IEEE's 802.15.4e amendment introduces enhancements and modifications to the MAC layer of IEEE 802.15.4 in order to overcome these limitations. It improves the old standard by introducing mechanisms such as time slotted access, multichannel communication and channel hopping . 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.
Functional Enhancements
The following enhancements were introduced in the 802.15.4e standard:
- Low Energy (LE) for applications that can trade latency for energy efficiency. It allows a node to operate with a very low duty cycle (1% or below) while appearing to be always on to the upper layers. This enables the IoT paradigm, which uses Internet protocols that assume that hosts are always on.
- Information Elements (IE) are an extensible mechanism to exchange information at the MAC sublayer.
- Enhanced Beacons (EB) are an extension of the 802.15.4 beacon frames and is more flexible. It allows the creation of application-specific frames by including relevant IEs.
- Multipurpose frame provides a flexible frame format that addresses a number of MAC operations. It is based on IEs.
- MAC Performance Metric is a mechanism to provide feedback on the channel quality to the network and upper layers, so that appropriate decision can be taken, e.g., the IP protocol may implement dynamic fragmentation of datagrams depending on the channel conditions.
- Fast Association (FastA). The 802.15.4 association procedure introduces a significant delay in order to save energy. For time critical applications, reducing latency is more important than energy efficiency. The FastA mechanism allows a node to associate in a shorter time.
MAC Behaviour Modes
There are five new modes:
- Time Slotted Channel Hopping (TSCH) is intended for process automation applications, particularly equipment and process monitoring. Typical segments for TSCH application are the oil and gas industry, food and beverage products, chemical products, pharmaceutical products, water/waste water treatments, green energy production, climate control.
It provides support for multi-hop and multi-channel communications using TDMA. TSCH combines time slotted access with multi-channel and channel hopping capabilities. Time slotted access increases the potential throughput, by eliminating collision, and provides deterministic latency to applications. Multi-channel allows more nodes to exchange their frames at the same time (i.e., in the same timeslot), by using different channel offsets. Hence, it increases the network capacity.
Channel hopping reduces the effects of interference and multi-path fading, thus improving the communication reliability. TSCH provides increased network capacity, high reliability and predictable latency, while maintaining very low duty cycles (i.e., energy efficient) due to the time slotted access mode. TSCH is also topology independent, i.e., it can be used to form any network topology (star, tree, partial or full mesh). It is suitable for multi-hop networks where frequency hopping allows for efficient use of the available resources.
- Deterministic and Synchronous Multi-channel Extension (DSME) supports industrial and commercial applications with stringent timeliness and reliability requirements, in addition to flexibility and adaptability to time-varying traffic and operating conditions. It is designed for multi-hop and mesh networks. It combines contention-based and time division medium access, and offers two channel diversity modes. DSME is suitable for many industrial, commercial and healthcare applications, such as factory automation, home automation, smart metering, smart buildings and patient monitoring.
DSME derives from the BE mode of the IEEE 802.15.4 standard, while introducing many enhancements. Time is divided into Contention Access Periods (CAPs) and Collision Free Periods (CFPs). During CAPs, nodes use a slotted CSMA-CA (or ALOHA) algorithm for channel access, while during CFPs they use Guaranteed Time Slots (GTSs), in a TDMA style. DSME extends the number of GTS timeslots and increases the number of frequency channels (previously limited to only one). DSME's versatile multi-superframe structure ensures the required flexibility to accommodate periodic and aperiodic (event-driven) traffic, even in large multi-hop networks.
Its two-channel diversity strategy allows DSME to select dynamically the best communication channels in order to guarantee robustness and high reliability even in time-varying channel conditions.
- Low Latency Deterministic Network (LLDN) is designed for single-hop and single-channel networks. It is designed for the industrial automation application domain, where a large number of devices observe and control the factory production. It supports factory automation, where applications require very low latency. In this context, wireless communication represents a valid alternative to the cabling of industrial sensors, which are expensive, time-consuming and cumbersome).
These applications require low latency and high cyclic determinism. LLDN allows data to be transmitted from 20 different sensor nodes every 10 ms (the IEEE 802.15.4 standard is unable to fulfill this stringent requirement). LLDN defines a fine granular deterministic TDMA access.
- Asynchronous multi-channel adaptation (AMCA) supports application domains that involves large deployments, such as smart utility networks, infrastructure monitoring networks, and process control networks. In these networks, using only one channel for communication may not connect all nodes in the same PAN. Moreover, the variance of channel quality is usually large, and link asymmetry may occur between two neighbouring nodes (i.e., a node may be able to transmit to a neighbour but unable to receive from it).
The AMCA mode relies on asynchronous multi-channel adaptation and can be used only in non Beacon-Enabled PANs. In this mode, each device selects the channel with the best local link quality as its designated listening channel and starts listening on that channel. As soon as two nodes have to exchange packets, the sender device switches to the designated listening channel of the receiver device, in a fully asynchronous way. After transmitting the data packet, the sender switches back to its own designated listening channel and continues listening. Nodes can exchange information about their designated listening channels by requiring beacon transmissions to coordinators or by sending special Hello packets.
- Radio Frequency Identification Blink (BLINK) is designed for application domains such as item/people identification, location and tracking. Specifically, it allows a node to communicate its ID to other nodes without prior association and acknowledgement. BLINK packets are generally sent by ‘transmit only’ devices using the Aloha protocol.
The 802.15.4e standard provides only a brief description about AMCA and BLINK.
Source
- D. De Guglielmo, S. Brienza, G. Anastasi, (2016). IEEE 802.15.4e: A Survey, Computer Communications, Volume 88
- A.N. Kim, F. Hekland, S. Petersen, P. Doyle. When HART Goes Wireless: Understanding and Implementing the WirelessHART Standard
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