# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### IEEE 802.15.4  
IEEE 802.15.4 is a subgroup of features that refers to physical and medium access control layers that can support ZigBee and 6 LoWPAN upper. IEEE 802.15.4 focuses on physical and data link layer specifications while ZigBee Alliance aims to provide the upper characteristics [33]. It is a standard that defines PHY and MAC layers for personal area networks that demand low rate and low cost applications. This also called a LR-WPAN protocol and has some advantages. Among them are a simple and flexible protocol stack, low cost, low energy consumption, short-range operation, reliable data transfer, and ease of operation [34]. These features are more important when operating in the Personal Operating Space (POS) also defined as Personal Area Network (PAN) that involves the human body. An IEEE 802.15.4 device address can be a short 16-bit or 64-bit address [35]. In addition, IEEE 802.15.4 uses a Direct Sequence Spread Spectrum (DSSS) access mode and operates on 2450 MHz, 915 MHz, and 868 MHz ISM bands working with 16 channels, 10 channels, and one channel, respectively. The main limitations of the radio interfaces working in the ISM frequency band are the small sizes and the narrow bandwidths. Small sizes and narrow bandwidths consequently cause a reduction in output power transmission. With these radio interface characteristics, it is possible to obtain from 20 to 250 Kbps shared among all the nodes using the same channel [36; 37]. The physical layer provides an interface between the physical data service and the PHY management service accessed through service access points (SAPs). This layer is also responsible for activation, and deactivation of the radio transceiver, energy detection (ED) within the current channel, link quality indication (LQI) for received packets, clear channel assessment (CCA) for carrier sense multiple access with collision avoidance (CSMA-CA), channel frequency selection, and data transmission and reception. The energy saving aspects of IEEE 802.15.4 are mainly addressed to this layer by the ED, LQI and CCA functionalities [35]. According to the PHY layer specifications, and following the spectrum utilization of each region, the distances between the nodes based on IEEE 802.15.4 can be up to 100 m. This range depends on propagation environment obstacles and the maximum transmission power levels defined by the IEEE 802.15.4 standard, illustrated in Figure 10[3; 38]. The IEEE 802.15.4 MAC layer provides access to the physical channel for all upper layers, providing two kinds of services: the MAC data service and the MAC management service. MAC data and MAC management are services that are also provided to other layers enabling them to access the PHY layer  
Figure 9: Bluetooth basic rate versus Bluetooth low energy. resources. The standard defines two different methods of channel access that are a beacon enabled (BE) mode and a non-beacon-enabled (NBE) mode. The MAC sub-layer, which is responsible for beacon management, is also responsible for channel access, Guaranteed Time Slots (GTS) management, frame validation, delivered frame acknowledgement, and association/disassociation activities [33]. An IEEE 802.2 Logical Link Control (LLC) can access the IEEE 802.15.4 MAC sub-layer through the Service Specific Convergence Sub-layer (SSCS). The SSCS IEEE 802.2 convergence sub-layer exists in a conceptual perspective, on the top of the MCPS (MAC Common Part Sub-layer). SSCS provides a link between the IEEE 802.2 LLC sub-layer and the IEEE 802.15.4 MCPS in the data service plane through the MCPS-SAP (MCPS-Service Access Point). On the management plane, driving the layer management functions, the MMLE (MAC sub-layer Management Entity) provides the service functions assembling capability through an interface between the SSCS and PHY. MMLE is also responsible for maintaining a PIB (PAN Information Base) which contains the data base of the managed objects belonging to the MAC sub-layer [3]. The IEEE 802.15.4 BE and NBE operational modes have being strongly investigated over recent years.