## 2 Survey of Existing HCWSNs  
This section provides a comprehensive literature survey of some previously proposed HCWSNs. Specifically, we survey the CodeBlue [13], MEDiSN [19], and MASN [20] HCWSNs because they appear as the most cited projects in the literature, and with sufficient body of knowledge on the five key system architecture requirements that form the central theme of this paper. _CodeBlue_[13]: CodeBlue is a prototype health care wireless sensor network that defines an architecture for hardware and a framework for software. The hardware architecture design allows for the integration of a pulse oximeter, electrocardiogram (ECG), and motion analysis sensor board onto the MicaZ [17] and Telos [18] motes. The software framework provides protocols for device discovery, a publish and subscribe routing layer, and a simple query interface that allows caregivers to request data from groups of patients [13]. Figure 1 provides an illustration of the CodeBlue architecture and how it operates. In Figure 1, each patient is equipped with a sensor more that is used to monitor health status. A simple device discovery protocol is employed so that the motes in the network can discover each other. Specifically, each node periodically publishes its node ID and the sensor types it supports to a specific broadcast channel using a broadcast beacon. End-user devices, such as personal data assistant (PDA) devices operated by medical professionals (e.g., doctors or nurses), issue network wide queriesto request information of patients that are monitored by a group of sensor motes that possess specific biomedical sensing capabilities. In order to facilitate a query and response communication process, the TinyADMR multicast routing protocol is used to establish multicast routes between the data publishing sensors and the end-user devices that subscribe interest to that data. The main purpose of TinyADMR is to deliver queries and responses under the effect of node mobility, multiple simultaneous paths, and an error-prone communication channel. For the querying mechanism, an interface is designed to allow a receiving device to request data from specific biomedical sensors based on their physical node address, sensor type, and whether or not it meets the requirements of a specified filter. CodeBlue is also equipped with a RF-based localization system that is used to track the location of patients and caregivers, a capability that is especially valuable in large hospital settings. _MEDiSN_[19]: The Medical Emergency Detection in Sensor Networks (MEDiSN) project utilizes a wireless sensor network composed of a network gateway, physiological monitors (PMs), and relay points (RPs), to monitor the health and transmit physiological data of patients. Figure 2 provides an illustrative overview of the MEDiSN architecture and how the various components (e.g., PMs, RPs, _etc._) operate. The PMs are sensor devices which collect, encrypt and sign patients' physiological data (e.g., blood oxygen level, pulse, ECG, _etc._) before transmitting them to a network of relay points that eventually forwards the data to the network gateway. The RPs self organize into a routing tree that facilitates the reliable delivery of periodic data and alerts from the PM to the network gateway, and also from the network gateway to individual PMs. The data received by the network gateway is stored persistently at a backend server, where clients can use a graphical user interface (GUI) to access the data through different queries. Unlike CodeBlue in which the PMs generate and forward data, only the RPs in MEDiSN are responsible for relaying data (_i.e._, PMs in MEDiSN only generate data and are not  
Figure 1: Overview of CodeBlue architecture and operation. involved with data forwarding). As a result, the RPs can use hop-by-hop retransmissions to assure the reliable delivery of bidirectional data traffic that is prone to packet collision and corruption.