An apparatus for receiving a multicast digital message while operating on a channel in a wireless communication medium, the apparatus comprising a receiver for receiving said multicast digital message transmitted over said wireless communication medium, said multicast digital message having a format comprising: a duration field specifying a time period for which a transmitter of said multicast message desires to reserve said wireless communication medium; a multicast receiver address field specifying a multicast wireless medium access control address of at least two apparatus of which said apparatus; a multicast transmitter address field specifying a MAC address of said transmitter of said multicast message; a multicast clear-to-send slot number field specifying transmission time slots in a contention period to be used by said apparatus to send a positive reply signal; and a multicast not clear-to-send slot number field specifying transmission time slots in the contention period to be used by said apparatus to send a negative reply signal; and said apparatus further comprising: a transmitter for transmitting either a clear-to-send message or a not clear-to message by utilizing said transmission time slots in said contention period to send said positive reply signal or said transmission time slots in said contention period to send said negative reply signal respectively in response to said received multicast digital message; and a processor configured to resets a network allocation vector of the apparatus when no transmission on the channel is received in a time interval T between times T1 and T2, wherein the time T1 occurs at a time comprising a sum of: a length of a time interval for receiving the multicast digital message; a length of a time interval for transmission of the clear-to-send message multiplied by a number of clear-to-send message slots; a length of a time interval for transmission of the not clear-to-send message multiplied by a number of not clear-to-send message slots; and a length of a time interval for interframe guard intervals; and wherein the time T2 occurs at the time T1 added to a wait time, wherein the wait time is a parameter to wait to make sure that there is no transmission over the wireless medium a duration field specifying a time period for which a transmitter of said multicast message desires to reserve said wireless communication medium; a multicast receiver address field specifying a multicast wireless medium access control address of at least two apparatus of which said apparatus; a multicast transmitter address field specifying a MAC address of said transmitter of said multicast message; a multicast clear-to-send slot number field specifying transmission time slots in a contention period to be used by said apparatus to send a positive reply signal; and a multicast not clear-to-send slot number field specifying transmission time slots in the contention period to be used by said apparatus to send a negative reply signal; and a transmitter for transmitting either a clear-to-send message or a not clear-to message by utilizing said transmission time slots in said contention period to send said positive reply signal or said transmission time slots in said contention period to send said negative reply signal respectively in response to said received multicast digital message; and a processor configured to resets a network allocation vector of the apparatus when no transmission on the channel is received in a time interval T between times T1 and T2, wherein the time T1 occurs at a time comprising a sum of: a length of a time interval for receiving the multicast digital message; a length of a time interval for transmission of the clear-to-send message multiplied by a number of clear-to-send message slots; a length of a time interval for transmission of the not clear-to-send message multiplied by a number of not clear-to-send message slots; and a length of a time interval for interframe guard intervals; and wherein the time T2 occurs at the time T1 added to a wait time, wherein the wait time is a parameter to wait to make sure that there is no transmission over the wireless medium