rity association, then multiple connection payloads are multiplexed prior to encryption and the multiplexed payload is jointly encrypted. The receiver performs the decryption and data authentication on the multiplexed payload, as specified by the security association. The receiver determines whether the payload in the MAC PDU is encrypted by verifying the Flow ID in the AGMH. The encryption information needed to decrypt the multiplexed payload at the receiving station is contained in the beginning of the first connection payload and at the end of the last connection payload. As an example, the AES CCM, PN, and EKS are present in the beginning of the connection payload and the ICV is appended at the end of the connection payload in the MAC PDU, as shown in Figure 7-15. 7.4 3GPP LTE RADIO LINK CONTROL AND MEDIUM ACCESS CONTROL SUB-LAYERS The Layer 2 functions in LTE are classified into the following categories: Medium Access Control (MAC) functions; Radio Link Control (RLC) functions; and Packet Data Convergence Protocol (PDCP) functions [9-12]. Figures 7-16 and 7-17 illustrate the structure of Layer 2 in LTE downlink and uplink. The Service Access Point (SAP) for peer-to-peer communication is marked with a small oval 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 [9]. 7.4.1 3GPP LTE MAC Sub-Layer The services and functions provided by the MAC sub-layer can be summarized as follows: Mapping between logical channels and transport channels; Multiplexing/de-multiplexing of RLC protocol data units corresponding to one or different radio bearers into/from transport blocks delivered to/from the physical layer on transport channels; Traffic volume measurement reporting; Error correction through HARQ; P