# An Energy Efficient Routing Approach for Cloud-Assisted Green Industrial IoT Networks
## 4 Preliminaries
### IEEE 802.15.4 Based Low-Power Wireless Networks  
The IEEE 802.15.4 standard is considered to be fit for constrained networks in the IIoT domain because it allows low-rate low-cost communications [46]. It defines the media access control (MAC) and PHY layers for low-rate wireless personal area networks. This standard is the basis for many other technologies, such as WirelessHART and ZigBee. Furthermore, it is also used with RPL and 6 LoWPAN for energy efficient operation. IEEE 802.15.4 based network supports two types of node full function devices and reduced function devices and can form a star, point-to-point, and cluster-tree topology. Principally, IEEE 802.15.4 defines two modes of operation, i.e., beacon-enabled and non-beacon, in which both operations make use of the carrier sense multiple access/collision avoidance (CSMA/CA) mechanism for channel access. In non-beacon-enabled mode, each node access the channel using a typical unslotted CSMA/CA technique, whereas beacon-enabled mode employs the superframe structure for channel access with slotted CSMA/CA mechanism. In our proposition, we take into account the beacon-enabled mode of operation because it enables an active and inactive section of the beacon interval for energy-efficient operation. Hence, in the following, we only describe slotted CSMA/CA and the superframe structure. Even though IEEE 802.15.4 protocol supports three different network topologies, beacon-enabled mode functions only in star or cluster-tree topology. The router or coordinator periodically transmits beacon frames to end devices to ensure node synchronization. This synchronization allows the device to sleep between coordinated transmissions to avoid idle listening, which results in increasing the life of the network. Since the beacon frames are broadcasted periodically, each beacon interval is divided into an active period, called a superframe, and an inactive period in which the node is in a low power state. Based on the duration of these two sections, the duty cycle of a node is determined. Figure 1 show the superframe structure of IEEE 802.15.4. The superframe structure is bounded by beacons and divided into equal 16 slots. The coordinator or router defines the format of the superframe based on the two fundamental parameters, which are beacon interval (BI) and superframe duration (SD). The coordinator transmits the next network beacons at the end of the BI, so BI is the time duration between two consecutive beacons. The BI is divided into two portions active and inactive period. The active portion of the BI duration is known as the SD in which frames can be transmitted. Further, the SD is composed of the fixed sized slots of a contention access period (CAP) and a contention free period (CFP). During CAP, any node wishing to communicate shall compete to access the channel using a slotted CSMA/CA mechanism. On the other hand, during CFP, no transmission use CSMA/CA to access the channel. IEEE 802.15.4 standard allows nodes to use their reserved slots by providing guaranteed time slots (GTS) for QoS demanding applications. In our proposition, to provide the emergency monitoring service guarantee we use GTS for transmitting real-time traffic. To save the energy nodes can sleep during inactive period and the router define this period by selecting BI must be greater than SD (\(BI\succ SD\)). The length of BI and SD is determined by the beacon order (BO) and superframe order (SO) as follows. \[ \begin{array}{l}BI\ =\ aBaseSuperframeDuration\times 2^{SO}\ 0\leq BO\leq 14\\ SD\ =\ aBaseSuperframeDuration\times 2^{SO}\ 0\leq SO\leq 14\end{array}  \tag{1}\]  
where _aBaseSuperframeDuration_ is the number of symbols defined by the standard based on the data rate and frequency. Therefore, it denotes the minimum superframe duration corresponding to \(SO\ =\ 0\), which is 15.36 ms while assuming 2.4 GHz frequency band and 250 kbps rate. Therefore, in our proposal, we consider this for defining the superframe duration.