## 3 Proposed Reliable Routing Mechanism
### Reliable Intra-cluster Routing  
The intra-cluster routing is based on slightly modified table-driven routing mechanisms like Destination Sequence Distance Vector (DSDV). In this routing mechanism, the node periodically exchange routing information. A node broadcasts its table to its neighbors once its routing table changes. This approach limits the exchange range within the cluster and its next hop to reduce the control overhead and interference with the shared media. Every node maintains routing information of other nodes within its cluster, including destination, next hop, cluster id of destination node, metric, sequence number and accumulating the reliability of each node between the source and destination during route discovery. Moreover, the border node in each adjacent cluster is added to the local routing table, enabling routing to adjacent clusters. The above routing information is adopted for routing selection in intra-cluster routing. No cluster head is elected in charge of transmission and routing maintain, thus avoiding the bottlenecks and reducing the control packets of choosing the cluster head. Figure 7 illustrates the intra-cluster routing algorithm.  
Table 1 shows the routing information of node N\({}_{33}\) given in Figure 8. Node N\({}_{33}\) can easily deliver the packets to node N\({}_{38}\) by the routing information in Table 1. When the packets are transmitted to N\({}_{13}\), which is located on the adjacent cluster C1, node N\({}_{33}\) has no route to N\({}_{13}\) in its routing table.  
Figure 7: Intra-cluster routing algorithm.  
However, node N\({}_{33}\) knows that destination node N\({}_{13}\) belongs to cluster C1, and discovers that the route to C1 is available in the second entry of Table 1 (selected by the best reliable path). Accordingly, node N\({}_{33}\) sends the packets to the next hop N\({}_{31}\). The packets are sent to node N\({}_{15}\) followed by node N\({}_{13}\) when this step is applied iteratively. The Intra-cluster routing of Cluster C1 is similarly adopted to convey the packets to N\({}_{19}\) iteratively.  
Moreover, these gateways, which are the border nodes to other clusters, are selected while the intra-cluster routing scheme is being built. Hence, no extra effort is required for gateway selection, and overhead is reduced. Figure 9 shows an example of this scheme.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cluster ID** & **Dest. ID** & **Next** & **Metric** & **Seq. No** & **Reliability** \\ \hline C2 & N\({}_{24}\) & N\({}_{34}\) & 2 & N\({}_{24}\)-200 & 0.94 \\ C1 & N\({}_{15}\) & N\({}_{31}\) & 2 & N\({}_{15}\)-100 & 0.96 \\ C1 & N\({}_{16}\) & N\({}_{31}\) & 2 & N\({}_{16}\)-110 & 0.95 \\ C4 & N\({}_{44}\) & N\({}_{35}\) & 3 & N\({}_{44}\)-200 & 0.93 \\ C4 & N\({}_{44}\) & N\({}_{36}\) & 4 & N\({}_{44}\)-320 & 0.90 \\ C3 & N\({}_{31}\) & N\({}_{31}\) & 1 & N\({}_{33}\)-110 & 0.97 \\ \(\cdots\) & \(\cdots\) & \(\cdots\) & \(\cdots\) & \(\cdots\) & \\ \hline \hline \end{tabular}
\end{table}
Table 1: N33’s routing information.  
Figure 8: Intra-cluster routing.  
Cluster C3 can communicate with C1 _via_ gateway N15, and with C4 _via_ N44. Moreover, two adjacent clusters may have more than one border node, which constitute multi-path gateways to adjacent clusters. Multi-path gateways enhance the fault tolerance, since the sensor node failure caused by blockage due to power depletion, physical damage or the power-saving schedule would not affect the inter-cluster communication in the sensor network. Additionally, multi-path gateways enhance the load balance, avoiding wear problems. Figure 9 shows that cluster C3 can communicate with C1 by passing either gateway N15 or N16. The proposed algorithm selects the best reliable node by default. The multi-path routing enhances the robustness of the wireless sensor network. If one gateway fails, then the packets can be sent _via_ another gateway.