What are the major limitations of IEEE 802.15 4 standard?

Wireless sensor and actuator networks [WSANs] will play a key role in the realization of the future Internet of Things [IoT] [1], [2], [3] since they represent the main way through which any computational system can interact with the physical world [4], [5]. In a WSAN many sensor and actuator devices are placed in the same physical environment for monitoring and control operations. Physical quantities, such as temperature, pressure and light intensity are continuously measured by sensor devices. Then, the acquired data are sent to a central controller using wireless links. The controller analyzes the received information and, if needed, changes the behavior of the physical environment through actuator devices. WSANs are already used in many application domains, ranging from traditional environmental monitoring and location/tracking applications to critical applications such as those in the industrial, smart grid and healthcare domain [6]. In the industrial field, WSAN applications include real-time monitoring of machinery health, factory automation, distributed and process control, detection of liquid/gas leakage, radiation check and so on. In the healthcare domain, WSANs are used for the monitoring of physiological data in chronic patients and transparent interaction with the healthcare system. In smart grid, WSANs have been recognized as a promising technology to achieve seamless, energy efficient, reliable, and low-cost remote monitoring and control of the electric power system. The real-time information gathered from WSANs can be analyzed to diagnose problems early and serve as a basis for taking remedial actions [7], [8].

Energy efficiency is usually the main concern in the design of a WSAN. This is because sensor/actuator devices are typically powered by batteries with a limited energy budget and their replacement can be very expensive or even impossible [9]. However, in many application domains, additional requirements such as reliability, timeliness and scalability need to be considered as well [9], [10], [11]. Reliability and timeliness are very critical issues for industrial and healthcare applications. If data packets are not delivered to the final destination, correctly and within a pre-defined deadline, the correct behavior of the system [e.g., the timely detection of a critical event] may be compromised. The maximum allowed latency depends on the specific application and ranges from tens of milliseconds [e.g., for discrete manufacturing and factory automation], to seconds [e.g., for process control], and even minutes [e.g., for asset monitoring]. Finally, scalability is fundamental as WSANs can be composed of hundreds to thousands of nodes.

In recent years many standards have been issued by international bodies to support the development of WSANs in different application domains. They include IEEE 802.15.4 [12], ZigBee [13], Bluetooth [14], WirelessHART [15] and ISA-100.11a [16]. At the same time, the Internet Engineering Task Force [IETF] has defined a number of protocols to integrate smart objects [i.e., sensor/actuator devices] into the Internet [17]. The most important of them are the IPv6 over Low power WPAN [6LoWPAN] [18] adaptation layer protocol, the Routing Protocol for Low power and Lossy networks [RPL] [19], [20], and the Constrained Application Protocol [CoAP] [21] that enables web applications on smart objects.

The IEEE 802.15.4 standard [12] defines the physical and MAC [Medium Access Control] layers of the protocol stack and is considered the reference standard for commercial WSNs. In fact, many products compliant to this standard are available today. Many studies have investigated the IEEE 802.15.4 performance in WSANs [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. These works highlighted that IEEE 802.15.4 has a number of limitations [such as low communication reliability and no protection against interferences/fading] that make it unsuitable for applications having stringent requirements in terms of latency, reliability, scalability or operating in harsh environments [32]. In order to overcome such limitations, in 2008 the IEEE set up a Working Group [named 802.15 Task Group 4e] with the aim of enhancing and adding functionality to the 802.15.4 MAC, so as to address the emerging needs of embedded applications. The final result was the release of the 802.15.4e standard in 2012 [32]. The 802.15.4e improves the old standard by introducing mechanisms such as time slotted access, multichannel communication and channel hopping. Specifically, it defines five new MAC protocols [called MAC behavior modes] to support specific application domains and some general functional enhancements that are not designed for specific applications. In this regard, the 802.15.4e standard document assumes that readers are quite familiar with the original 802.15.4 technology and presents a significant amount of references to the original standard. Hence, it is absolutely not easy to read for an inexpert reader. The main goal of this paper is to provide a clear and structured overview of all the new 802.15.4e mechanisms. After a general introduction to the 802.15.4e standard, we devote special attention to describe the details of the main 802.15.4e MAC behavior modes, namely Time Slotted Channel Hopping [TSCH], Deterministic and Synchronous Multi-channel Extension [DSME], and Low Latency Deterministic Network [LLDN]. For each of them, we provide an in-depth description and highlight the main features as well as possible application domains. In addition, we survey the main research works present in the literature and summarize open research issues.

The rest of the paper is structured as follows. In Section 2, we briefly describe the original 802.15.4 standard and highlight the main limitations that motivated the development of the new standard. In Section 3, we present the new 802.15.4e standard and we give a general overview of the introduced enhancements. Then, we focus on the new 802.15.4e MAC behavior modes. Specifically, in Section 4, we describe the TSCH MAC behavior mode. In Section 5, we focus on DSME, whereas Section 6 is devoted to describe LLDN. Finally, Section 7 concludes the paper.

IEEE 802.15.4 standard

IEEE 802.15.4 [12] is a standard for low-rate, low-power, and low-cost Personal Area Networks [PANs]. A PAN is formed by one PAN coordinator which is in charge of managing the whole network, and, optionally, by one or more coordinators that are responsible for a subset of network nodes. Regular nodes must associate with a [PAN] coordinator in order to communicate. The supported network topologies are star [single-hop], cluster-tree and mesh [multi-hop].

The standard defines two different channel

In 2008, the IEEE created the 802.15 Task Group 4e with the aim to redesign the existing 802.15.4 MAC protocol so as to overcome its limitations. The goal was to define a low-power multi-hop MAC protocol, capable of addressing the emerging needs of embedded [industrial] applications. The final result was the IEEE 802.15.4e MAC Enhancement Standard document [32], approved in 2012. 802.15.4e borrows many ideas from existing standards for industrial applications [i.e., WirelessHART [15] and ISA

TSCH [time slotted channel hopping]

The Time Slotted Channel Hopping [TSCH] mode is mainly intended for the support of process automation applications with a particular focus on equipment and process monitoring. Typical segments of the TSCH application domain include oil and gas industry, food and beverage products, chemical products, pharmaceutical products, water/waste water treatments, green energy production, climate control [32].

TSCH combines time slotted access with multi-channel and channel hopping capabilities. Time

DSME [deterministic and synchronous multi-channel extension]

The Deterministic and Synchronous Multi-channel Extension [DSME] has been designed for all those critical applications that require deterministic delay and high reliability, in addition to flexibility and adaptability to time-varying traffic and operating conditions. In this perspective, DSME is particularly suitable for many industrial, commercial and healthcare applications, such as factory automation, home automation, smart metering, smart buildings and patient monitoring.

DSME derives from

LLDN [low latency deterministic network]

The LLDN mode specifically addresses the industrial automation application domain, where a large number of devices observe and control the factory production. In this context, wireless communication represents a valid alternative to the cabling of industrial sensors [typically expensive, time-consuming and cumbersome] and also provides advantages in case of mobility and retrofit situations. As an example, LLDN devices can be located on robots, cranes, and portable tools in the automotive

Conclusions

The IEEE has recently released the 802.15.4e amendment that introduces a number of enhancements/modifications to the MAC layer of the original 802.15.4 standard to overcome its limitations, i.e., low reliability, unbounded packet delays and no protection against interference/fading. The 802.15.4e standard document assumes that readers are quite familiar with the 802.15.4 technology and presents many references to the original standard resulting a not easy-to-follow document for an inexpert

Acknowledgment

This work has been partially supported by the University of Pisa, in the framework of the PRA 2015 program.

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