## 1 Overview of BSNs
### BSN Requirements  
BSNs have tremendous potential to transform how people interact with and benefit from information technology, but their practical adoption must overcome formidable technical and social challenges (e.g., form factor, battery life, reliability, safety, security, privacy, interoperability, ease of use, _etc._). These challenges have far-reaching implications but offer many immediate opportunities for  
Figure 1: TEMPO packaged (cover removed) and wrist mounted. system design and implementation [5]. Although BSNs share many of these challenges and opportunities with general wireless sensor networks (WSNs)--and can therefore build off the body of knowledge associated with them--many BSN-specific research and design questions have emerged that require new lines of inquiry. Unlike generic WSNs that have many nodes doing the same thing, BSNs are likely to have a small number (\(<\)10) of nodes with each node dedicated to a specific task. For example, a sensor node monitoring acceleration at the ankle for gait analysis clearly cannot also measure brainwaves using an EEG since both the location and sensing hardware are so different. To achieve widespread adoption, BSN nodes must be extremely noninvasive, which means that the nodes must have a small form factor that is not overly inconvenient to use. Smaller nodes imply smaller batteries, creating strict tradeoffs between BSN node energy consumption and the fidelity, throughput, and latency requirements of BSN applications. Therefore, while the diverse BSN application space results in wide ranging system requirements, all BSN applications--whether real-time or delay insensitive, continuous high data rate streaming or infrequent small packet bursts, _etc._--demand energy efficiency while meeting data fidelity requirements. The battery size _versus_ battery life tradeoff plays a major role in defining any BSN system, and applying design techniques to reduce energy consumption can improve both size and lifetime. If energy consumption can be reduced far enough, perpetual operation on harvested energy becomes a possibility. Thus, BSN node sensing, processing, storage, and wireless transmission must all be done in a way that reliably delivers the important data but with the lowest possible energy consumption, thus minimizing battery size (which dominates BSN node form factor) and maximizing time between battery recharge (which is a key factor in wearability), both of which can impact the performance and practicality of possible applications. The best approach for optimizing the tradeoff between energy consumption and other requirements varies depending on the specific BSN application. To illustrate this, we can consider the tradeoff between battery lifetime (e.g., the application requirement for how long the system must work between re-charges) and effective wireless communication data rate (e.g., the average rate at which data from the node must reach the base station) across different applications. Figure 2 estimates how different applications map to this tradeoff space. Some applications like pulse oximetry (measures saturation of peripheral oxygen, SPO\({}_{2}\)), ambulatory blood pressure (ABP), or electromyography (EMG) for muscle activity require monitoring lifetimes between an hour (e.g., in the clinic) and a day or two (assuming sensors can be recharged at night), but the quantity of data that must be transferred varies dramatically. Continuous glucose monitoring (CGM) sensors may need to have lifetimes approaching a month, but they do not need to send much data on average. Some RFID-like sensors may only need to work for a second after being queried by a base station acting as a reader, but some long term sensors implanted in the body or incorporated into clothing may need to last for years. This great variety in requirements defies a single solution to solve the energy constraint problem.