## 1 Introduction
\begin{table}
\begin{tabular}{l l c l l l} \hline
**Symbols** & **Description** & **Eq.No.* & **Symbol** & **Description** & **Eq.No.* \\ \hline
**T(n)** & Threshold value & (1) & _Avg_ & Average number of 2-hopneighbors & (14) \\ \(P_{CH}\) & Desired percentage of CHs & (1) & _Thresh\({}_{tumor}\)_ & Lower threshold of cluster size & (15) \\ \(r_{e}\) & Current round number & (1) & _E\({}_{e\text{-}CH}\)_ & Remaining energy of CH & (16) \\ \(E_{e}\) & Current energy level & (2) & _D\({}_{N\text{-}CH}\)_ & Distance of node to CH & (17) \\ \(E_{i}\) & Initial energy level & (2) & _D\({}_{CH\text{-}BS}\)_ & Distance of CH to BS & (17) \\ \(r\) & Last round number & (4) & _MAX\({}_{N\text{-}BS}\)_ & Maximum distance from all CHs to BS & (17) \\ \(\text{E}_{r\text{-}dispute}\) & Node dissipated energy & (4) & _MIN\({}_{N\text{-}BS}\)_ & Minimum distance of all CHs to BS & (17) \\ \(\text{E}_{r\text{-}average}\) & Node average initial energy & (4) & _MAX\({}_{N\text{-}CH}\)_ & The distance of farthest CH in & (17) \\ \(\text{E}_{r}\) & Node remaining energy & (4) & _E\({}_{i\text{-}CH}\)_ & Initial energy of CH & (17) \\ \(N_{\theta}\) & Number of neighbors & (5) & _A\({}_{tumb}\)_ & Expected level of reliability by user & (18) \\ \(E_{\theta}\) & Initial energy & (5) & \(p\) & Ratio of failure rate to repair rate & (18) \\ \(d_{BS}\) & Node distance to BS & (6) & \(l\) & Failure rate & (19) \\ \(R\) & Cluster radius & (6) & _p\({}_{d}\)_ & Node density in the field & (20) \\ \(P_{r}\) & Probability & (6) & _\(\delta_{e}\)_ & Constant factor for dropping speed & (20) \\ \(E_{tumor}\) & Initial energy & (7) & _A\({}_{g_{a}}\)_ & Aggregation ratio & (20) \\  & & & & Minimum aggregation convergence & \\ \(a_{e}\) & Constant weight & (7) & _r\({}_{min}\)_ & point & (20) \\ \(\delta\) & Time duration of CH selection phase & (7) & _E\({}_{dice}\)_ & Energy of radio transceiver & (21) \\ \(p\) & Some random number & (8) & _E\({}_{g_{b}}\)_, _e\({}_{amp}\)_ & Amplifier energy consumption & (21) \\ \(T\) & Predefined maximum time of CH & (8) & \(l\) & Size of message in bits & (21) \\  & competing duration & (8) & _E\({}_{TX}\)_ & Transmitting energy & (21) \\ \(j\) & Neighboring node ID & (8) & _E\({}_{RX}\)_ & Receiving energy & (22) \\ \(E_{rj}\) & Residual energy of neighbor node j & (8) & \(d\) & Transmission distance & (23) \\ \(d_{BS}\) & Distance of nodes to the BS & (9) & _w\({}_{l}\)_ & Weight of node & (24) \\ \(K_{opt}\) & Optimum number of CHs & (11) & _Q\({}_{critical}\)_ & Critical cluster size & (26) \\ \(N\) & Number of neighbors in the & (12) & _E\({}_{BM}\)_ & Energy consumption of data & (27) \\ \(Node_{neighbor}\) & Number of foreign nodes located in & (12) & _f\({}_{o}\)f\({}_{e}\)f\({}_{B}\)_ & Fitness parameters & (27) \\  & other clusters & (13) & _d\({}_{dwar}\)_ & Radio characteristic distance & (28) \\ \(E_{max}\) & Maximum energy & (13) & \(e\) & Energy level & (28) \\  & Upper threshold of cluster size & (14) & _d\({}_{a}\)_ & Upstream link length & (28) \\  & Combined set of 1-hop and 2- hop & (14) & _d\({}_{a}\)_ & Upstream link length & (28) \\ \(N_{t2}\) & neighbors of node & (14) & \(\beta\) & Direction of link between (-\(\pi\), \(\pi\)) & (28) \\ \hline \end{tabular}
* The Equation numbers are according to the first occurrence of symbols.  
\end{table} 
Table 1: List of symbols in order of the occurrence. approaches and some related open research areas for the design of cluster-based routing protocols are presented too.