noise, electromagnetic interference, and complex coordination among devices that pose unique challenges for wireless connectivity. IIoT systems often require support of seamless communication and diverse objectives of functions for a wide range of applications. Therefore, wireless communication protocol design that meets these performance criteria is essential to fulfill service QoS and ensure prolonged lifetime.
MAC layer is a key sublayer of the data link layer that exploits use of specific protocols to administer nodes privilege to access shared wireless medium according to application requirements. MAC protocol controls the radio and channel sensing scheme and defines nodes duty cycle, communication mode between devices, data rates, transmission power, and range. Given that radio is the foremost source of power consumption in networks, MAC protocols could significantly impact nodes’ overall power consumption and their lifetime [188]. In addition to the aforementioned key features, the MAC layer also controls other wireless settings such as frames synchronization, source–destination address management, error detection for physical layer transmission, collision reduction, and mitigation of idle listening.
With the goal of meeting the requirements of IIoT applications, the existing protocols could be adapted, evolved, or developed for different performance characteristics. MAC protocol schemes can be categorized into two broad classes: scheduled based and contention based [189]. Hybrid schemes are also proposed as a combination of these schemes.
1.7.1 Scheduled-Based Schemes
In scheduled-based protocols, also known as fixed reservation–based schemes, a fixed duration of time, frequency, or other domain is scheduled and assigned to nodes for network resources access. The scheduling assignment algorithm is conducted by a centralized base station and aims at avoiding channel collisions. In addition to a collision-free schedule, a device is simply set to sleep when it is not using its time slot to prevent idle listening and message overhearing [190]. This scheme is more suitable for networks that deploy low-mobility nodes and require infrequent topology changes and scheduling adjustment. It also tends to be more predictable and offers deterministic E2E delay. However, in dense networks, nodes should wait to gain access to the wireless medium, and additional queuing delay shall be incurred. Synchronization is an important issue in this approach leading to higher complexity and additional traffic due to additional control packets. The following multiple access schemes are utilized in typical multi-user wireless communication systems.
TDMA: Time is divided among nodes for a given and identical frequency channel. Therefore, a fixed portion of time is assigned to every node to transmit data. For successful TDMA slot assignment and collision-free communication, tight clock synchronization should be established between nodes. GinMAC [191] and wireless arbitration (WirArb) [192] are some collision-free TDMA-based MAC protocols in time-sensitive IIoT. For instance, GinMAC provides reliable data delivery as well as deterministic time delay for industrial process automation such as closed-loop control systems.17 Given that only one node is allowed to transmit data during the scheduled time slots, TDMA suffers from relatively high delay.
FDMA: As the name infers, accessible frequency bandwidth is partitioned into non-overlapped sub-channels, where each individual sub-channel is adequate to accommodate transmission of a signal spectrum. Ideally, through proper frequency assignment algorithms, a unique physical frequency is dedicated to every node to offer a collision-free protocol. The FDMA-based protocols support multiple frequencies and require more costly hardware. They generally are not useful for IoT systems because of a high level of power consumption and more complicated design [193].
CDMA: A MAC channel access method that enables transmission of multiple signals in a single transmission channel. A combination of special encoding scheme and spreading spectrum technology is exploited to send multiple signals through a single channel. The basic principle is that users have access to the whole bandwidth for the entire duration, but they utilize different CDMA codes; this assists the receiver to distinguish among different users. Given that the entire bandwidth is allocated to a CDMA channel, this scheme suffers from limited flexibility in adapting bandwidth, particularly for M2M communication in IIoT systems.
OFDMA: A multiple access scheme that divides the entire channel resources into small time-frequency resource units. Since the available bandwidth is divided into multiple mutually orthogonal narrowband sub-carriers, several users could share these sub-carriers and simultaneously transmit data. In other words, the signal is first split into multiple smaller sub-signals, and resource units are allocated to them. Then, each data stream is modulated and transmitted through the assigned resource units. OFDMA allows several users with various bandwidth requirements simultaneously to transmit data at different (orthogonal) frequencies. Therefore, channel resources can be assigned with much more flexibility for different types of traffic. In addition to high spectral efficiency, OFDMA can effectively overcome interference and frequency- selective fading caused by multipath. OFDMA is a promising multiple access scheme adopted for wide range of mobile broadband wireless networks such as LTE, Wi-Fi6, and 5G [194–196].
1.7.2 Contention-Based Schemes
In contention-based protocols, nodes perform random access competition with each other to access medium on demand. Before transmission, nodes perform channel sensing to verify whether the medium is clear and wait for a specified backoff period to transmit data. Each node performs the channel sensing independently, and the allocated channel will be accessible for the required duration. Once data communication is complete, the occupied channel is released. Compared with scheduled-based protocols, this class of protocols does not require centralized control and precise time synchronization [197]. Moreover, these protocols are adequately simple, adaptive toward change in network topology, and robust to variation in nodes traffic load and density [198]. They also do not require extra message exchange overhead, thanks to the independent decision-making process for channel access. There are several protocols under this class, such as ALOHA [199] and variants of CSMA [200]. BREATH is a self-adapting CSMA MAC protocol that provides reliable, energy efficient, and timely data transmission for industrial control applications [52].
A consequential drawback of contention-based schemes is that the probability of collisions and idle listening grows with increased node density, leading to unacceptable performance in terms of latency. To alleviate the effect of collisions, additional control packets may be added to the MAC protocol, resulting in noticeable control overhead in IIoT systems. Contention-based MAC protocols could be performed through synchronous and asynchronous protocols.
Synchronous protocols: This class of schemes employs local time synchronization between nodes to alternately switch their operation mode between active and sleep modes. In these protocols, a node operates in active mode for packets listening or sleeping mode to decrease overhearing and idle listening. To prevent overload from frequent synchronization messages, the protocol could use infrequent synchronization, although it may decrease network adaptability to nodes mobility [189].
Asynchronous protocols: Unlike synchronous protocols, this method does not require explicit scheduling between nodes. Instead, a low power listening (LPL) concept could be employed, where each node transmits data with a long enough preamble so that receiver is guaranteed to wake up during preamble transmission [201]. Basically, the receiver is often in sleep mode and wakes up shortly to sense the channel for every preamble. If a sender has data, it will send preamble to the receiver until it is awake and properly acquires the preamble. Then, the receiver remains in active mode to receive incoming data. After the transmission or reception period, all nodes check their data queue before going to sleep mode. Duty cycle and idle listening of asynchronous protocols could be decreased through dynamic preamble sampling [202]. The advantages of asynchronous protocols are flexibility to topology changes, less synchronization overhead, and a reduction in a receiver’s idle listening. Nevertheless, asynchronous protocols suffer from transmitters’ overemission before sending data, extra power consumption in unintentional receivers, and increased latency [189]. It also does not fully resolve
