## 1 Introduction  
A Wireless Sensor Network (WSN) refers to a distributed network, consisting of dispersed and autonomous sensing stations. Each sensing station--also known as a sensor node--consists of a microcomputer (computing component), transceiver (communication component), a power source (normally a battery), and some sensor(s) depending upon the application area. Some smart sensors are equipped with an actuator [1]--an electro-mechanical device used to control different components of the system. These nodes self-organize themselves upon deployment and form a network, which is typically comprised of several to thousands of such sensor nodes. Upon forming the network, sensor nodes sense, measure and gather information from the surrounding environment for some activity and report the sensed data to a special station called base station or sink in a multi-hop fashion.  
WSNs can be deployed on a large or small scale (depending on the requirements and applications) to detect and collect the required information from the surrounding environment. They can be deployed either in a pre-planned manner or on random basis. WSNs have the ability to change people's lifestyle [2]. They can be used for different applications [3], such as environmental monitoring and seismic sensing [4], natural disaster relief [5], bio-medical health monitoring [6; 7; 8; 9], and military target tracking, national defense and surveillance [10; 11].  
According to Department of Economic and Social Affairs of United Nations Secretariat [12], by year 2025, people of 65+ age will be account for about 15% of the overall worldwide population, which will be nearly 761 millions [13]. In developed countries the elderly population will be high, _i.e._, 20% [14] as compared to developing and under-developed countries. Since the elderly aged people are more vulnerable to different health issues and diseases, they require frequent medical check-up, which results in high healthcare costs [15; 16]. These statistics demand major changes towards proactive management of these issues by focusing on the prevention and early detection and treatment of different diseases [17].  
Wireless Body Sensor Networks (WBSNs) are a subset of wireless sensor networks, which can offer this paradigm shift and can be used for early detection of the different diseases. They can collect and analyze the vital sign-related data of patients by deploying different types of bio-medical sensors (for example: body temperature, heartbeat, blood pressure, electrocardiogram (ECG), electro encephalogram (EEG), _etc._ sensors) for a long period of time, thus reducing the healthcare costs. The bio-medical sensor node can either be suitably placed on the body or implanted inside the body. These bio-medical sensor nodes send the sensed information to a coordinator (base station), located on or near the body. The coordinator (base station) is responsible for forwarding the collected information to the sink node. The sink node will send the received data to the health care center or any other destination.  
In this paper a comprehensive study of the existing data routing approaches proposed during the last decade is provided, along with a critical analysis of each protocol. Section 2 covers the general architecture of the wireless body sensor networks, while in Section 3 the different routing issues and challenges of WBSNs are discussed. Based on the nature and structure of the existing routing protocols, they are classified into different classes and discussed in Section 4. Finally, this paper is concluded in Section 5.