## 3 Existing Chain Oriented Topologies  
Chain oriented topologies have been used by researchers in designing various protocols, among which data broadcasting protocols, data collection/gathering protocols and routing protocols are the major instances. Chain topologies are mainly used in these protocols to reduce the total energy consumption, and thus to increase the lifetime of the network. This section discusses different protocols, which use chain oriented topologies. [PERSON] and [PERSON] present several chain oriented data broadcasting and data collection/gathering protocols for sensor networks [27,37]. They investigate broadcast problems in sensor networks and adopt a chain oriented approach for situation awareness systems, where networked sensors track critical events via coordination. They propose a linear-chain scheme for all-to-all broadcasting and data gathering. They also propose a binary-combining scheme for data gathering, which divides each communication round into levels in order to balance the energy dissipation in sensor networks. For broadcasting, the linear-chain scheme starts data transmission with a packet at the beginning of a chain. Each node along the chain attaches its own data to this packet. Eventually, information from the entire network reaches the end of the chain. The same procedure runs in the reverse direction to complete all-to-all broadcasting. The linear-chain scheme can also be applied to gather data in sensor networks. To gather data, each node senses and transfers information along the chain to reach one particular node that will send data to a remote BS. Such a scheme is named PEGASIS (Power-Efficient Gathering in Sensor Information Systems) [27]. PEGASIS is the first protocol that uses chain oriented topology for periodic data collection from the target field. PEGASIS forms a chain of the sensor nodes and uses this chain as the basis for data aggregation. In PEGASIS, the chain is formed using a greedy approach, starting from the node farthest to the sink. The nearest node to this is added as the next node in the chain. This procedure is continued until all the nodes are included in the chain. A node can be in the chain at only one position. Figure 2(a) shows the chain creation method. In this figure, the node \(C0\) lies furthest from the BS. Chain construction starts from the node \(C0\), which connects to the node \(C1\) as \(C1\) is the closest node to \(C0\). The node \(C1\) then connects to its closest node \(C2\), and so on. In this fashion a chain \(C0-C1-C2-C3-C4-C5\) is created. Figure 2(b) shows the data collection strategy adopted by PEGASIS. In the constructed chain, a leader node for each round is selected randomly. The authors argue that randomly selecting a head node is beneficial as nodes are more likely to die at random locations, thus providing robust network. All nodes send their data to the leader node, and then, the leader node sends the data to the BS. For example, in Figure 2(b), \(C3\) is selected as the leader node. The node \(C5\) passes its data to the leader node \(C3\) via the node \(C4\). PEGASIS suffers from several problems. First, in this protocol the role of the leader node changes in every round of data collection. This causes extra overhead. Moreover, when a node is selected as the leader, the protocol considers neither the distance of the node from the BS nor its energy level. Additionally, the chain in PEGASIS is constructed by a greedy algorithm. Using this chain causes some problems, such as an unexpectedly long transmission time, and non-directional transmission to the BS. These problems adversely affect the energy efficiency of WSNs. All nodes in sensor networks transmit their data in order. Therefore, the delay increases linearly as the number of nodes increases. Thus, PEGASIS is not scalable for large-scale WSNs. PEGASIS also causes redundant transmission of data as a result of having a single leader. To resolve the delay problem of PEGASIS, a 3-level PEGASIS was proposed. In 3-level PEGASIS, the chain is cut into several chains.