# Body Area Networks in Healthcare: A Brief State of the Art
## Data Link Layer Properties
### Sustainability  
Awareness of the environment is increasing in recent years. Big and small companies are developing environmental friendly products. Computing systems contribute to the environment with two aspects: (1) energy consumption and (2) use of renewable materials. Energy efficiency is an essential feature in BAN applications. Batteries used in these applications should not be replaced in many years. This goal can be achieved by using strategies for optimal energy consumption. Usually, communication is the action that drains more energy. Energy consumption can be optimized using energy efficient medium access protocols. These protocols can turn off the transmitter when it is not required. Another technique is allotting slots dynamically. If a transition fails, allocated slots prevent waste of energy and time. Energy for powering nodes in a BAN system can come from different sources. These sources can be solar power, pressure variation, vibrations, air flow, human power and temperature. A human produces body heat that is equivalent to a power density of 20 mW/cm\({}^{2}\)[47]. This energy can be gathered by thermoelectric generators. These generators can store energy in a battery which powers the node. The use of human body in BAN applications can produce more energy efficient solutions. It is also possible to use the human body as a communication channel, and some studies are being developed to improve this technology. There are different strategies for reducing the nodes' energy requirements:  
* Workload management ensures that the tasks are balanced among all nodes of the system. This prevents overloading states causing a higher power consumption. It is also possible to delay a work (not affecting the requirements) until the energy source is ready. * Power management helps to reduce energy consumption. This involves the administration of sleep and idle modes in the processor when it is not required. The same process can be applied to communication devices. * The energy demanded is typically due to non-computing processes. An efficient management of these processes can reduce significantly power requirements of the application. Preserving the environment is also a great concern. A good design is mandatory to reduce the pollution in BAN applications. Two strategies are used for protecting the environment when developing BAN solutions: (1) using eco-friendly materials that are recycled with a minimum impact and (2) designing durable nodes, avoiding partial or total replacement, thus reducing waste. The use of green energy for powering a BAN application may also lead to problems. Usually, the available energy is not sufficient to operate the devices. This problem is caused by unknown load characteristics. Sustainability research proposes three ways to overcome this problem [15]:  
1. The first one is to improve the efficiency of energy harvesting from the environment,
2. The second one is to adjust the energy source to the duty cycling of the devices,
3. Finally, the use of energy-efficient computing. The aim of these techniques is to achieve an energy balance. A system reaches energy neutrality when it consumes the same energy harvested. Harvesting theory is an analytic method that is used to solve this problem. This theory uses a model-based approach and stochastic models of the harvesting source and the device. The harvesting theory can be used to find the best duty cycle of the device while maintaining energy balance. Energy consideration is still a hot topic in BANs. A recent paper intends to find the optimal QoS and energy consumption to increase profits [48]. A proper design is fundamental to achieve this balance. The design methodology has three phases: (1) profiling phase, (2) modeling phase, and (3) analysis and design phase. MBE has similar phases and both methodologies can be implemented at the same time. Both methodologies require gathering the requirements for modeling the components interactions and the evaluation of the expected results.