# Novel Extensions to Enhance Scalability and Reliability of the IEEE 802.15.4-DSME Protocol
## 1 Introduction  
Presently, a wide range of industrial and automation applications adopt Industrial Wireless Sensor Networks (IWSNs) to benefit from the deployment flexibility and the support for a large number of mobile nodes that these networks offer. In this context, the IEEE 802.15.4 standard [1] is one of the most widely used technologies, as it supports different kinds of traffic through different medium access strategies, such as TDMA, CSMA/CA and prioritized channel access (PCA) [2]. In 2012, the IEEE 802.15.4e [3] amendment introduced three new Medium Access Control (MAC) protocols, namely the Deterministic and Synchronous Multichannel Extension (DSME), the Time Slotted Channel Hopping (TSCH) [4] and the Low Latency Deterministic Network (LLDN) [5]. Later, DSME and TSCH were included in the IEEE 802.15.4-2015 standard. DSME is a MAC protocol designed for critical application domains with stringent requirements, such as, deterministic delay, high reliability, and adaptability to changes in the network traffic and operating conditions. DSME uses frequency hopping to mitigate the effects of external interference [6]. As discussed in [7, 8], DSME is particularly suitable for several industrial, commercial and healthcare applications, such as factory automation, home automation, smart metering, smart buildings, and patient monitoring. DSME can be successfully used in solar tower power plant industries [9], in WSN applications such as outdoor monitoring (as those reported in [10, 11]) and in otherapplications [12] in which the number of network nodes or the generated data flows change over time [7] in response to an external event. For instance, the work in [13] discusses the use of DSME in surveillance systems, such as temperature reading or low-resolution images and video, in which devices send a small amount of data. In particular, in the addressed use cases, the bitrate of a video stream increases upon movement detection and the sample time of a pollution monitoring system decreases when the guard levels are exceeded. DSME uses a tree topology made up of a root node (named the _PAN-Coordinator_), one or multiple _coordinators_, and one or multiple _end-nodes_[1]. The end-nodes receive (transmit) data only from (to) their own coordinators. The end nodes use beacons to synchronize with the network time. The network time is divided into multiple _beacon intervals_, each one containing \(m=2^{\text{BO}-\text{MO}}\) multi-superframes. Each multi-superframe is divided, in turn, into \(n=2^{\text{MO}-\text{SO}}\) superframes. The parameters BO, MO and SO (named _Beacon order_, _Multi-superframe order_ and _Superframe order_, respectively) are configurable. Each superframe, as it is shown in Figure 1, includes the beacon slot and two sections for data transmission, i.e., the Contention Access Period (CAP) and the Contention-Free Period (CFP). The CAP is used to transmit the aperiodic traffic. During the CAP, no scheduling of source-destination node pairs is used, therefore each node can start the transmission according to the CSMA/CA access mechanism. Conversely, the CFP is divided into multiple GTSs that are used to transmit periodic traffic in a TDMA-like strategy. The duration of the CAP and of the CFP slots is customizable. DSME provides diverse additional options, such as the CAP reduction and the Group Acknowledgement (ACK). The former enables CAP only in the first superframe of a multi-superframe, thus providing the remaining superframes in the multi-superframe with a longer CFP, i.e., 15 GTSs instead of 7. The latter option, instead, allows aggregating the ACK of multiple frames into a single ACK frame, thus improving the energy efficiency. The Group ACK also allows specifying a GTS for the retransmissions of the frames not correctly received and reduces the delay, as it provides a retransmission opportunity within the same multi-superframe in which the original frame was transmitted. **Motivation. ** In DSME, each GTS within the multi-superframe can be assigned to at most one flow.