# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### IEEE 802.15.4
Thus, some limitations have been addressed and the most important ones are the unbounded delay, low communication efficiency, low interference robustness, and/or fading and main powered relay nodes [39; 40; 41; 42; 43]. The IEEE 802.15.4 Task Group 4e was chartered to define a MAC amendment to the standard 802.15.4-2006 in order to evolve and add important functions to the 802.15.4-206 MAC protocol to enhance MAC to PHY functionalities interaction [44]. IEEE 802.15.4e supports five new categories of MAC enhancements also called MAC behaviors: Time Slotted Channel Hopping (TSCH), Deterministic and Synchronous Multi-channel Extension (DSME), Low Latency Deterministic Network (LLDN), Asynchronous Multi-channel Adaptation (AMCA), and Radio Frequency Blink (BLINK) [45]. Some general enhancements were also included as follows: Low Energy (LE), Information Elements (IE), Enhanced Beacons (EB), Multipurpose Frame, MAC Performance Metrics, and Fast Association (FastA) mechanism [46]. Figure 11 compares IEEE 802.15.4 stack with the OSI reference model. Figure 11: IEEE 802.15.4 compared with OSI reference model. Figure 10: IEEE 802.15.4 maximum transmission power levels according to regions. The use of multipurpose sensor networks becomes as a natural network environment with a focus on how to reduce the implementation of the operation costs through the reuse of resources. Ordinary traffic generated by regular applications such as environmental monitoring will not require the same treatment as the traffic response required for applications that use information queries time sensitive [47] characteristics. A heterogeneous traffic coexistence, with different QoS (Quality of Service) requirements is not handled well by IEEE 802.15.4. One option to improve QoS is the adoption of multiple transmission queues in both 802.15.4 and 802.15.4e considering the urgency level of the different traffic. This process separates the different QoS traffic in different classes and handles them on four transport queues with differential treatments [36; 48]. Mobility can be treatable by IEEE 802.15.4 but can hardly degrade the network performance [49] and happens when an orphan device that loses its coordinator association attempts to synchronize with network coordinators. The IEEE 802.15.4 terminal uses the common transmission channel to search for coordinators. On the topology aspect, the coordinators that belong to the same Personal Area Network Identification (PANId) are connected to a Super Coordinator that can handle the device mobility from one coordinator to another. It is important to point out that, during this re-association, the service data are interrupted. Some authors enlarged their visions beyond IEEE 802.15.4 limits suggesting inter-technology mobility. For example, to allow a mobile node to move through the cells in a various cluster tree network. Coordinators have both IEEE 802.15.4 wireless and wired connections\" [50] to expand the mobility domain. Energy performance is boosted by channel hopping based on multi-channel support. The development of IEEE 802.15.4 is a joint work between IEEE and ZigBee Alliance. The stack consists of layers one and two of the IEEE 802.15.4 standard as the basis for other protocols such as ZigBee itself, 6 LoWPAN, Thread, and ISA100 [51; 52]. Due the fact that these protocols are derived from the IEEE 802.15.4 PHY and MAC layers, they belong to the network layer and are out of context for this comparison.