## 5 Data Communication Phase
### Intra-Cluster Transmission
#### 5.1.1 Single-Hop  
In the LEACH algorithm, once the clusters are formed, the CH broadcasts a TDMA schedule to cluster member giving the order in which they can transmit their sensed data. One cycle of data transmission by all cluster members is called a frame and every node is dedicated one slot of the frame. After the finish of data communication by the node in the last slot, the schedule is repeated from the first slot until the current round ends. However, the uneven necessity of the nodes to have access to the transmission channel motivates the authors of [57] to address the issue of fixed allocation of TDMA slots to the cluster members by CHs, proposed by the LEACH algorithm. The paper introduces a dynamic TDMA schedule, which adapts to changing load conditions. In the proposed algorithm, sensor nodes are allocated varying time slots according to their traffic in the previous round. Therefore, the nodes with higher rate of data traffic in the current round will be allocated more time slots for the next round, thus they can accommodate high data traffic requirements. On the other hand, the nodes with less data traffic will save their energy with shorter time slots or longer sleeping times. The algorithm saves the energy expenditure of the nodes in idle mode for the nodes with less traffic, while improves the network real-time transmission by the decrease of data-buffering time in sensor nodes with high level of data traffic. However, the paper's contribution is based on the high likelihood of the continuous of the same traffic load of the nodes in current round for the future rounds, which may not be applicable to all scenarios.  
Due to the type of implementation, sensor nodes usually scattered non-uniformly within the network field. Weighted LEACH (W-LEACH) [128] is able to handle non-uniform networks and increases the network lifetime. The main contribution of this protocol is to modify the intra-cluster data communication phase of LEACH to make it adaptable to the non-uniform distribution of sensor nodes. W-LEACH assigns weights to each sensor nodes based on the number of its neighbouring sensors (node density of that area) and its remaining energy. The algorithm uses nodes' weights to decide on the competency of the nodes to send data to their CHs. It is important to ensure that sensor readings of the areas having low densities are not overlooked because of fewer numbers of transmitting nodes. To address all the mentioned issues, the weight of each node (w\({}_{i}\)) is defined in Equation (24):  
\[w_{i}=\left\{\begin{array}{ll}E_{r-i}*d_{i},&\mbox{ if }d_{i}>d_{thresh}\\ d_{i}&\mbox{ otherwise}\end{array}\right. \tag{24}\]where \(d_{i}=(1+\text{number of alive sensors in range r})\)(number of alive sensor nodes) is the node density in \(r\) radius of node \(i\), E\({}_{r+i}\) is remaining energy of node \(i\), and \(d_{\text{thresh}}\) is the density threshold of defining low density areas. Based on Equation (24), the sensor nodes with low weights in each cluster are chosen to send data to the CH. Therefore, the chance of the node from low-density areas to be selected to send their data is more. Using this approach, a uniform data gathering scheme is provided and the level data redundancy decreases extremely. The only drawback of the algorithm is the more frequent selection of the nodes from the areas with node density lower than \(d_{\text{thresh}}\). This may lead to faster demise of these nodes and hence creation of uncovered areas. The solution presented for this issue by the paper is to sustain data transmission of this type of nodes by some probabilities for some rounds. However, this solution is in contrast with the main objective of the paper and it may result in loss of sensor readings of an area of the network.