# A Robust and Energy-Efficient Weighted Clustering Algorithm on Mobile Ad Hoc Sensor Networks +
## 4 Communication and Topology Maintenance
### Inter-Cluster and Intra-Cluster Communication  
In the description of RE\({}^{2}\)WCA, we assume single-hop communication among CHs and their registered cluster members. This is desirable in source-driven networks, where reports are periodically transmitted by nodes. Hence, a Time Discrete Multiple Address (TDMA) frame may be constructed at each \(CH\) to eliminate interference within a cluster. Obviously, constructing TDMA frames need well synchronization and in a lightly-loaded network, adopting TDMA frames may waste frequent resources. Therefore, a better way is to use channel contention. Multipath routing can increase channel capacity in this case. In intra-cluster communication, in order to reduce the collisions of data, each \(CH\) will create a TDMA scheme for its nodes and broadcasts it among cluster members. Every node sends its data to its \(CH\) according to specified TDMA schedule. In order to avoid the interference caused by signals from a nearby cluster, \(CH\) can set the Code Discrete Multiple Address (CDMA) code of all nodes within the cluster. The CDMA code, along with the TDMA, are sent at scheduled times [2; 3; 19; 34]. However, in intra-cluster communication, for example if two nodes from different cluster need to communication, then the multi-path routing may work better. Clearly, the collisions caused by data forwarding are nearly unavoidable, therefore the capacity and throughput will decrease dramatically. Additionally, if it adopts a way of saving stable routing table, the great communication overhead will degrade the network performance. In order to solve this problem, some routing protocols are proposed, such as Directed Diffusion or Dynamic Sourcing Routing (DSR) [36; 31]. The protocols are mainly to reduce communication overhead through increasing the computing cost since computing cost is relatively much less compared with communication overhead. As is shown in Figure 7, TDMA schemes are adopted for the intra-cluster communication. Meanwhile, as is shown in Figure 8, the inter-cluster communication adopts the CDMA schemes.  
Figure 8: CDMA schemes adopted for the inter-cluster communication.  
Figure 7: TDMA schemes adopted for the intra-cluster communication.  
In RE\({}^{2}\)WCA, the proposed two fault tolerant algorithms which are fault detection algorithm and _CH_ backup algorithm respectively need to consider the synchronization. When _CH_ find fault node, it needs to transmit data to sink node through inter-cluster communication. If some clusters failed to accomplish the process of fault detection, then the data forwarding cannot succeed. In Figure 6, only when all nodes finish the detection process during _time_fn2_, the topology maintenance will be triggered. Hence, a suitable way is to send Hello message packets among all CHs to ensure every node is in a waiting state (in fact has nothing to do). Actually, the waiting of other CHs will degrade the instantaneity of the network because if just one _CH_ has not implemented the fault detection, all nodes need to wait. But if a cognitive routing algorithm is adopted, the synchronization will be solved. The _CH_ can find a best dynamic route to transmit the data. However, whatever the routing protocol has been adopted, the synchronization should not be a hard problem. On the one hand, the time waiting is not worthy of paying much attention because when _CH_ are in waiting state, the energy consumption is nearly limited to zero. Actually, this is a tradeoff between instantaneity and reliability. On the other hand, the great difference among clusters is not common in real situations. After sink node receiving the fault data CHs forward, the topology maintenance process will be triggered. Sink node collects the fault information of nodes so that upload to higher class network. Meanwhile, sink node will broadcast packets to relevant clusters to tell them re-clustering.