## 1 Introduction  
In recent years, rapid advancements in micro-electro-mechanical systems (MEMS) [1] have inspired the adoption of collaborative sensor networking for the acquisition of health related information. This type of networking is known as wireless body area networks (WBANs) or simply body sensor networks (BSNs) within the wireless research community. Low-power and inexpensive sensors with low computation and communication capacity are widely used in WBANs. The basic applications of WBANs include long-term health-care, patient monitoring in a hospital environment, collection of health-related information for workers in hostile and life-threatening areas (e.g., soldiers, deep-sea and space explorers, _etc._) [2]. It is noticeable that the WBAN applications necessitate both low-rate periodic monitoring (_i.e._, observation) data and high-rate traffic including event-triggered bursts. Therefore, communication protocols for WBANs should allow for dynamic operation in response to the such fluctuations and variations in application traffic. In a typical WBAN, both implanted and body-attached physical sensors are deployed inside and outside of the human body that collaboratively monitor a user's health condition and transmit the observed data to a powerful external gateway such as a PDA, smart phone, smart watch, _etc._, in order to facilitate timely and effective action. The collaborative WBAN model could have either single-hop star topology [3] or multi-hop tree and mesh topologies [4; 5; 6; 7; 8] which are rooted at the gateway. However, the medium access control (MAC) operations of the nodes in such WBAN topologies are very challenging for several reasons. First, the resource-constrained sensors have difficulty in dealing with the considerable variation in the data generation rate due to the scarcity of node energy and computation power. For example, in addition to low observation traffic, few low-rate physical sensors used for monitoring heart beat, blood pressure, glucose level, body temperature, _etc._, often generate time-critical emergency data. Furthermore, the sensor used for real-time streaming of ECG signals always generates data at a high rate. Second, some low-rate applications require energy conservation whereas the event-triggered and high-rate applications demand high-throughput, and minimal delay is expected in both cases. Hence, energy conservation should be the primary concern during low-traffic periods, whereas a balance between satisfying high-throughput demand and efficient energy usage is necessary during high-traffic times. Finally, the limited channel capacity and internal and external interferences in a dynamic wireless environment impose an unexpected and excessive burden on the execution of MAC operations. Therefore, a WBAN medium access control protocol needs to be designed in a scalable and dynamic fashion in order to simultaneously achieve _better energy-efficiency_ and _high throughput_ while _minimizing the delay_. The existing efficient MAC protocols are broadly categorized in [9] as _low power listening_ (LPL), _contention-based_ and _time division multiple access_ (TDMA). In LPL protocols [10; 11], a receiver periodically wakes up from inactive (sleep) mode and checks for a potential preamble signal from the sender. If the preamble is available then receiver stays awake for data reception, otherwise it goes back to sleep. The LPL protocols are ineffective for WBANs since the preamble transmission and periodic sampling are not suitable for energy constraint sensors. However, the receiver-initiated concepts [12; 13] have gained a favorable reputation because it facilitates energy conservation by minimizing the medium occupancy time of the sender's preamble. In a contention-based protocol such as CSMA/CA, the mainsources of energy inefficiency are idle listening, overhearing, collision and protocol overhead [14].