## 6 Signature Based Routing
Please note that \(\varphi\) value for every PAN is updated every time a communication takes place through their respective gateway. This update process takes place at the stage shown in Figure 14b and 14d. However, the \(\varphi\) is not updated if the latency is more than the signature value. This action keeps the signature either equal or less than the predefined (or application required) latency level (latency level is represented by the value of signature). The value of \(\varphi\) is also updated after the communication takes place through the new routing path (_i.e.,_ path forced by the gateways). However, signature \(S\) is recomputed only after the \(\varphi\) values are shared among gateways. Figure 13: SBR process taking place at gateways. \(S\) is the signature, \(X\) represents the threshold value and \(Y\) represents the current packet count. This sharing can be periodic or after every \(\varphi\) update. The update of \(\varphi\) and re-computation of \(S\) makes sure that the recent latency is added to the signature and makes the signature adaptive. If we analyze Figure 15, we notice that the initial signature value is 0.19. We have taken initial signature to the worst latency value shown in our experiments. The signature value is adaptive and is adjusted with every signature re-computation. We can notice that the node 3 has the latency equal to the signature value; at this stage gateways start monitoring the communication between the sender and the receiver. We will later see in the phase II that how gateways decide to enforce the new path. We can find few other nodes having value more than the signature; gateways monitor all those nodes and keep track of the latency. Please note that the signature is being continuously computed with every latency value. Figure 14: Stepwise overview of the SBR scheme (Phase 1: Hotline assisted Routing). Figure 15: Simulation results showing the end-to-end latency in the case packets are sent using hotline assisted routing. Signature value is also shown. We now review phase II. This phase deals with the process after the gateways start monitoring the nodes with latency greater or equal to the signature. As we saw node 3 had latency greater than signature; it gives the gateway right to decide if it wants to enforce the new SBR path. In order to do that gateway first needs to compare the node positions and see if the nodes are close enough (_i.e.,_ in terms of number of hops or distance). If nodes are close then the gateway enforces a new communication path for node 3 (as shown in Figure 16c). Similar procedure will be activated for all those nodes having latency greater than or equal to signature. However, in our simulation node 30 was the only other node that was enforced with a new path (after considering the topological information). Figure 17 shows the results of the simulation after implementing SBR. We can notice that the new path for node 3 (even for node 31) gives a better and more reliable alternative where latency is much less than the previous routing path. Figure 16: Stepwise overview of the SBR scheme (Phase II: New Path Enforcement). Figure 17: Simulation results showing the end-to-end latency in the case packets are sent using SBR. Signature value is also being shown. We can conclude that the SBR provides a good addition to our hotline approach [5] where we not only have reliable communication from source to the sink but also with less latency.