## 3 Problem Description and Motivation  
As of now, only the single-hop based star topology is considered in existing WBAN MAC protocols, such as those described in [3, 17] and IEEE 802.15.4. However, the sensors in this type of topology are bounded to make long range direct transmissions to the gateway, thereby consuming more energy. As the sensors are typically deployed throughout the body, the distance between each sensor and the gateway varies significantly. It is obvious and usual that the amount of energy required for data transmission increases as the distance between each sensor and the gateway increases [4, 8], since transmission energy consumption is directly proportional to the distance. In contrast, a multi-hop communication pattern can avoid such long range transmission oriented energy loss and potentially can be implemented in WBAN arena for energy efficiency. It is noteworthy that consideration of multi-hop communication is still unrevealed in designing the WBAN MAC. However, the protocols those discussed in [4, 7, 8] can potentially form a multi-hop clustered communication model for WBAN. Furthermore, the recent studies in [6, 8] justify the multi-hop communication model for WBAN with the idea of \"intra-BAN communication\" comprising of _communications between body sensors_ and _communications between sensor and gateway_. Motivating by these observations, in this study we are encouraged to implement the multi-hop clustered communication pattern for a WBAN MAC protocol. The shortcomings of the IEEE 802.15.4 protocol are evident in the recently developed TDMA-based LDTA-MAC protocol [3]. As shown in Figure 1, the superframe structure of LDTA-MAC includes a fixed contention access period (CAP), a time-slotted fixed contention free period (CFP) along with a dynamic extended CFP and an inactive (sleep) period. From the gateway node, a _beacon_ at the beginning of each superframe synchronizes the other nodes and a _notification_ after the first CFP allocates the time-slots of the extended CFP (_i.e._, upon receipt of the CFP slot request during CAP by the sensors). Although the LDTA-MAC protocol provisions for the quick transmission of emergency traffic, the given approach seems to be inefficient in several aspects, particularly in terms of adaptability to the changes in the traffic load. During the CAP in LDTA-MAC, sensors are allowed to send the data along with the CFP slot request to the gateway and maintain the inactive period. However, since the CAP is fixed for LDTA-MAC, this would lead to unfair and inappropriate allocation of the time slots of extended CFP during high traffic periods, because at such network instance some sensors might not be able to send the CFP slot request due to the heavy traffic oriented contentions. Hence, LDTA-MAC exhibits low throughput and low channel utilization when the traffic load is high. Additionally, when the traffic load is low, the fixed CAP remains under-utilized unless it is appropriately set to be a very short period prior to network setup, which is logically not feasible. Moreover, if the CAP is set to be moderately long, this could lead to wasted energy at the gateway due to unnecessary medium listening under low traffic condition. Finally, as mentioned above, the LDTA-MAC is limited to perform in the single-hop star topology, experiencing more energy cost for long range direct transmissions [4, 8]. The shortcomings of LDTA-MAC highlight the fact that the superframe components of the IEEE 802.15.4 MAC protocol should be adjusted to operate adaptively according to the traffic load characteristics and network demands (e.g., energy efficiency, capacity utilization, _etc._). These observations on existing WBAN MAC protocols have encouraged us to design the ATLAS protocol as a more generalized and diversified traffic load adaptive MAC solution for WBAN. This protocol is geared toward minimizing energy consumption during low-rate traffic and maximizing the data throughput during high-rate traffic condition in WBAN.