In this work it is considered a ultra wideband cognitive radio network where multiple secondary users (SUs) communicate with an access point (AP) in an opportunistic way. The SUs adopt a distributed medium access control (MAC) scheme based on time contention through random medium access. One or multiple primary users (PUs) may become active in the entire band or in a part of it, which may impact in the communications of the SUs. Departing from a classical random access scheme for the SUs, it is derived a solution to estimate the level of activity of the PUs through the observation of different data availble at the MAC layer of each SU. Differently from the majority of the works already proposed, no physical spectrum sensing is needed to estimate the activity of the PUs since the estimation only relies on data collected at the MAC layer. Based on the level of PUs' activity detected by each SU, it is proposed a solution to regulate the SU's medium access in order to guarantee a given level of protection to the PUs. The formal methodology is evaluated through simulation and several results confirm the effectiveness of our proposal.