## 4 The WBANs Communication Stack
### Medium Access Control Layer
#### 4.2.3 Existing State-of-the-Art on the MAC Protocols  
Medium access is based on either TDMA or CSMA mechanisms. Table 8 presents a brief overview with reference to the performance of these mechanisms. Different performance metrics are presented and it can be seen that both mechanisms are effective based on the specific constraints. Most of the existing MAC protocols consider energy efficiency and delay as the prime objectives for most of the WBAN-based applications. However, in this section we will also explore more design constraints through the various studies that exist in the literature about the MAC protocols for WBAN. We have classified various MAC protocols in different categories. It includes IEEE 802.15.4-based general and WBAN-specific MAC protocols, TDMA-based low duty cycle, hybrid and cross-layer protocols, traffic-aware adaptive protocols and IEEE 802.15.6-based WBAN MAC protocols, below we will explore detailed insight of each category. _IEEE 802.15.4-based Protocols_: IEEE 802.15.4 is a low-power, low data rate standard developed for typical wireless sensor network-based applications. Initially it also has been the main focus of researchers in the context of WBAN. Below we will present a brief survey of the IEEE 802.15.4-based WBAN MAC protocols. The performance in terms of energy efficiency of a non-beacon IEEE 802.15.4 was investigated in [49]. The authors concluded that the non-beacon IEEE 802.15.4 results in 10 to 15 years sensor lifetime for low data rate and asymmetric WBAN traffic. However, their work considered data transmission based on periodic intervals, which is not a perfect scenario in a real WBAN. In [50], three different access schemes are evaluated through several metrics. Considering the coexistence of contention access period (CAP) and contention free period (CFP), authors also study the mutual influences of these two traffics. The results show that the unslotted mode has better performance than the slotted one in terms of throughput and latency but with the cost of more power consumption. Further, an improved IEEE 802.15.4 MAC protocol is present in [51], with an adjustment of the beacon traffic according to packet arrival rate. The presented scheme performs better in terms of energy consumption with regards to classical IEEE 802.15.4. Performance analysis of IEEE 802.15.4 and IEEE 802.11e for WBANs applications is presented in [52]. This paper studied the energy efficiency and QoS performance of IEEE 802.15.4 and IEEE 802.11e MAC protocols for WBANs-based applications. Authors simulated a WBAN, as well as co-existence scenarios where the body sensors operate in the presence of voice, video, and IT traffic. Their results indicate that although IEEE 802.15.4 and IEEE 802.11 e can provide an acceptable compromise between power consumption and QoS in some scenarios, there are situations (e.g., co-existence with video and heavy data traffic) in which both performance criteria cannot be met simultaneously. This highlights the need for improving existing MAC protocols or designing new solutions that can provide both extremely low power and QoS for WBANs. Both [52,53] concluded that although 802.15.4 can provide the required QoS for BAN, however, the technology is not adaptable, scalable and power efficient to meet the requirements of various WBAN applications. It can be considered for quick and easy implementation, but the results are poor because IEEE 802.15.4 was not designed for WBANs. Therefore, a specialized MAC protocol is needed to meet all the requirements. _IEEE 802.15.4-based WBAN-Specific MAC Protocols:_ The authors in [54] proposed an energy efficient BodyMAC protocol. It uses flexible bandwidth allocation to improve node energy efficiency by reducing the possibility of packet collisions and by reducing radio transmission times, idle listening, and control packets overhead. BodyMAC is based on a Downlink and Uplink scheme in which the Contention Free Part in the Uplink sub-frame is completely collision free.