# Wake-Up Receiver-Based Routing for Clustered Multihop Wireless Sensor Networks
## 3 Related Work
The data communication in this strategy is based on forwarding of WuPts. This means that if the destination cannot be woken up directly, the relays forward the WuPt. If the destination is woken up, the source starts sending a data packet. Unlike our approach, where all routing information is stored at the Fog node, here, all sensor nodes have to determine the routes opportunistically on their own. This requires a large number of packets to be sent, which has a negative impact on the overall efficiency of the network. Since the relays have to receive and retransmit WuPts, another drawback in time and energy consumption occurs. The work published in [28] is the Load Balancing Parent Selection (LoBaPS) protocol that supports opportunistic parent selection. In this strategy, the WuPt contains information about the node's own rank (in a layered topology) together with a unique application ID (enumeration of WuPts). Each node in the communication range of the sender will receive this WuPt and compare the received rank with its own rank, and only wakes up its main radio if the former is higher than the own rank. In this case, each sensor node receives the DataPt sent by the source. However, only the sensor node that has the shortest CSMA (Carrier Sense Multiple Access) backoff forwards the WuPt followed by the data packet. The significant disadvantage of this strategy is the amount of energy wasted in listening mode when all the feasible successors wake up their main radio. Since only one sensor node per rank is included in the data forwarding process, but all sensor nodes in this rank receive the WuPt as well as the data packet, this leads to significant energy consumption. In addition, the most reliable route is chosen every transmission round. This leads to an uneven distribution of energy consumption and, therefore, most used nodes acting as relays can prematurely run out of power. In contrast, the approach we have developed has the advantage that sensor nodes, which are actually necessary for data forwarding, are specifically awakened. As the entire communication route is already known, it enables the Fog node to realise a time-efficient and energy-saving wake-up and data transmission. Authors in [29] propose a strategy for multi-hop wake-up relay for CTP-WuR in which the Collection Tree Protocol is modified to work with Wake-up Radios. The feature of this approach is that the intermediate nodes are not woken up; they only forward the wake-up signal. If a node wants to wake up its parents two wake-up signals need to be transmitted. The first signal contains the address of the node via which the wake-up signal must be forwarded by means of the WuRx. The second signal contains the address of the destination node that the message should ultimately reach. If the address contained in the second signal does not match with the address of a node, the node sends its own pair of WuPt signals to act as a relay. The disadvantage of this strategy is that multiple WuPts have to be sent to wake up a single sensor node. This leads to unnecessary sending procedures, which are energy and time inefficient. Compared to our work, it is not necessary to send WuPt several times. With our addressing mechanism, we are able to wake up individual sensor nodes in an energy-efficient manner. On the one hand, this means that fewer packets are sent, which leads to reduce the risk of collisions. On the other hand, our approach offers a time and energy saving advantage by sending the WuPt once. The presented protocol, called G-WHARP [30], is a wake-up radio-based forwarding strategy for wireless networks equipped with energy harvesting capabilities aiming to minimise energy consumption and end-to-end latency. Wake-up semantic addressing (energy-related aspects based on a Markov Decision Process (MDP)) is used to avoid waking up devices with no forwarding availability. In this strategy, the initiator node sends a broadcast WuPt that wakes up all sensor nodes with the corresponding semantic.