This paper considers the medium access problem in the IEEE 802.11 standard. Although the transmission bit rates have clearly increased, some MAC related problems remain yet unsolved. The random channel contention suffers from short term unfairness and from the considerable reduction of the effective throughput due to the collision probability that increases with the number of contending nodes in the network. Our objective is to define a coordinated access mechanism that improves the effective throughput and grants a fair sharing of network resources among the different nodes. We propose Transmit And Reserve (TAR), a novel packet based coordinated channel access mechanism that combines advantages of random access and channel assignment. The idea of TAR mechanism is simple: Prior to transmitting a packet, the sender node selects in advance the backoff value for its next packet and advertises this selection within the currently transmitted packet. The neighbor nodes avoid then choosing the same backoff value as the advertised one. The simulation results showed that TAR leads to higher throughput and fairness, and lower collision rate than IEEE 802.11. TAR adapts fast to the network load and to the number of active nodes.