## 3 MAC Protocols for CRBANs
### Asynchronous MAC Protocol for Spectrum Agility  
Energy consumption plays a major role in the design of wireless body area sensor networks (WBASNs). The wireless spectrum in ISM bands becomes crowded and causes coexisting interference. High coexisting interference in networks causes high energy consumption, increased packet delay and low network throughput. An energy-efficient MAC protocol, the cognitive-receiver initiated cycled receiver (C-RICER), was designed for WBASNs in high interference environments [17]. C-RICER adjusts both channel frequency and transmission power to reduce interference and energy consumption. The main purpose of applying CR is to maintain the SNR for data exchange and to overcome the disadvantages of high coexisting interference in WBANs. By dynamically scanning and switching channels, WBANs can avoid coexisting interference, save energy and reduce packet drops and packet delays. The energy required for the cognitive task of C-RICER is mainly consumed at the coordinator. The spectrum agility function of C-RICER is considered to be a very important characteristic of WBANs. The use of a transceiver-per-sensor node with energy constraints renders the design of C-RICER more challenging and different from traditional cognitive MAC protocols. Traditional MAC protocols are divided into two families depending on how a node gains medium access: contention-based and reservation-based protocols. The contention-based MAC is more suitable for WSNs, because of limits on traffic and energy [18]. Receiver-initiated protocols in the contention-based family seem more suited to the context of WBANs. A well-known protocol in this family is the RICER protocol; RICER3b and RICER5b are variants of RICER according to the number of required handshakes. In the RICER3b protocol, the destination node with no data packet to transmit wakes up periodically and transmits a short wakeup beacon to indicate that it is awake. Having transmitted the beacon, it monitors the channel for a response; if there is no response, the node goes back to sleep mode. A source node with data to transmit stays awake, monitors the channel and awaits a wakeup beacon from the destination nodes. The use of beacons in RICER3b is different than that in ZigBee [19]. In ZigBee, the beacons are used to synchronize the superframes of nodes. The data are transmitted using the carrier sense multiple access with collision avoidance (CSMA-CA) algorithm based on an appropriate back-off timer. However, in the case of RICER3b, having received a wakeup beacon, the source node instantly sends back a buzz signal and starts transmitting data. Having received the buzz signal, the destination stays awake and waits for data from the source node. The main objective of C-RICER is early detection of interference in its working channel. Channel sensing is periodically performed at the coordinator. According to the detected level of interference, the WBAN can adaptively switch to the channel with the lowest interference to prevent coexisting interference. The conceptual operation in C-RICER is shown in Figure 3. In C-RICER, sensing and switching is continuously performed during the operation. Channel inferences can be unpredictable, which prompts frequent channel switching and costs a considerable amount of energy. To address this, C-RICER uses power adaption methods prior to considering channel adaption. The working mechanism of C-RICER involves two tasks: a data exchange task and a cognitive task. The data exchange task is used for data collection, and the cognitive task is used to dynamically adapt both the transmission power and the channel according to the interference levels in question. Data communication in C-RICER is based on RICER3b, with the destination node as the coordinator and the source as the sensor node. The coordinator wakes up and sends a wakeup beacon. The sensor node receives the beacon and transmits a buzz signal to caution the coordinator to wait for data. Having received the buzz signal, the coordinator waits for data.