 initialisation, the CPB is empty. NOTE - After initialisation, the HRD is not initialised again by subsequent buffering period SEI messages. Each access unit is referred to as access unit n, where the number n identifies the particular access unit. The access unit that is associated with the buffering period SEI message that initialises the CPB is referred to as access unit 0. The value of n is incremented by 1 for each subsequent access unit in decoding order. The time at which the first bit of access unit n begins to enter the CPB is referred to as the initial arrival time tai(n). The initial arrival time of access units is derived as follows: Rec. ITU-T H.264 (01/2012) If the access unit is access unit 0, tai( = 0, Otherwise (the access unit is access unit n with n > 0), the following applies: If cbr flag[ SchedSelIdx ] is equal to 1, the initial arrival time for access unit n, is equal to the final arrival time (which is derived below) of access unit n - 1, i.e., (C-2) Otherwise (cbr_flag[ SchedSelIdx ] is equal to 0), the initial arrival time for access unit n is derived by tai(n)=Max(taf(n-1),t tai,earliest ) (C-3) where tai,earliest(n) is derived as follows: If access unit n is not the first access unit of a subsequent buffering period, tai,earliest( is derived as tai,earliest( =tr,(n) - ( initial_cpb_removal_delay[ SchedSelIdx + initial_cpb_removal_delay_offset[SchedSelldx])+ 90000 (C-4) with tr,n (n) being the nominal removal time of access unit n from the CPB as specified in clause C.1.2 and initial_cpb_removal_delay[ SchedSelIdx ] and initial_cpb_removal_delay_offset[SchedSelldx being specified in the previous buffering period SEI message. Otherwise (access unit n is the first access unit of a subsequent buffering period), tai,earliest(n) is derived as earliest(n)t(n)-(initial_cpb_removal_delay[SchedSelldx]90000) - (C-5) with initial_cpb_removal_delay[SchedSelldx ] being specified in the buffering period SEI message associated with access unit n. The final arrival time for access unit n is derived by 