## 1 Introduction  
The rapid development of electronic technology and wireless communication enables many kinds of medical sensor devices, which are designed for monitoring the human body. This paper focuses on swallowable medical devices like wireless capsule endoscopes (WCEs) [1]. They are swallowed by patients and go through the gastrointestinal (GI) tract to collect the vital signs of the human body, and send them to a data receiver which is attached onto the abdomen of the human body [2; 3]. It can be imagined that more than one medical device can be swallowed and they cooperate to finish some common task, like image segmentation capturing, long term and short term monitoring, _etc._[4]. In these scenarios, the sensors and the data receiver form a kind of body sensor network (BSN) [5]. Figure 1 shows a simple architecture of this swallowable BSN. Several sensor devices are moving or staying in the GI tract. They collect the sensing data and send them to the coordinator in a single-hop or multi-hop way. The coordinator then transfers the data to a nearby or remote monitor for diagnosis by doctors. Energy efficiency is the major concern during the design of BSNs. Normally the sensors are powered by batteries and the batteries cannot be replaced if they are inside the human body. The demand for higher resolution and higher sampling frequency of the sensing data makes sensor batteries a big challenge. On the other hand, the path-loss of the body tissue is much larger than through the air, therefore the sensors need larger transmission power to send data. It is well known that the medium access control (MAC) protocol plays an important role in energy management. It can effectively reduce the energy consumption of sensor nodes and prolong the network lifetime. One can focus on the MAC protocol design of BSNs to improve the energy efficiency. The BSN domain can benefit considerably from the appropriate modification and application of modern advances in protocols, techniques and methodologies which have recently been proposed for WSNs pursuing performance improvement in energy efficient routing, QoS, congestion control, congestion avoidance, connectivity and coverage as described in [6; 7; 8; 9; 10; 11]. Many energy efficient MAC protocols have already been proposed for WSNs [12; 13; 14; 15], however, BSNs have their own attributes which make them different from WSNs [16]. Firstly, WSNs  
Figure 1: A simple architecture of body sensor networks. cover a monitoring environment of meters or kilometers, while the scale of BSNs is only as large as the human body (centimeters). The feature makes the coordinator easily reach every sensor node directly. Secondly, WSNs have greater numbers of nodes, ranging from tens to thousands. BSNs have normally less than 10 sensor nodes. Thirdly, BSNs have more energy and processing resource constraints than WSNs. Compared with general BSNs, swallowable BSNs have two main specific features. One is that all the communications are _in-vivo_. The path-loss through body tissue is different from through the air. The other one is that some of the sensor nodes are mobile. The path-loss between sensor nodes varies. The routing may change. It is necessary to consider the above issue and define the specific MAC protocols for specific BSNs. Several energy efficient MAC protocols for BSNs have been proposed by researchers as surveyed in [17, 18, 19]. [PERSON] _et al._, proposed a traffic-aware dynamic MAC (TAD-MAC) which is considered as an addition in the class of preamble sampling MAC protocols [20]. Every node adapts its wake up interval dynamically with the amount of traffic it receives and consequently optimizes the energy consumption. A traffic status register bank which contains the traffic statistics is used to continuously update the wakeup interval of the receive node with respect to the data transmission rate of transmit nodes. However, this preamble-based protocol is not suitable for BSNs since in most of the applications of BSNs sensors transmit data outside the human body.