## 2 Related Works  
The continuous monitoring and analysis of vital signs is the key to detecting potential health risks in otherwise healthy-looking patients. There are presently several projects around the world whose goal is to monitor the patient health. The authors in [4] describe the BASUMA project, which focuses on developing a robust and energy efficient platform for human wireless body sensor networks to provide at-home monitoring of chronically ill patients. The initial goals of the BASUMA project are: to improve the treatment of obstructive pulmonary disease and provide support for female breast cancer patients undergoing chemotherapy. In [5], the author describes how to implement a personal sensornetwork to monitor patients and help provide health care. This project combines several intelligent sensors and an integrated control node that functions in conjunction with a Bluetooth network. In [6], the authors present a system based on wireless sensor network technology. This project describes an architecture composed of medical sensors incorporated around the human body using the ZigBee standard. The WHAM-Bios project in [7] proposes telemedicine applications to provide real-time emergency medical services. The WHAM-Bios project is based on a device the authors call \"Human Body Gateway,\" where the sensor nodes provide the information needed to produce instantaneous monitoring results. Real-time monitoring requires algorithms that facilitate contention-free communication in order to reduce the power needed to transmit data. The long-term health effects of the electromagnetic fields caused by in-home monitoring has been a cause of concern. The author in [8], focuses on the safety of body sensor networks and wireless communications in close and constant proximity to humans. In [9, 10], the objective is to incorporate technologies into clothes or common accessories (for example, watches, bracelets, _etc._) to measure, register and transmit different physiological parameters, including: heart rate, body temperature and movement. The authors in [11] describe a prototype that monitors diabetic patients and the authors in [12] describe another prototype of a retinal prosthesis, based on embedded implanted intelligent sensors. There are various projects that use 802.15.4 and ZigBee to transfer patient information. The authors in [13] present ZUPS. This is an ultrasound based position system that provides multi-cell coverage. The system uses ZigBee and ultrasound to measure distances between mobile devices carrying tags and beacons with known locations; however, it uses proximity and multi-lateration localization methods simultaneously. This combination reduces the infrastructure needs for the ultrasound system and also provides accurate information even in very short distances; this enables the system to provide guidance and spatial orientation training inside buildings for the elderly and people with disabilities. Projects presenting plural layer architecture have also been developed. In [14] a three-layer network structure for a pervasive medical supervision system is proposed. The first layer is the medical sensor network that provides oximetry, heart rate and blood pressure information, as well as contextual data such as temperature and the patient's video/picture in emergency situations. This network is configured in a star topology with a gateway node. The second layer provides a reliable transmission stream that permits data to be transmitted to the nearest wireless node located in the house. The transmission can then be relayed to a PC with an Internet connection inside the house. The third layer of the system is responsible for the compiling physiological data in a remote medical center for analysis to provide feedback to the patient by means of a standard mobile phone, PDA or web services. Some projects developed aim to significantly reduce long-term network power consumption for monitoring applications where there is no need to alert the patient or health care provider about a threatening event.