## 3 MAC Protocols for CRBANs
### Asynchronous MAC Protocol for Spectrum Agility
The sensor node sends the data and waits for the ACK signal. The coordinator receives the data and sends the ACK to the sensor node. The cognitive task of C-RICER has three main features: channel sensing, power adaption and channel adaption. Channel sensing is the initial step in detecting interference. The received signal strength indicator (RSSI) is used to measure the interference level in different channels. In WBASNs, the coordinator is the node in the network most suited to channel sensing. It is the only node that has less energy constraints than sensor nodes in the network; thus, exploiting the energy at the coordinator instead of sensor nodes can prolong the network's lifetime. Communication is only established after a transmitter has successfully received a wakeup beacon from a receiver. The coordinator of C-RICER periodically senses the interference level only at its current channel instead of in the entire frequency band in order to reduce the required sensing energy. The coordinator scans for the interference level of the remaining channels only when the interference level of its current channel is greater than a threshold value. Not only can increasing the transmission power enhance SNR, it can also reduce retransmissions. As shown in Figure 4, transmission powers (TxPowers) _P1_ and _P2_ (_P1_ \(<\)_P2_) and interference threshold values _Thres1_ and _Thres2_ are specified in the transceiver at each node. The transceiver of each sensor node will work at its default transmission power _P1_ if the RSSI of the current working channel is less than _Thres1_. When the RSSI of the current channel is greater than _Thres1_, but less than _Thres2_, TxPower increases to _P2_. Once _P2_ is used, the coordinator will adaptively change its scanning cycle by _Trescan cycle_ seconds to rescan the channel. If the RSSI of the current channel is less than _Thres1_, the network will return to using _P1_. The coordinator scans the remaining channels to build the interference map when the RSSI is higher than _Thres1_. If the RSSI of the current working channel is greater than _Thres2_, the coordinator also scans the remaining channels to build the interference map. The WBASN will switch to the channel with the lowest interference level without changing transmission power. The coordinator utilizes the wakeup beacon message to broadcast TxPower information. The sensor nodes extract the bit from the beacon message. If the bit is zero, _P1_ is used; otherwise, _P2_ is used. Figure 3: Conceptual operation in cognitive-receiver initiated cycled receiver (C-RICER). When the interference level of the current working channel is greater than _Thres2_, the WBASN decides to switch to a new channel. The C-RICER coordinator sends the channel switching information to the sensor nodes. Two transceivers are used at each node: one for data transfer and the other for channel information [18]. The wakeup function of the sensor nodes depends on the demand to send data. Sensor nodes with different sensing functions will wake up randomly and will not receive the switching information simultaneously from a coordinator. A checklist table is used by the coordinator to gather information regarding sensor nodes that successfully received the switching information. Figure 5 shows the channel switching algorithm. Once a channel switching decision is made, the index of the channel with the lowest RSSI is attached to the channel switching message. The coordinator periodically broadcasts the channel switching message and waits for the ACK message from the sensor nodes. The channel switching checklist table of the coordinator is filled with successfully received ACK messages from a sensor node. If there is a data packet to transmit, the sensor nodes will wake up and wait for the wakeup beacon form the coordinator. Figure 4: Power adaption strategy in C-RICER. RSSI, received signal strength indicator; TxPower, transmission power. Figure 5: Channel switching in C-RICER.