of their actual load of traffic, energy consumption of IEEE 802.15.4 is high [60]. In addition, the single channel property of the IEEE 802.15.4 MAC layer is not reliable and suffers from a high level of signal fading and destructive interference, particularly in multi-hop scenarios.
To overcome such limitations, IEEE 802.15.4e has been proposed as an extension of this standard [59]. It modifies the existing MAC sublayer to better comply with WSN and emerging IoT applications, with the same PHY layer. Therefore, it is compatible with IEEE 802.15.4 hardware and can be deployed without any adjustment. Various MAC channel access protocols are established by IEEE 802.15.4e standard and present different MAC layer modes. Among them, time-slotted channel hopping (TSCH) becomes highly appropriate for industrial process automation and control applications [61]. Dust Network initially adopted TSCH for its proprietary MAC [62]. Because of TSCH time and frequency diversity characteristics, its core idea was later adopted in several industrial standards: WirelessHART [63], ISA100.11a [64], and WIA-PA [58]. An open standard protocol stack called 6TiSCH was later designed that combined lower layers of IEEE 802.15.4e with higher layers of the Internet Engineering Task Force (IETF) to fulfill stringent requirements of industrial IoT networks with respect to ultra-low jitter, low latency, and high reliability [65, 66]. To summarize, the IEEE 802.15.4e standard enhances its performance in terms of reliability, deterministic latency, network capacity, and power consumption to support seamless communication of embedded devices in design of IoT systems [67].
In the following, we elaborate some well-known industrial variants of the IEEE 802.15.4 standard.
Zigbee. Developed by Zigbee Alliance and primarily targeted residential and commercial applications to provide wireless connectivity [68], this wireless technology is based on PHY and MAC specifications of IEEE 802.15.4. However, it has its own specifications for all upper layers of the protocol reference model [69]. Zigbee defines an application framework (AF) for application layer that demultiplexes incoming data between registered applications. Network nodes can operate as either FFDs or RFDs and exclusively transmit data toward the router or coordinator. Since it offers various features for protocols of all the layers above MAC and PHY, it can be properly adapted to large deployments.
Zigbee operates on the frequency band of 2.4 GHz (i.e., the Industrial Scientific and Medical [ISM] band) and shares the spectrum with IEEE 802.15.1 and IEEE 802.11. This makes it susceptible to high mutual interference and noise and leads to frequent backoff for Zigbee MAC protocol [70]. Zigbee is not highly suited to provisioning easy channel access and delivery of data in delay-sensitive applications. Consequently, it is not suitable for industrial applications that require deterministic delay and high reliability [71].
To combat limitations of its MAC layer, Zigbee Alliance developed a variant called Zigbee PRO, which specifically supports industrial process and control applications [72]. Zigbee PRO can change network operating channels and enhances security features if it faces significant levels of interference or noise [73]. Zigbee Smart Energy (Zigbee SE) is another related protocol that relies on Zigbee IP and effectively manages the power consumption of nodes. Considering the cost efficiency, low-powered structure, and redundancy capability of Zigbee SE, it is suitable for demand–response and load control systems such as smart grids [74].
WirelessHART. This open standard specifically targets wireless instrumentation for factory automation [75]. The main motive behind WirelessHART was to deliver feasible solutions that address stringent timing requirements and severe interference conditions of the industrial ecosystem [76]. The PHY layer of WirelessHART is based on IEEE 802.15.4; however, it applies TDMA channel access for MAC protocol to guarantee collision-free channel access [62]. Therefore, WirelessHART communication is scheduled and offers strict time slots as well as network-wide time synchronization due to the adoption of the TDMA-based MAC layer. WirelessHART MAC protocol features some prominent properties such as channel blacklisting and channel hopping schemes, which enhance data bandwidth and system robustness. More importantly, the WirelessHART network layer supports self-organizing mesh networks to assist in automatic configuration, optimization, diagnostician, and healing networks.
Different from generic WSNs, the WirelessHART network design and employment includes eight different types of devices. The sensors and actuators are denoted as field devices, which are usually attached to the plant equipment or processes to collect data such as pressure, humidity, and fluid flow from physical environment. The remaining seven are deployed to assist in network management and functionalities such as network interoperation, security, and optimization. WirelessHART is mainly considered as a centralized wireless network and utilizes a central network management to keep communication and route scheduling up to date. Consequently, it is more suited to industrial applications where network graphs should be continuously adapted to network changes and demands. WirelessHART can serve a large number of devices and high network data rates by using multiple gateways connected to a HART over IP backbone or multiple access points [77].
ISA100.11a. The International Society of Automation (ISA) developed industry standards that offer reliable and secure systems for automation and control applications. ISA100.11a is an industrial wireless communication standard ratified in 2009 and operates in a 2.4 GHz band (ISM band) [64]. A PHY layer of ISA100.11a is based on the IEEE 802.15.4 standard; however, its MAC layer is built on a modified, noncompliant MAC protocol of IEEE 802.15.4 and utilizes a combination of contention- and scheduled-based MAC scheme. To achieve real-time networking, a MAC protocol of ISA100.11.a exploits TDMA and carrier-sense multiple access (CSMA) along with additional spatial, frequency, and temporal diversity. Channel blacklisting and frequency hopping are also leveraged to address mutual interference from coexisting wireless systems and enhance network robustness [70].
The network of ISA100.11a consists of field and infrastructure devices. Both classes of devices are further divided into multiple types to assist in achieving a proper network architecture. Similar to WirelessHART, it exploits self-healing networks. The configuration of monitoring (e.g., slot allocation and scheduling), network runtime configurations, and execution of security standards policies are performed by the system manager as an infrastructure device. ISA100.11a connects field and plant networks via gateway devices. It embraces either distributed or centralized management; however, the proposed distributed management does not specify how network resources should be coordinated [78]. Different from WirelessHART, ISA100.11a provides graph and source routing while also offering options for configuration-based time-slot sizes, explicit congestion notification, and dual acknowledgment [77].
Wireless networks for industrial automation-process automation (WIA-PA). The Chinese Industrial Wireless Alliance introduced WIA-PA in 2008 as the national standard that proposes architecture and communication specifications for industrial automation and process use cases. Later, this standard was approved by the IEC (International Electrotechnical Commission) [58]. The WIA-PA PHY layer complies with the IEEE 802.15.4 standard, though its MAC layer applies a hybrid scheme (a combination of scheduled- and contention- based mechanisms) on IEEE 802.15.4 MAC protocol. It also exploits TDMA, CSMA, and frequency-division multiple access (FDMA) approaches for channel access. Given that WIA-PA MAC layer leverages adaptive frequency hopping, time slot hopping, and adaptive frequency switching, it can cope with varying network conditions and is considered a self-healing network [78]. Similar to WirelessHART and ISA100.11a, this standard employs a reactive approach that exploits redundant routing and gateway devices to prevent failures in networks, further enhancing its reliability and self-organizing characteristics [78].
Physical devices in WIA-PA networks are categorized into five classes [76]: (1) handheld devices to monitor and control the production plants and configure network devices; (2) field devices (sensors and actuators) located in the field to control or monitor industrial processes; (3) routing devices; (4) gateway devices that connect WIA-PA networks to various plant networks; and (5) a host computer as the user interface for management and maintenance. Both centralized and distributed mechanisms are deployed in WIA-PA networks to perform network and security management. Typically, network manager configures the network, schedules communication, handles routing tables, and protects
