In this paper we investigate the MAC layer sensing schemes in cognitive radio. Reactive and proactive sensing are both considered and assessed. The assessment of these sensing has been held via the obtained idle channel search delay in each case. The simulation results conclude that with proactive sensing we achieve lower idle channel search delay as compared with reactive sensing, but on the other hand we waste some of our resources in sensing which is higher than in reactive scheme. Thus, in proactive sensing we have observable computational tasks to be done by the network nodes.