads.