Essay: LEACH

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LEACH is completely distributed, requiring no control information from the base station. Nodes do not require knowledge of the global network. It runs with many rounds. Each round begins with a set up phase. When the clusters are organized, it is followed by a steady state phase when data are transferred from the node to the cluster head and on to the base station.
Steady-State Frame Round
Figure 4.2 Time line showing operation of LEACH
The steady state phase duration is usually much longer than set up phase duration. However,the first phase is more important, in which sensor nodes are allowed to elect themselves as cluster heads randomly and then divided into clusters.Each node that becomes the cluster head will create a TDMA schedule for the sensor nodes within the cluster.That allows the radio components of each non cluster head node to be turned off all times except during their transmit time.
LEACH is the most popular hierarchical cluster based routing protocol for a wireless sensor network .In LEACH, the nodes in the deployed area are organized into local clusters and the communication process is divided into rounds with each round including set-up and steady-state phases.
Set-Up Phase
After finishing the deployment of sensor nodes, each node in the monitor field decides independently of other nodes whether it can become a cluster head in the current round. During the phase, each node generates a random number between 0 and 1 and then compares the threshold value with there is the percentage of cluster heads over all nodes in the network, is the number of rounds of selection, and is the set of nodes that have not been selected as cluster heads in round . The node whose number is larger than the threshold will select itself as a cluster head and then broadcasts the message to its surround sensor nodes. In this phase, a node may receive more than one broadcast message from different cluster heads, but the node can judge its distance to a cluster from the strength of received broadcast signal; the stronger the signal, the closer to a cluster. So the node whose number is smaller than threshold will only send request message containing its ID to the cluster which has the strongest signal strength for saving energy spent on the transmitting distance. Once the cluster head receives request message coming from one node, it records the node’s ID and proclaims it as its member node. After the message exchanges between cluster heads and normal nodes, each CH gets its own member nodes’ information about IDs and each normal node gets which cluster it belongs to. Based on the message it records, the CH creates a TDMA schedule table and broadcasts it to the cluster members. Therefore, all the member nodes get their idle slots for data transmission, and then the steady-state phase starts.
Steady ‘state phase The establishment of a cluster head in each cluster during the set-up phase provides a guarantee for the data transmission in a steady-state phase. In normal circumstances, member nodes can turn off their radio until they sense the necessary environment data. If there are some data in need to transmit, they will send the data to CH during the idle slots recorded in the TDMA schedule table. As for the CHs, they have to keep up communication status at all times so as to receive the data from different member nodes. After receiving all the data sent by their members, CHs will aggregate them firstly and then send them to BS. Because some sensor nodes may sense similar environment data, the aggregation on the cluster head can diminish unnecessary bandwidth cost and communication traffic, which has a positive reflection to the energy’s consumption. Also, the data transmission distance becomes shorter comparing with transmitting to BS separately for each member node, which can save some energy for the member nodes. However, the heavy tasks executing on CH can lead to too much energy consumption. In order to avoid making the CHs die early and cause the cascade effect in the network, a new round begins and new clusters will be rebuilt in the whole network.
Based on the LEACH, LEACH-C also organizes the sensor nodes into clusters with each cluster a cluster head and divides a round into set-up and steady-state phases. It differs from LEACH only in that it uses a high-energy base station to finish the choice of cluster heads. In the set-up phase of each round, every sensor node sends its information about energy to remote BS. Then the BS selects the cluster heads based on the energy information and broadcasts the IDs of cluster heads to other member nodes. This method can make the nodes with more energy and more chance to become the cluster head in the current round. But in this phase, every sensor node needs to send its ID and energy information to remote BS to compete for the role of cluster heads, which causes energy consumption on the long distance transition.
Although LEACH and LEACH-C protocols act in a good manner, they also suffer from many drawbacks like the following
(i)CHs’ selection is random, which does not take into account the residual energy of every node or need the support of BS.
(ii)The high frequency of reclustering wastes a certain amount of energy.
(iii)It cannot cover a large area.
(iv)CHs are not uniformly distributed, where CHs can be located at the edge of the cluster.
Figure 4.3 Flow Chart of Leach CC
4.4.1. INTRODUCTION TO LEACH CC: Leach CC(Centralized with Chain)Based on the research of LEACH protocol, a low energy-consumption chain-based routing protocol LEACH-CC. Leach CC, each node will send information about its current location and energy level to the base station. The base station runs the simulated annealing algorithm to determine the clusters for that round. Then a chain routing between clusters is established to reduce the amount of nodes which communicate with the base station. LEACH-CC performs better than LEACH protocol. It not only extends the lifetime of the network, but also improves the energy efficiency.
Set-up: During the set-up phase of LEACH-CC, each node sends information about its current location and energy level to the base station. The base station runs an optimization algorithm to determine the clusters for that round. The clusters formed by the base station will in general be better than those formed using the distributed algorithm. However, LEACH-CC requires that each node transmit information about its location to the base station at the beginning of each round. This information may be obtained by using a global positioning system (GPS) receiver that is activated at the beginning of each round to get the node’s current location.
In addition to determining good clusters, the base station needs to ensure that the energy load is
evenly distributed among all the nodes. To do this, the base station computes the average node
energy, and whichever nodes have energy below this average cannot be cluster-heads for the current round. Using the remaining nodes as possible cluster-heads, the base station runs a simulated annealing algorithm to determine the best k nodes to be cluster-heads for the current round. This algorithm minimizes the amount of energy the non-cluster-head nodes will have to use to transmit their data to the cluster-head, by minimizing the total sum of squared distance between all the non-cluster-heads and the closest cluster-head.
Once the optimal cluster-heads and associated clusters are found, the base station transmits this
information back to all the nodes in the network. This is done by broadcasting a message that
contains the cluster-head ID for each node. If a node’s cluster-head ID matches its own ID, that
node takes on the cluster-head role; otherwise, the node determines its TDMA slot for data
transmission and goes to sleep until it is time to transmit data to its cluster-head.
The Steady-State:The steady-state operation is broken into frames, where nodes send their data to the cluster-head at most once per frame during their allocated transmission slot. The cluster-head must keep its receiver on to receive all the data from the nodes in the cluster. Once the cluster-head receives all the data, it can aggregate the data. Since the base station may be far away, the difference from LEACH is to use multi-hop routing by forming chains between cluster-heads and selecting only one cluster-head to transmit to the base station instead of using multiple cluster-heads to reduce the dissipation of the energy.
Fig. 4.5 shows the time-line for a single round of LEACH-CC, from the time clusters are formed during the set-up phase, through the steady-state operation when data are transferred from the nodes to the cluster-heads and forwarded to cluster-head to cluster-head, finally to the base station.
Set-up Steady -state
Cluster Formed Slot For Node i Slot For Node i Slot For Node i
Clusters Chain
Fig. 4.4 Time-line showing LEACH-CC operation
The main idea in LEACH-CC is for each cluster-head to receive form and transmit to close
neighbor cluster-head and take turns being the leader for transmission to the base station. This
approach will distribute the energy load evenly among the sensor nodes in the network. For
constructing the chain, we assume that all nodes have global knowledge of the network and employ greedy algorithm. The greedy approach to constructing the chain works well and this is done at the first frame of steady-phase. We choose the cluster-head which has the highest energy level as the leader at the current round of communication. This is important for cluster-heads to complete the high-energy dissipation task. As shown in Fig. 4 cluster-head c0 passes its data to cluster-head c1.
Cluster-head c1 aggregates cluster-head c0’s data with its own and then transmits to the leader.
After cluster-head c2 passes the token to cluster-head c4, cluster-head c4 transmits its data to
cluster-head c3. Cluster-head c3 aggregates cluster-head c4’s data with its own and then transmits to the leader. Cluster-head c2 waits to receive data form both cluster-heads and then aggregates its data with its neighbors’ data. Finally, cluster-head c2 transmit one message to the base station.

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