cks, respectively. In the C-plane, the Non-Access Stratum (NAS) functional block is used for network attachment, authentication, setting up bearers, and mobility management. All NAS messages are ciphered and FIGURE 3-20 The user-plane protocol stack [29] CHAPTER 3 IEEE 802.16m Reference Model and Protocol Structure FIGURE 3-21 The control-plane protocol stack [29] Radio Bearers Security Security Security Security Segmentation Segmentation Segmentation Segmentation Logical Channels Scheduling / Priority Handling Multiplexing UE1 Multiplexing UEn Transport Channels FIGURE 3-22 The 3GPP LTE Layer 2 structure in the downlink [29] integrity protected by the Mobility Management Entity (MME) and User Equipment (UE), i.e., 3GPP LTE mobile station [28]. The Layer 2 functions in 3GPP LTE are classified into the following categories: Medium Access Control (MAC); Radio Link Control (RLC); and Packet Data Convergence Protocol (PDCP) functions [29]. Figures 3-22 and 3-23 illustrate the structure of Layer 2 protocols in 3GPP LTE downlink and 3.3 3GPP LTE/LTE-advanced protocol structure Radio Bearers Security Security Segmentation Segmentation Logical Channels Scheduling / Priority Handling Multiplexing Transport Channels FIGURE 3-23 The 3GPP LTE Layer 2 structure in the uplink [29] uplink. The SAP for peer-to-peer communication is marked with circles at the interface between the sub-layers. The SAP between the physical layer and the MAC sub-layer provides the transport channels. The SAP between the MAC sub-layer and the RLC sub-layer provide the logical channels. The multiplexing of several logical channels (i.e., radio bearers) on the same transport channel (i.e., transport block) is performed by the MAC sub-layer. Each logical channel is defined by type of information that is transferred. The logical channels are generally classified into two groups: (1) control channels (for the transfer of control-plane information); and (2) traffic channels (for the transfer of user-plane information). As shown in Figure 3-21, the RRC laye