# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### IEEE 802.11 a/b/g/n/ah  
Certainly, one of the most discussed and exploited standards in its functionalities and applications is IEEE 802.11. Its design has as an impulse the demand for high data transfer rates. Standardized by the IEEE as protocol for WLAN, its technology has evolved to meet the needs of increasingly specific demands. This evolution has initiated a group of IEEE 802.11 standards that have been merged, and named Wireless Fidelity (Wi-Fi). This group is the Wi-Fi Alliance [72] that certifies Wi-Fi products. In order to ensure that the Wi-Fi products meet the standards, this facility was named the WLAN System Toolbox which guarantees the compatibility of the market products in the PHY layer parameters. In addition, it contributes to the exploitation of the various different regional implementations, thus contributing to protocol evolution. The standard defines that communication devices are referred to as Stations (STAs) and can behave independently. Communication is directly between the two devices forming an ad hoc topology. The star topology happens when a certain STA is defined to be the traffic concentration point of other STAs, becoming an Access Point (AP). A STA-AP has a defined coverage area called the Basic Service Area (BSA) that allows it to associate with several STAs, forming a Basic Service Set (BSS). The STA-AP is usually connected to the internet or to a WAN network through a wired connection. It is also possible to have a Distributed System (DS) connecting the various STA-APs of the same LAN by forming a transport backbone infrastructure called the Extended Service Set (ESS) [73]. The IEEE 802.11 protocol stack follows the OSI reference model on its PHY and MAC layers. While the IEEE focuses on defining the PHY and MAC layers of the protocol as a grouped context, the Wi-Fi Alliance aims to work on the physical layer to facilitate peer-to-peer communications. The IEEE 802.11 standard has evolved since its first release in 1997. The PHY layer has evolved to work on the 2.4 GHz and 5 GHz ISM frequency bands with direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM). The channelization was segmented from 20 MHz for IEEE 802.11 up to 40 MHz for IEEE 802.11 n. The multiple transmission beam-forming technology improves the transmission and reception parameters and, consequently, transmission rates with greater values from 2 Mbps on IEEE 802.11 up to 600 Mbps for IEEE 802.11 n.  
The MAC layer also had to adapt to this evolution. The MAC layer of IEEE 802.11 has as its main differential the mechanism of access to carrier sense multiple access with collision avoidance (CSMA/CA) as a medium access method. Features such as MAC level acknowledgments, fragmentation and reassembly, inter frame gaps and exponential back-off algorithm, roaming, synchronism, security and power saving mechanisms are adopted by IEEE 802.11. These techniques guarantee communication in a frequency band with a lot of spectral pollution. The 802.11a operates at the 5 Ghz band, with 52 orthogonal frequency-division multiplexing (OFDM) reaching less than 54 Mbps. Its transmission rate is fragmented on 48, 36, 24, 18, 12, 9 or 6 Mbps. As the 2.4 GHz band is more common and has more spectral pollution, the 5 GHz band offers an advantage over the IEEE 802.11a standard, due its spectrum low interference. Devices using IEEE 802.11a have a low transmission performance compared with IEEE 802.11b when dealing with obstacles. The IEEE 802.11b revision of the original standard was ratified in 1999 with a maximum transmission speed of 11 Mbpsand uses the same CSMA/CA access method defined in the original standard. IEEE 802.11b standard uses the same 2.4 GHz, operating at a maximum theoretical speed of 54 Mbps. With Machine to Machine (M2M) communications emerging, it was necessary to adjust the IEEE 802.11 standard that was primarily designed for computer communication.