## 5 Data Communication Phase
### Intra-Cluster Transmission
#### 5.1.2 Multi-Hop  
The protocol presented in [129], also addresses the issue of identical data transmission by the cluster members of a CH. The main objective of the algorithm is to minimize the average energy expenditure of clusters, while it restricts sensing and data transmission tasks to the nodes having higher level of energy in each cluster. To implement the solution, the proposed algorithm uses _Meta-data_, and adds two stages to the data communication phase. Meta-data is defined as an indicator of data type of the source node having smaller packet size than the sensing data. In this respect, the data collection process of the scheme starts by transmission of the meta-data of all cluster members to their corresponding CH. After collection of meta-data, according to the data correlation of the meta-data, the highest energized sensor node among each group of the nodes with identical meta-data is selected by CH as the representative of the group. Then, the CH informs the elected node to transmit the real sensing data and forward the received data to the BS. Performance of the algorithm is dependent on two factors of the ratio of the length between meta-data and sensing data packet, and level of data correlation. However, considering two extra phases of meta-data transmission and representative node selection, the algorithm is only beneficial for the type of applications which monitor phenomena with limited changes and high levels of correlation. A Multi-hop Two-phase clustering (TPC) scheme is proposed in [126] to minimize energy consumption in collecting sensed data while meeting delay constraints. The paper uses multi-hop transmission for sending sensed data to CH to distribute the CH's workload by reducing the number of direct-link communications of a CH with its cluster members. The clustering process is divided into two phases; in the first phase, like other clustering algorithms, the optimum clusters are constructed by CHs and in the second phase, the direct-link star-graph connection between CH and cluster members transforms to a multi-hop data relay communication link. The main idea of the paper is built upon the fact that the sensor nodes are densely deployed within the network field and the generated data of cluster members are highly correlated. Therefore, the data aggregation process can be done in the path towards the CH rather than in CHs only. However, delay constraint is an issue regarding multi-hop transmissions. The paper addresses this issue by using an _n-relay_ control message, which is broadcast by CH to cluster members. According to this _n-relay_ threshold value and forwarding index of the packet, each node decides whether to transmit the packet to the next RN or directly transmits it to the CH. Moreover, the algorithm implements CDMA/CA (code division multiple access/collision avoidance) as the medium access layer protocol instead of TDMA, because of relatively low cost of implementation and less scheduling problems of the cluster members by CDMA/CA over TDMA in a multi-hop communication algorithm. Based on the restriction of forwarding angle, an intra-cluster multi-hop routing algorithm is implemented in [124]. In this protocol, an intermediate node is selected according to the following conditions: first, it should satisfy the \"triangle inequality\", and second, it should belong to a limited-angle area, which is a restricting angle used to limit the forwarding scope of data packets. In wireless sensor networks, the energy consumption does not always satisfy \"triangle inequality\". Therefore, it is important to note that multi-hop transmission is not necessarily an energy saving process. Furthermore, the delay caused by relaying a packet through multiple hops is more than with direct communication. Thus, by considering the energy transmission model in Equation (21), multi-hop transmission is acceptable when the energy condition presented in Equation (25) is satisfied.