## 3 General Strategies for Energy Efficient BSN Hardware
### Communication versus _Computation_
Therefore, it is worthwhile to understand the requirements for BSN radios. BSNs have slightly different radio requirements compared to radios in typical WSNs with low data rate communication for monitoring environmental conditions. For BSN applications, the network is probably arranged as a star-hub topology with the hub acting as a base station [1]. All communication from the nodes is to the hub, which can be assumed to have substantially more resources than the nodes (e.g., the hub may be a smart phone). Therefore, the nodes need a small, low power, and short range (1-2 m) radio. This means that a BSN radio could be optimized to operate at a much lower transmission power compared with radios designed for WSN applications. Judging from Figure 2, the ability to accommodate variable communication data rates will be important. Also, since we have observed that processing to reduce the communication data rate allows us to save power by using the radio less often, the radio should save energy when operating at lower data rates or should allow energy efficient transitions to and from active mode. We note that turning off the radio for longer times creates the need to re-synchronize the radio when it turns back on. Since the hub has more resources than the nodes, it can remain active permanently to listen for communication from the nodes. Most radio traffic in a BSN system is from the nodes to the hub, alleviating the need for the nodes to run a receiver continuously looking for messages from the hub. However, the cost of synchronization may still be significant depending on the specific BSN application and communication protocol. BSN applications can span a large range of data rates from a few bits per minute to almost 1 Mbps depending on the application [5]. Currently, there are a few low power radio and radio protocols such as ANT and Zigbee that are commonly used in wireless sensor networks. However, these radios and protocols can only operate with data rates of 10 kbps and 150 kbps, respectively. This severely limits their usefulness for the upper range of BSN applications such as motion assessment, ECG (electrocardiogram), EMG (electromyogram), and EEG (electroencephalogram). Conversely, the high data rate COTS radios and protocols, such as Wi-Fi have data rates which easily cover the entire span of BSN applications. However, these radios and protocols consume so much energy that they are impractical for use on BSNs with longer battery life requirements. Bluetooth is a radio and protocol that sits somewhere in between high data rate and low data rate radios. The protocol uses a large amount of energy due to the fact that it was designed as a very general purpose radio for applications spanning outside of the area of BSN. Bluetooth is convenient for BSN development platform purposes because of its widespread adoption, but its relatively poor energy efficiency leaves room for optimization with custom radios and protocols. Nevertheless, Bluetooth is a convenient and viable option for lower lifetime BSN applications. Due to inefficiencies in existing radios and protocols, there are other protocols that are being developed to accommodate the area of BSNs such as 802.15.6. This protocol specifically targets body sensor devices and the medical applications that can span a wide range of data rates [12].This new protocol supports data rates greater than 850 kbps and allows flexibility for the PHY layers supporting ultra wide band, narrow band, medical implant communication bands, and human body communication PHY. While this new protocol is not a standard yet and is still in working group, it is being designed to be more efficient for BSNs compared to existing options, and their main challenges will be providing efficient access for the broad range of BSN applications (see Figure 2) and achieving the pervasiveness that Bluetooth and Wi-Fi have achieved in smart phones and other personal computing devices.