## 4 Network Communication
### Channel Characterization
#### 4.1.4 MAC Layer  
The original purpose of the BSN MAC layer was to achieve maximum throughput, minimum delay, and to maximize network lifetime by controlling the main sources of energy waste, _i.e._, collisions, idle listening, overhearing, and packet cost. Generally, MAC protocols are grouped into contention protocols, schedule protocols, and contention-schedule protocols [31]. In contention MAC protocols, such as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocols, nodes contend for the channels to transmit data. If the channel is busy, nodes will defer their transmission until the channel becomes idle. These protocols are not demanding on strict time synchronization, but they cause extra cost. [PERSON] _et al._ studied the behavior of CSMA/CA protocol in BSNs and concluded that CSMA/CA protocol encounter serious collision problems for high traffic nodes [119]. In schedule protocols such as Time Division Multiple Access (TDMA) protocols, the channel is divided into several time slots. These slots are assigned to nodes and each node transmits during its corresponding slot. These protocols are energy conserving protocols. There is no contention, idle listening, and overhearing problems, but these protocols require frequent time synchronization which is somewhat difficult to achieve. An enhanced Distributed Queuing Medium Access Protocol (DQ MAC) based on TDMA has been proposed in Reference [102]. It proposes a remarkable improvement of overall network energy efficiency, which scales well for very dense BSNs and is particularly suitable in medical scenarios. H-MAC is also a TDMA-based MAC protocol designed for BSNs, which aims to improve energy efficiency by exploiting heartbeat rhythm information to perform time synchronization, unlike other general MAC protocols which achieve synchronization by sending beacons [104]. WhMAC, a signal- oriented TDMA-based protocol of low power, utilizes WBSN-specific features and a novel ultra-low power wake-up receiver, to achieve flexible wireless data transfer of physiological signals [120]. BodyMAC, another TDMA-based MAC protocol, with a flexible bandwidth allocation strategy, improves node energy efficiency by reducing radio transmission, idle listening, and control packets overhead times, to realize the smallest collision possibility [121]. Contention-schedule MAC protocols combine the characteristics of the former two mentioned above. The authors of [122] designed a traffic load aware sensor MAC for collaborative BSNs, called ATLAS. The protocol takes the superframe structure of 802.15.4, while it makes full use of its Contention Access Period (CAP), Contention Free Period (CFP), and Inactive Period (IP). The protocol can provide better energy efficiency, higher capacity-utilization, and less delay based on adaptive traffic load evaluation.  
In addition, part of the information of a BSN is correlated. For instance, a patient suffering from a fever triggers temperature, blood pressure, and respiration sensors at the same time [101]. The instant data increase may result in serious network congestion. In this case, a CSMA/CA protocol encounters serious collisions and extra energy consumption. Besides, nodes are required to perform Clear Channel Assessment (CCA) before transmission, which will make the situation worse, but TDMA-based protocols can provide good solutions to the issues mentioned above [31].