# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### IEEE 802.11 a/b/g/n/ah
M2M communication demands distinct characteristics such as transmission range above 1 Km, transmission rates higher than 100 Kbps and low power consumption. There is also a need to have a network that supports a large number of nodes, operating under a policy of lower power consumption. In an attempt to meet these requirements, the IEEE 802.11 ah Task Group (TGah) [74] has guided the necessary improvements in the PHY and MAC layers of the IEEE 802.11 protocol to suit this scenario. The IEEE 802.11 ah can be used not only as WSN but as well as back-haul infrastructure to connect the sensors to the data collectors. This is possible due to its large coverage and data rates capability. The IEEE 802.11 ah amendment comes to solve some important limitations encountered to use Wi-Fi IEEE 802.11a/b/g/n when used to M2M communications. Its physical layer operates on ISM at 863-868.6 MHz in Europe, 950.8-957.6 MHz in Japan, 314-316 MHz, 430-432 MHz, 433.00-434.79 MHz in China, 917-923.5 MHz in Korea, and 902-928 MHz in USA. It uses an orthogonal frequency division multiplexing (OFDM) modulation scheme to achieve larger areas of coverage and increases the number of simultaneously operable stations. Frequency operations sub 1 GHz depend on local regulations for each region. For this reason, the bandwidth occupation is usually 1 MHz or 2 MHz, but, in some countries, broader configurations using 4, 8 and 16 MHz are also allowed. In the physical layer, the transmission is OFDM-based, working with 32 or 64 sub-carriers with 31.25 KHz spacing. The supported modulation techniques include BPSK, QPSK and 16 to 256 QAM with transmission/reception characteristics. These modulations are enhanced with beam forming by using a multi-input multi-output (MIMO) antenna scheme for single-users, and downlink multi-user MIMO [75]. However, the IEEE 802.11 ah physical layer PHY BW channelization can be split into:  
* Bandwidth of 1 MHz: Used to extended range of applications especially IoT or M2M applications that work with short burst low data rates. Range extension is obtained when using new Modulation and Coding Scheme index 10 (MCS 10) added to the previous 802.11 MCSs. * Bandwidths of 2 MHz and more: This mode is oriented to data rates higher than those obtained with the 1 MHz bandwidth, using up to 16 MHz bandwidth with different Modulation and Coding Scheme (MCS) options. MIMO can be used to compose this solution to improve its performance [76]. The PHY layer of IEEE 802.11 ah follows the evolution of the IEEE 802.11 standard for IEEE 802.11 ac that uses Orthogonal Frequency Division Multiplexing (OFDM) modulation. Its Transmission System now has uplink MIMO antennas and downlink Multi-User MIMO (DL MU-MIMO) antennas. The previous 10, 20, 40, 80 and 160 MHz bandwidths were reduced to a 10 fold scale, resulting in channels with 1, 2, 4, 8, and 16 MHz bandwidths in the IEEE standard 802.11 ah. At the same time, keeping the same number of carriers in each channel as previous versions, except for the 1 MHz channel as the guard band value between the sub-carriers cannot be reduced. In this way, the transmission of a symbol lasts 10 times longer than the previous standards. The increase in transmission range is a consequence of the combination of some other improvement factors in the PHY layer. Due to its operation in the frequency range below 1 GHz, the transmission lost 8.5 dB of its link budget, in the LOS condition. Reducing 10 times the transmission channel bandwidth also reduces the noise level in the transmission. This increases the signal-to-noise ratio (SNR) by 10 dBs. With the adoption of a 1 MHz bandwidth channel, an increment of 3 dBs is achieved in the SNR, when compared to the 2 MHz bandwidth channel. Another 3 dBs of gain is improved on the SNR, with the 1 MHz channel supporting the repetition coding scheme for binary phase shift keying (BPSK) modulation, used with the 1/2 coding rate. The sum of the above gains brings us a total gain in the link budget of 24 dBs when compared to previous standards that operate at 2.4 GHz.