# Wake-Up Receiver-Based Routing for Clustered Multihop Wireless Sensor Networks
## 4 Proposed Approach  
In our proposed system, there are two types of nodes: powerful nodes (in aspects of energy and transmission power) that act as the sink or Fog Nodes and energy-critical cluster nodes that act as cluster members. Each Fog Node is main powered and is able to send messages directly to every node in its cluster. Whereas, battery powered cluster members use very low transmitting power. For this reason, when a given cluster member node A is placed faraway from the Fog Node, intermediate nodes are needed to relay the data from that node A to its corresponding Fog node. There is hence an _asymmetric link_ between each Fog Node and its cluster members.  
The sensor nodes in each cluster are equipped with a WuRx and are only activated when required by a special RF signal (WuPt). If these sensor nodes are not activated, the entire sensor node remains in sleeping mode and thus does not consume any energy. As already mentioned, data communication within a wireless sensor network is a very energy-consuming process, which is why the individual sensor nodes use a reduced transmission power around \(-34\) dBm. This ensures that these energy-critical sensors send data in an energy-efficient way.  
To return to the description of an individual cluster, as shown in Figure 2, we go into the formation in more detail. It should be noted that each cluster is assigned to a single Fog Node, to which eight sensor nodes are assigned. The Fog Node is the only network participant that enables communication between the sink and the individual sensor nodes of a cluster. However, data communication within a cluster is possible via multi-hop. This means that the individual sensor nodes can communicate with each other and send data packets to the Fog Node using cluster nodes as relays.  
Focusing on an energy-efficient and delay-minimising communication strategy using WuRx, we have developed a novel energy- and time-efficient strategy. Since, in a WuRx-based WSN, the sensor nodes are in sleeping mode, it is necessary to wake up these nodes in order to use them for data transmission. The most trivial way is that each node sends a wake up message to the next node that will be used as the relay until reaching the destination. However, such a strategy is too time consuming because, in each step, we need to wait for the intermediate node to wake up and become ready to transmit the packet. In health care scenarios, such as in an elderly or Alzheimer's patient's household, the supervision of the patient is needed to have very short reaction time when a patient faces health problems. Therefore, even a few additional milliseconds make the difference in the health status of the patient. Thus, the said strategy is inconvenient for time-restricted application.  
Our routing strategy aims to overcome this limit by allowing the Fog Node to wake up multiple sensor nodes simultaneously. This ensures a timely waking up of the nodes  
Figure 2: Scenario of asymmetric communication using multicast WuPt transmission using 1, 2, or 3 cluster Nodes as relay.  
that will act as relays from the source to the Fog Node. To enable these multiple wake-ups, the Fog Node has a list of the addresses of all associated cluster members. To know which are the relay nodes that will be woken up, the Fog Node uses source routing. It maintains in its routing table the list of nodes that will participate in the routing process.