## 4 Descriptions of Different Topologies of WSNs
### Chain-Based Topology  
In this topology, the protocols construct transmission chain(s) connecting the deployed sensor nodes to save energy dissipation of data transmission. A leader is selected in a chain that acts as the sink. All sensor nodes communicate with each other along the chain. A node sends data to the next node, which is called successor node of the former node, towards the leader node. A successor node, receiving data from the predecessor node, forwards the data to its successor node towards the leader. In this fashion, all sensor nodes send their sensed data to the leader node(s). This way of communication facilitates the data aggregation.  
PEGASIS [71] is an example protocol based on chain topology. In PEGASIS, every node in chain senses the data, receives data from its predecessor, fuses with received the predecessor's data and transmits to next node in chain. Data aggregation performs in-network fusion of data packets, coming from different sensors en-route to the base station, in an attempt to minimize the number and the size of data transmissions and thus save sensor energy.  
The basic idea of the PEGASIS protocol is that in order to extend network lifetime, nodes only need to communicate with their closest neighnours, and they take turns in communicating with the BS. When the round of all nodes communicating with the BS ends, a new round starts, and so on. This reduces the power required to transmit data per round as the power draining is spread uniformly over all nodes. Hence, PEGASIS has two main objectives. First, increase the lifetime of each node by using collaborative techniques. Second, allow only local coordination between nodes that are close together so that the bandwidth consumed in communication is reduced. The chain construction is performed in a greedy fashion. Simulation results showed that PEGASIS is able to increase the lifetime of the network to twice that under the LEACH protocol. Such performance gain is achieved through the elimination of the overhead caused by dynamic cluster formation in LEACH, and decreasing the number of transmissions and reception by using data aggregation.  
Although the clustering overhead is avoided, the protocol PEGASIS still requires dynamic topology adjustment since a sensor node needs to know about the energy status of its neighnours in order to know where to route its data. Such topology adjustment can introduce significant overhead, especially for highly utilized networks. On the other hand, PEGASIS introduces excessive delay for distant nodes on the chain. In addition, the single leader can become a bottleneck. Finally, although in most scenarios sensors will be fixed or immobile as assumed in PEGASIS, some sensors may be allowed to move and hence affect the protocol functionality.  
Figure 4 shows the chain-based topology used in PEGASIS. The circles represent sensor nodes whereas a bidirectional line between two nodes represents successor-predecessor relationship.  
Besides PEGASIS, there are also other protocols such as COSEN and CHIRON that use chain-based topologies. COSEN is probably the first chain-oriented topology that used multiple chains instead of a single chain. Here we summarize the advantages and disadvantages of chain-based topology  
Here advantages and disadvantages of chain-oriented topology are summarized.  
Figure 4: Chain-oriented topology architecture (used in PEGASIS).