he previous higher layer. On the upward direction, it is the data unit sent to the next higher layer. 3.1 The IEEE 802.16m reference model This chapter provides a top-down systematic description of IEEE 802.16m reference model and protocol structure, starting at the most general level and working toward details or specifics of the protocol layers, their functional constituents and interconnections. An overview of 3GPP LTE protocol structure is further provided to enable readers to contrast the corresponding protocols and functionalities. It must be noted that while the IEEE 802.16 standard does define a generic network reference model (or a network abstraction model), the mobile WiMAX systems use the specific network reference model and system architecture that were described in Chapter 2 to achieve interoperability. Therefore, the network reference model and associated components and interfaces described in Section 3.1 are only informative, and they should not be interpreted as normative for implementation and deployment of the IEEE 802.16m systems. 3.1 THE IEEE 802.16M REFERENCE MODEL Figure 3-3 illustrates the IEEE 802.16 reference model [3]. The data link layer of IEEE 802.16 standard comprises three sub-layers. The service-specific convergence sub-layer (CS) provides any IEEE 802.16 Entity CS SAP Service Specific Convergence Sublayer CS Management/Configuration MAC SAP MAC Common Part Sublayer (MAC CPS) MAC Management/Configuration Security Sublayer Management Information Base (MIB) PHY SAP Physical Layer (PHY) PHY Management/Configuration Data/Control Plane Management Plane FIGURE 3-3 The IEEE 802.16 reference model [3] CHAPTER 3 IEEE 802. 16m Reference Model and Protocol Structure transformation or mapping of network-layer data packets into MAC SDUs. On the transmitter side, the CS receives the data packets through the CS Service Access Point (SAP) and delivers MAC SDUs to the MAC Common Part Sub-layer (MAC CPS) through the MAC SAP. This includes classifying network-layer SDUs and associating them with the