Читать книгу Industry 4.0 Vision for the Supply of Energy and Materials - Группа авторов - Страница 63

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 the issue of channel collisions.