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The IEEE 802.11 standard defines the multicast transport to deliver one content to many receivers at the same time. However, no retry policy is defined for this transport mode. Therefore, missing packets are not retransmitted and are definitely lost. In IEEE 802.11 networks, transmission failures can be caused by many factors: collisions, path loss, interference, etc. Based on our measurements, packets losses caused by collisions may exceed 40% of the transmitted multicast packets. In this paper, we study the collision rate in a WLAN network and we introduce the Busy Symbol (BS) mechanism to prevent the collision of the multicast packets. The BS is perfectly compliant with the standard and requires no modifications to be deployed except at the AP. Moreover, we show that this mechanism guarantees a fair sharing of the medium between the unicast and multicast flows.