rminal InUse instance associated with Route 1 Air Interface InUse instance associated with Route n Sector Access Node Access Network n Figure 6.1 Wideband architecture reference model. Reproduced with permission from © 2008 IEEE [4] 6.4.2 Protocol Architecture for Unicast Traffic The air interface has been designed in layers, with interfaces defined for each layer, as well as for each protocol within each layer. Each sublayer or plane consists of one or more protocols that perform the sublayer's functionality. The layering in IEEE 802.20 is grouped into the physical layer and the MAC layer, as shown in Figure 6.2. Figure 6.2 describes the layering architecture for each route of the air interface that carries unicast data. This layering architecture forms a well-defined and compatible lower layer set, upon which layers 3 and above of the ISO 7-layer model can operate. The layered architecture also allows future modifications to a layer or to a protocol to be isolated. Service Sublayer Security Functions Radio Link Sublayer Lower MAC Sublayer Physical Layer Figure 6.2 Unicast Route Layering architecture. Reproduced with permission from C 2008 IEEE [4] IEEE 802.20 Mobile-Fi The MAC layer further consists of Lower MAC, Radio Link and Service Sublayers. It also consists of three control planes namely Route Control Plane, Session Control Plane, and Connection Control Plane. In addition there is a Management Information Base (MIB) function for both physical and MAC layers. 6.4.2.1 Physical Layer It provides the channel structure, frequency, power output, modulation, and encoding specifications for the Forward and Reverse Channels. 6.4.2.2 Lower MAC Sublayer The Lower MAC sublayer defines the procedures used to receive and to transmit over the physical layer. 6.4.2.3 Radio Link Sublayer The Radio Link Sublayer provides QoS for IP packets via Basic QoS Management Protocol. Seg- mentation, reassembly, retransmission (if needed) and duplicate detection of higher sublayer packets is provided by Basic Radio Link Protocol (RLP