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\begin{aligned} &\mathrm{E}(\mathrm{N})=\frac{1}{1-\mathrm{e}^{-(1-\rho)}} \\ &\sigma_{\mathrm{N}}^{2}=\frac{\mathrm{e}^{-(1-\rho)}}{\left(1-\mathrm{e}^{-(1-\rho)}\right)^{2}} \\ &\text { where, } \rho=\frac{\text { mean service time }}{\text { mean inter-arrival time }}=\frac{2 \lambda}{\mu(b+a)} \text { for all } \mathrm{a}, \mathrm{b} > 0 \text { and } \mathrm{b} > \mathrm{a} \text {. } \end{aligned}