## 2 WirelessHART  
As the WirelessHART technology is the basis for the present paper, in this section, we will present a brief history of the technology and how it is organized. Furthermore, we will present the TDMA technique and the IEEE 802.15.4 physical layer, since those are common traits among WirelessHART and the main IWSN nowadays (WIA-PA and ISA100.11a). Then, we will proceed to the main focus of the present paper, by emphasizing the specific traits of WirelessHART routing and scheduling.  
WirelessHART is an extension of the HART protocol to support wireless communication. In September, 2008, the WirelessHART specification (HART 7.1) was approved by the International Electrotechnical Commission (IEC) as a publicly available specification (IEC 62591) [6]. In April 2010, WirelessHART was the first industrial wireless communication technology to attain this level of international recognition, as can be seen in Figure 1[16]. A WirelessHART IEC update is expected for 2015.  
WirelessHART technology implements five layers: a physical layer, a data link layer, a network layer, a transport layer and an application layer. Thus, when compared to the traditional OSI (Open Systems Interconnection) stack model, the presentation layer and session layer do not have direct equivalents in the WirelessHART stack [41], as can be seen in Figure 2. Figure 2 also presents the equivalences and differences of the traditional wired HART technology and the WirelessHART. It is worth noting that WirelessHART devices are fully compatible with a traditional HART network [7].  
WirelessHART defines eight types of devices: network managers, network security devices, gateways, access points, field devices, adapters, routers and handheld devices. All devices are connected to the wireless network to build a mesh-type topology (see Figure 3) and implement basic mechanisms to support network formation, maintenance, routing, security and reliability. The communications are  
Figure 1: Historical development of the industrial WSN (IWSN) technologies.  
organized by the network manager, which centralizes the information from all of the network devices (including joining devices) to make the routing and scheduling function.  
Although WirelessHART has a physical layer based on IEEE 802.15.4, it implements its own medium access control (MAC) sublayer. The MAC layer uses the frame structure described in Section 2.1, but naming it a superframe. Superframes are composed of slots; the number of slots indicates the periodicity of the superframe. In order to support different transmission intervals, a WirelessHART network can use multiple superframes with different numbers of slots. Each slot has a fixed duration of 10 ms, which is enough time to transmit a packet and receive an acknowledgment message (the maximum packet size is 133 bytes, including headers). A slot supports up to 15 channels. Thus, theoretically, 15 devices can simultaneously transmit in the same slot time. The standard uses a mechanism of frequency hopping and  
Figure 3: WirelessHART devices.  
Figure 2: WirelessHART architecture compared to wired HART and the OSI stack. OSI, Open Systems Interconnection.  
a blacklist channel to minimize the influence of noise in network operation and consequently increase the reliability of the communication. As the protocol is TDMA-based, the source and destination stations have specific times to transmit and sense the medium. More details about the values of those times can be found in [6].