## 3 MAC Protocols for CRBANs
### CR-Based MAC Protocol for Cognitive Wireless Sensor Body Area Networking  
In the CR-based MAC (CR-MAC) protocol for wireless sensor body area networks [15], a separate type of sensor node is used for critical health information and non-critical health information. In this protocol, the network nodes are assumed to have the capability to dynamically adjust their transmission power according to the level of urgency of the traffic. The CR-MAC protocol regulates access by critical and non-critical packets to the transmission medium by transmitting packets with higher and lower transmission powers, respectively. The receiver circuit at the PAN coordination node enables the successful reception of critical packets with higher power, even though other, lower priority packets with a lower power are simultaneously received. In CR-MAC, three transmission power levels of _P1_, _P2_ and _P3_ are assigned to non-critical traffic, moderately urgent traffic and critical traffic, respectively, where _P1_ \(<\)_P2_ \(<\)_P3_. Critical traffic has the highest priority, and moderately urgent traffic has the second-highest. The successful transmission of a moderately urgent packet using power level P2 occurs as long as there are no transmitted packets of higher priority in the traffic. Non-critical traffic is subsequently transmitted using power level P1, and the successful transmission of packets requires only a single packet in a time slot. The CR-MAC protocol was analyzed mathematically for two transmission power levels in [15]. In the analysis, a star topology composed of a number of different sensor nodes and a single network coordinator was used. The different sensor nodes are composed of reduced function devices (RFDs), and the coordinator is a full function device (FFD). The RFD nodes are classified into critical and non-critical sensor nodes according to their assigned physiological variable of measurement, such as EEG, ECG, heartbeat, blood pressure, temperature, _etc._ The RFD nodes periodically report the measured physiological values to the FFD coordinator node, which transfers the data to the server through other networks, such as cellular systems or wireless local area networks (WLANs), where the data are monitored and analyzed. Critical traffic throughput is first analyzed because critical traffic transmission is independent of non-critical traffic transmission. The output is then applied to the analysis of non-critical traffic throughput, which depends on the transmission of critical traffic. As an essential parameter, the traffic rejection rate for critical and non-critical packets was also analyzed in [15]. The probability of critical traffic rejection depends on the number of allowed critical packet retransmissions. For the transmission of the first packet, the probability of packet transmission failure is \(\left(1-p_{c}\right),\) where \(p_{c}\) is the probability of a successful critical packet transmission in a particular time slot. The probability of critical packet transmission failure is \(\left(1-p_{c}\right)^{\tau_{c}+1},\) where \(\tau_{c}\) is the number of allowed packet retransmissions for critical nodes. In the CR-MAC protocol, packet collision scenarios are defined in three states, as shown in Figure 1:  
* S1: One critical packet arrival of transmission power level P2 and one or more non-critical packet arrivals of P1;
* S2: More than one non-critical packet arrival;
* S3: More than one critical packet arrival. In S1, the collision is partial due to the critical packet transmission being considered successful because of the cognitive radio transmission feature of CR-MAC. However, collisions occur between non-critical packets, as well. The states S2 and S3 involve the simultaneous arrival of more than one non-critical packet and more than one critical packet, respectively.