 3.2 The IEEE 802. 16m protocol structure The multi-radio coexistence block within the RS coordinates collocated multi-radio operation of the subordinate mobile stations in coordination with the BS. Based on the earlier description of the functions and protocols performed by a relay station, the control- and data-plane protocol termination is shown in Figure 3-17. The convergence sub-layer protocols are terminated at the MS and the BS. However, some of the MAC CPS protocols are terminated at the RS. Due to the use of a distributed security model, the security functions including encryption and packet validation are terminated at the RS on the relay and access links. 3.2.3 Protocol Structure for Support of Multi-Carrier Operation The generic protocol structure for support of multi-carrier operation is illustrated in Figure 3-18. A single MAC instance controls a number of physical layers spanning over multiple frequency bands. Some MAC messages transmitted over one RF carrier may also apply to other RF carriers. The RF channels may be of different bandwidths (e.g., 5, 10, and 20 MHz), and can be contiguous or non- Radio Resource Control and Management Network Layer Medium Access Control Functions Scheduling and Radio Resource Management PHY-SAP PHY-SAP PHY-SAP MAC Control Signaling Generic Multi-carrier Protocol Structure MAC Control MAC Control Signaling MAC Control Signaling Signaling Scheduling Scheduling Scheduling Network Network Layer Layer Network Layer Example Multi-carrier Operation (Mobile Stations with Different Capabilities) FIGURE 3-18 Multi-carrier operation using a single MAC instantiation [12] CHAPTER 3 IEEE 802. 16m Reference Model and Protocol Structure contiguous in frequency. The RF channels may support different duplexing schemes, e.g., frequency division duplex (FDD) mode, time division duplex (TDD) mode, or a combination of multicast and/or unicast RF carriers [12]. As shown in Figure 3-18, the MAC entity can provide simultaneous service to mobile stations with different bandwidth capabilitie