4.4.

LEACH, PEGASIS
LEACH
The LEACH protocol (Low-energy Adaptive Clustering Hierarchy) presented by Heinzelmanet al.
assumes a dense sensor network of homogeneous, energyconstrained nodes, whichshall report their data
to a sink node. In LEACH, a TDMA-based MAC protocol is integrated withclustering and a simple
―routing‖ protocol
LEACH partitions the nodes into clusters and in each cluster a dedicated node, the clusterhead, is
responsible for creating and maintaining a TDMA schedule; all the other nodes of a cluster aremember
nodes. To all member nodes, TDMA slots are assigned, which can be used to exchange data between the
member and the clusterhead; there is no peer-to-peer communication. With the exception of their time
slots, the members can spend their time in sleep state. The cluster head aggregates the data of its
members and transmits it to the sink node or to other nodes for further relaying. Since the sink is often
far away, the clusterhead must spend significant energy for this transmission. For a member, it is
typically much cheaper to reach the clusterhead than to transmitdirectly to the sink. The clusterheads role
is energy consuming since it is always switched on and is responsible for the long-range transmissions. If
a fixed node has this role, it would burnits energy quickly, and after it died, all its members would be
―headless‖ and therefore useless. Therefore, this burden is rotated among the nodes. Specifically, each
node decides independent of other nodes whether it becomes a clusterhead, and therefore there is no
signaling traffic related to clusterhead election (although signaling traffic is needed for subsequent
association of nodes to some clusterhead). This decision takes into account when the node served as
cluster head the last time, such that a node that has not been a clusterhead for a long time is more likely
to elect itself than a node serving just recently. The protocol is round based, that is, all nodes make their
decisions whether to become a clusterhead at the same time and the noncluster head nodes have to
associate to a clusterhead subsequently. The nonclusterheads choose their cluster head based on received
signal strengths. The network partitioning into clusters is time variable and the protocol assumes global
time synchronization. After the clusters have been formed, each clusterhead picks a random CDMA code
for its cluster, which it broadcasts and which its member nodes have to use subsequently. This avoids a
situation where a border node belonging to clusterhead A distorts transmissions directed to clusterhead
B, shown in Figure 5.10.
A critical network parameter is the percentage of nodes that are cluster heads. If there are only a few
clusterheads, the expected distance between a member node and its cluster head becomes longer and
therefore the member has to spend more energy to reach its cluster head while maintaining a given BER
target. On the other hand, if there are many clusterheads, there will be more energy expensive
transmissions from clusterheads to the sink and less aggregation.
Therefore, there exists an optimum percentage of clusterheads, which for the scenario investigated in
is≈5%. Ifthis optimum is chosen, LEACH can achieve a seven to eight times lower overall energy
dissipationcompared to the case where each node transmits its data directly to the sink, and between four
and eight times lower energy than in a scenario where packets are relayed in a multi hop fashion. In
addition, since LEACH distributes the clusterhead role fairly to all nodes, they tend to die at about the
same time.
The protocol is organized in rounds and each round is subdivided into a setup phase and asteady-state
phase (Figure 5.11). The setup phase starts with the self-election of nodes to cluster heads. In the
following advertisement phase, the clusterheads inform their neighborhood with an advertisement
packet. The clusterheads contend for the medium using a CSMA protocol with no further provision
against the hidden-terminal problem. The nonclusterhead nodes pick the advertisement packet with the
strongest received signal strength. In the following cluster-setup phase, the members inform their
clusterhead (―join‖), again using a CSMA protocol. After the cluster setup-phase, the clusterhead knows
the number of members and their identifiers It constructs a TDMA schedule, picks a CDMA code
randomly, and broadcasts this information in the broadcast schedule subphase. After this, the TDMA
steady-state phase begins
Because of collisions of advertisement or join packets, the protocol cannot guarantee that each
nonclusterhead node belongs to a cluster. However, it can guarantee that nodes belong to at most one
cluster.The clusterhead is switched on during the whole round and the member nodes have to be
switchedon during the setup phase and occasionally in the steady-state phase, according to their position
in the cluster‘s TDMA schedule.With the protocol described so far, LEACH would not be able to cover
large geographical areasof some square miles or more, because a clusterhead two miles away from the
sink likely does not have enough energy to reach the sink at all, not to mention achieving a low BER. If
it can be arranged that a clusterhead can use other clusterheads for forwarding, this limitation can be
mitigated.

PEGASIS
– Power Efficient Gathering in Sensor Information Systems is one such hierarchical routing protocol
which follows a chain based approach and a greedy algorithm. The sensor nodes organize themselves to
form a chain. If any node dies in between then the chain is reconstructed to bypass the dead node. A
leader or a cluster head node is assigned and it takes care of transmitting data to the base station/ sink
node. The main goal of PEGASIS is to receive and transmit data to and from the neighbour and take
turns being the cluster head for transmission to the Sink Node
PEGASIS (Power-Efficient GAthering in Sensor Information Systems) is a communication protocol
designed for Wireless Sensor Networks (WSNs). PEGASIS introduces a novel approach to data
gathering and transmission by organizing sensor nodes into a linear chain. This protocol aims to improve
energy efficiency and prolong the network's lifetime, making it suitable for applications with constrained
energy resources. Here are the key features and mechanisms of PEGASIS:
Linear Chain Formation:
1. Topology:
 Explanation: PEGASIS organizes sensor nodes into a linear chain structure. Each node in the
chain communicates with its neighboring nodes, forming a sequence through which data is
transmitted towards the base station.
2. Communication Flow:
 Explanation: Data is passed from one node to the next in the linear chain, creating a relay
mechanism. Each node aggregates its data with that of its neighbors before transmitting to the
next node in the chain.
3. Neighboring Node Interaction:
 Explanation: Nodes communicate only with their immediate neighbors in the chain. This
limited communication range reduces the energy consumption associated with long-range
transmissions.
Data Aggregation and Processing:
1. Aggregation along the Chain:
 Explanation: PEGASIS promotes data aggregation along the linear chain. As data is relayed
through the nodes, each node aggregates its data with the information received from its
 neighbors before passing it on.
2. Collaborative Processing:
 Explanation: Nodes in the chain have the ability to perform collaborative data processing. This
means that individual nodes can execute computations on the aggregated data before passing it
to the next node, allowing for in-network processing and reducing the need for centralized
processing.
Energy Efficiency and Trade-offs:
1. Reduced Communication Overhead:
 Explanation: PEGASIS minimizes communication overhead by limiting transmissions to
neighboring nodes in the linear chain. This approach reduces the energy consumption
associated with long-distance transmissions and enhances overall network efficiency.
2. Trade-off Between Latency and Energy:
 Explanation: PEGASIS involves a trade-off between communication latency and energy
efficiency. While the linear chain structure reduces communication delays, it introduces the
need for multi-hop communication. The trade-off depends on the specific requirements of the
application.
3. Adaptability to Node Failures:
 Explanation: PEGASIS is adaptive to node failures. If a node in the linear chain fails, the
communication paths can be rerouted dynamically, allowing the network to continue
functioning even in the presence of node failures.
Applications:
1. Surveillance and Monitoring:
 Explanation: PEGASIS is well-suited for applications such as surveillance and environmental
monitoring, where nodes are deployed along a linear path. The linear chain structure simplifies
the routing and communication patterns in such scenarios.
2. Energy-Constrained Environments:
 Explanation: PEGASIS is particularly useful in energy-constrained environments, where
prolonging the network's lifetime is crucial. By reducing communication overhead and enabling
in-network processing, PEGASIS helps conserve energy.
3. Applications with Linear Deployment:
 Explanation: PEGASIS is effective in scenarios where sensor nodes are deployed in a linear
fashion, such as monitoring a pipeline or a road. The protocol leverages the spatial deployment
pattern to optimize data gathering and transmission.
In summary, PEGASIS is a unique WSN protocol that introduces a linear chain structure for efficient data
gathering and transmission. By leveraging the collaborative processing capabilities of nodes and
minimizing communication overhead, PEGASIS aims to improve energy efficiency and prolong the
operational lifetime of wireless sensor networks. The protocol's suitability depends on the specific
requirements and characteristics of the target application.