ment. The ARQ mechanism may be disabled for some delay sensitive applications such as VoIP. Fragmentation is a process in which a MAC SDU is divided into one or more MAC SDU fragments. Packing is a process where multiple MAC SDUs are packed into a single MAC PDU payload. Both processes may be initiated by either a BS for a downlink connection or an MS for an uplink connection. Several MAC PDUs may be concatenated into a single transmission in the downlink or uplink. The MAC PDUs containing user data are processed by the physical layer for over-the-air trans- mission. It must be noted that the physical layer processing of the user traffic can be different in terms of the permissible MIMO modes or modulation and coding schemes that are used. 3.1.4 Control-Plane A set of Layer 2 control functions are needed to support various radio resource configuration, coordination, signaling, and management. This set of functions is collectively referred to as control- plane functions. The IEEE 802.1 16m control-plane entity comprises Radio Resource Control and Management (RRCM), MAC functional group, and physical layer protocols corresponding to control path. The RRCM functional class includes all control and management functions such as network entry/re-entry management, paging and idle mode management, multicast and broadcast service, etc. This group of functions is also known as Radio Resource Control (RRC) in other air interface standards such as 3GPP LTE. The MAC functional group consists of functions that perform physical layer control and signaling, scheduling services, QoS, etc. This functional group corresponds to Radio Link Control (RLC) and MAC layers in other air interface standards such as 3GPP LTE. Figure 3-9 illustrates the IEEE 802.16m control-plane entity. As shown in this figure, the RRCM performs control and management of lower-layer functions. The control information is communicated with the mobile station via MAC management messages. The underlying functional elements of the RRCM sub-layer will be described