it must ensure that the priority of the information access class is higher than or equal to the minimum access class advertised by BR channel configuration within the DL control message. If this is not the case, then the MS waits until the BR channel configuration within a DL control message advertises an access class with a priority less than or equal to that requested by the MS. When the MS access class is allowed, the MS appropriately sets its internal backoff window. The bandwidth request channel and bandwidth request preambles are used for contention-based random access. Each BR channel indicates a BR opportunity. The MS decides whether to send the BR preamble sequence only or to send the BR preamble sequence together with a quick access message for the random access based BR procedure. The three-step random-access-based BR procedure is illustrated in Figure 6-25. In step 1, the MS transmits a BR preamble sequence and a quick access message on a randomly selected BR opportunity. The BR-ACK A-MAP IE is sent in the next DL frame if the BS detects at least one BR preamble sequence in the BR opportunities in the previous frame. In this case, if the BR-ACK A-MAP IE is not sent in the next DL frame, the MS assumes an implicit NACK and may restart the BR procedure. The BR-ACK A-MAP IE indicates the decoding status of each BR opportunity in the previous frame, all correctly received BR preamble sequences in the BR opportunities of the previous UL 1-BR Preamble Sequence and 1-BR Preamble Sequence and Quick Access Message Quick Access Message BR-ACK A-MAP IE BR-ACK A-MAP IE 2-Grant for UL Transmission 2-Grant for Standalone BR Header 3-Scheduled UL Transmission 3-Standalone BR Header 4-Grant for UL Transmission 5-Scheduled UL Transmission FIGURE 6-25 Three- and five-step random access bandwidth request procedures CHAPTER 6 The IEEE 802. 16m MAC CPS (Part I) frame, and the decoding status of the quick access message for each correctly received BR preamble sequence. If the BR-ACK bitmap indicates no BR preamble sequence