# MAC Layer Protocols for Internet of Things: A Survey
## 2 Short Range MAC Layer Protocols
### Wireless-HART  
Wireless-HART (Highway Addressable Remote Transducer Protocol) is a variation of IEEE 802.15.4 design to work essentially as a centralized wireless network. IEEE 802.15.4 is designed to meet the requirements of industrial wireless applications with hard timing parameter restrictions, critically security issues, and severity on obstacle interferences. The Wireless-HART protocol has the same specifications as IEEE 802.15.4 PHY, but develops its own MAC layer based on the TDMA technique.  
Using Bluetooth, there is no guarantee to delay values on an end-to-end wireless communication. The absence of a hopping channel technique and a quasi-static star Bluetooth topology works against its scalability. These characteristics make them inappropriate to be used in industrial scenarios. Wireless HART comes as a solution for process control applications through the effort of some industrial organizations such as International Society of Automation 100 (ISA 100) [52], HART [53], Wireless Industrial Networking Alliance (WINA) [54] and ZigBee Alliance [55] to attend their specific requirements ratified by the HART Communication Foundation in 2007.  
Using the IEEE 802.15.4 PHY layer, Wireless-HART operates in the license-free ISM of 2.4-2.4835 GHz with 2 MHz bandwidth of each one of the 16 channels. The channels are numbered from 11 to 26 with a gap of 5 MHz between IEEE 802.11b/g adjacent channels, delivering up to 250 Kbps. Wireless-HART uses its own Time Division Multiplex Access (TDMA) on the MAC layer including the 10 ms synchronized time slot features. These characteristics allow the messages routing through a network topology obstacle and interference. This is possible due to the use of self-organizing and self-healing mesh networking techniques supported by the network layer. Even being essentially a centralized wireless network, Wireless-HART uses a network manager in its stack in order to provide routing and communication schedules. This can guarantee network performance and satisfy the wireless industrial applications. The focus of Wireless-HART is communication on a one-hop level and the network layer has its responsibility to the network devices vicinity allocation [3; 56; 57; 58].  
Differing from IEEE 802.15.4, Wireless-HART uses time-synchronized the TDMA technique combined with frequency hopping on its MAC layer, thus allowing multiple devices to transmit data at the same time along different channels. During the joining process of the devices onto networks, the network manager distributes the communication links and the channel hop patterns to the devices. It also manages the enabling or disabling of the use of channels that are frequently affected by considerable interference levels, calling this feature _channels blacklist,wHart-n-802-15-4e,petersen2011 wirelesshart_. The eight types of devices defined on Wireless-HART are: routers, gateways, adapters, network managers, network security devices, access points and field devices on a mesh topology. All of them support the implementation of features to attend network creation, maintenance issues, data and signaling routing capability, and a minimum of reliability. A comparison between the OSI reference model and Wireless-HART protocol layers and its main features is shown in Figure 12.  
Another addressable characteristic of Wireless-HART is the information blocks that each network device maintains on its memory. The information of neighbor nodes and the next reachable device is called a neighbor information block. The connection with the network layer is made through the block information, adding data to the network layer routing table. Working with TDMA as a medium access technique, the network devices have very stringent timing requirements to accomplish network synchronization premises. This happens because synchronization occurs both in the joining process and in normal operations [58].