nges as indicated by QoS requirement carried in the explicit signaling mechanisms. 3. Explicit, One Time Only: the GPI and Grant Size one-time-only change is triggered by explicit signaling by the MS, such as in piggyback bandwidth request, bandwidth request signaling, quick access message in bandwidth request channel, or the fast-feedback channel. If GPI_Secondary/ Grant_Size_Secondary is defined, the GPI and Grant Size are switched only once from GPI_Primary/Grant_Size_Primary to GPI_Secondary/Grant_Size_Secondary; otherwise, GPI and Grant Size change as indicated by QoS requirements contained in the explicit signaling mechanisms. 6.7 MAC management/control messages The mandatory QoS parameters of aGPS are the Maximum Sustained Traffic Rate, the Request/ Transmission Policy, Primary GPI, and Primary Grant Size. 6.7 MAC MANAGEMENT/CONTROL MESSAGES The connection-oriented MAC layers in the serving BS and the MS communicate using the MAC control messages to perform the control-plane functions. The MAC control messages are contained in MAC PDUs and are transported over broadcast, unicast or random access connections. There is a single unicast control connection. In order to improve reliability, the HARQ mechanism is used for MAC control messages that are sent over unicast control connections. Furthermore, encryption may be enabled for unicast MAC control messages. The IEEE 802.1 16m MAC control messages can be fragmented; however, a multiplexing extended header is not used for these messages. The IEEE 802.16m MAC management/control messages are listed in Table 6-8. The MAC management messages in IEEE 802.16m are distinguished from their legacy counterparts by an "AAI" prefix, denoting "Advanced Air Interface" messaging. Unlike the IEEE 802.16-2009 standard, the IEEE 802.16m MAC management/control messages are encoded in ASN. format. The encryption of MAC PDU is signaled through Encryption Control Bit in FEH extended header when it is set to one. As shown in Table 6-8, the MAC management messages may or may not be e