# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### IEEE 802.11 a/b/g/n/ah
When the concern is not the distance but the energy factor, the transmission power of the nodes can also be reduced using its low power mode operation. It reduces the power consumption, the cost of the device and, consequently, its size. A disadvantage of using a narrower channel is a more sensitive transmission to flat fading that can be deep in indoor environments. This challenge is overcome by using the transmission selection of the best sub-channels for transmission at that time. This technique can increase up to 7 dBs [77] gain for systems working in indoor environments. In cases of node displacement during transmission, the Doppler effect occurs. It is necessary to estimate the channel and the correction of the transmission during transmission. To do this, IEEE 802.11 ah changes the pilot carrier of each OFDM symbol [78]. The IEEE 802.11 ah MAC layer incorporates the majority of the IEEE 802.11 main characteristics or has improved some of them. This is to optimize M2M communications, to support a large outdoor IoT network and to support energy-efficient communications for sensors [79]. Considering a scenario of a network densely populated by nodes, some factors that imply the containment and the characteristics of access to the medium were implemented, for example, the techniques of restricted access window adjustment, synchronization frame and hierarchical, and traffic indication map (TIM). They are implemented in the MAC layer to mitigate the problems of transmission collisions between hidden nodes [80]. The time window that a group of nodes belonging to the same AP has to access the medium and transmit is called the restricted access window (RAW). RAW is divided into slots that are individually assigned to some transmitting nodes of this group during each RAW. The same group of nodes dispute access to the medium through the same RAW assigned to them and, therefore, is a shared resource. Information such as the number of slots, the duration of each slot and the start time of each RAW is served by the RAW Parameter Set (RPS). The RAW technique prevents transmissions between hidden nodes from overlapping, by limiting the time that a station uses when competing for the transmission medium. When a station needs to transmit, it must first detect and receive a complete and correct frame. Detection and identification of this frame causes the station to wait for the transmission window in order to avoid transmission collision. When attempting to access the medium, if the station does not receive a frame or is unable to identify the received frame, it must wait for a time interval called _ProbeDelay_ to make another attempt to access the medium. The disadvantage of this procedure is to generate a medium access delay, which is reduced with the aid of the AP and its medium access control modes [81]. At the beginning of each RAW slot, the AP transmits a SYNC frame. The AP can detect the availability of the medium and initiate its transmission, after the end of the reception of the SYNC frame, not having to wait for the _ProbeDelay_ timeout. Using the SYNC frame can reduce battery consumption by up to 30% [81]. Another improvement brought by IEEE 802.11 ah is in regard to the power consumption and latency control. They are caused by the traffic indication map (TIM) based communication process, used in the downlink transmission demand detection mechanism. This method works detecting the demand of downlink communication through to TIM transmitted in the beacon by the AP. Thus, there is also a need for the node to respond to the AP with a PS-Poll frame. This procedure is eliminated by adopting a predefined schedule of the future wake-up time. With the data stored in the buffer, ready to be transmitted, the AP transmits the data to that node in its predefined window, thus saving energy that the node would expend by mapping the TIM, negotiating access to the medium, and receiving data from the AP.