## 4 WBANs: The Key M2M Area Network Technology for mHealth Applications
### MAC Layer Design for WBANs
#### 4.4.1 Energy-Efficient Channel Access and Synchronization Mechanisms
The uplink is split into a Contention Access Part (CAP), during which the nodes can access the channel through CSMA/CA rules, and a Contention-Free Part (CFP), during which time slots are allocated dynamically to the nodes by the coordinator. In order to gain slot allocation, the nodes must notify their bandwidth requirements to the coordinator during the CAP period. Then, different bandwidth allocation schemes can be implemented for periodic or temporal slot assignments. Sleep mode is supported through a three-step handshake, initiated by the transmission of a sleep request packet by the node during the CAP. The coordinator acknowledges reception with an ACK packet and transmits a sleep mode schedule in the downlink period of the following superframe, containing the necessary sleep parameters. A different implementation of the same principle is adopted in Distributed Queuing Body Area Network (DQBAN) [83]. The superframe structure in DQBAN is again divided into three parts: (i) the access part, further divided into a number of access minislots; (ii) the scheduling part, further dividing into a number of scheduling minislots; and (iii) the data part, in which collision-free data transmission takes place. Nodes compete for channel access by transmitting small control packets in randomly selected access minislots, and a collision resolution algorithm is employed to resolve any collisions among access requests. After a successful access request, nodes enter a transmission queue and are enabled to transmit their packet in the data part in a collision-free manner. Finally, an energy-aware hybrid access scheme that exploits energy harvesting at the wireless nodes has been presented in [84, 85]. The proposed scheme, HEH-BMAC combines two different access methods: (i) a reserved, collision-free polling access phase for nodes with high energy level and more urgent data; and (ii) a probabilistic random access phase for nodes with low energy level. The key novelty of this scheme is that the duration of the two phases is dynamically determined, taking into account the energy level of the body nodes. Preamble sampling:Another class of protocols adopts a technique often referred to as preamble sampling [76]. The main concept is that the nodes follow independent sleep patterns, spending most of their time asleep, waking up only for short periods of time. While they are awake, they listen to the channel for any beacons transmitted by the coordinator. If a beacon is received, they proceed to transmit their data, otherwise they return to sleep mode. Energy consumption can be reduced by minimizing the idle listening time (_i.e._, the time spent for waiting to receive a beacon). This can be accomplished by adapting the beacon transmission intervals to the sleep pattern of the nodes. A mechanism to deal with this issue is based on the negotiation of sleep pattern between the coordinator and the nodes. In [86], for example, a master-slave architecture is assumed and the master keeps track of the sleeping pattern of each slave node. The slave remains in a sleep mode until the predefined sleep time elapses, and then wakes up and waits to be polled by the master. A new sleep time is then defined and the process is repeated. In Traffic-adaptive MAC (TaMAC) [87], the coordinator determines the wake-up patterns of the nodes and stores them in a table. By consulting this table, the coordinator can adjust its polling scheme accordingly and transmit beacons when a node is expected to wake up, thus minimizing idle listening times. In the previous schemes, the node wakeup times are known to the coordinator. Another approach has been adopted in Traffic-Aware Dynamic MAC (TAD-MAC) [88], in which the coordinator tries to learn the sleeping pattern of the nodes. TAD-MAC operation takes place in two phases. During the evolution phase, the coordinator polls the nodes at different times to see if they have packets to transmit, thus gathering statistics on their traffic load that are stored in a register bank.