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$$
U=N\cdot A_{{t}}\cdot ({\frac  {P_{{F}}}{A_{{t}}+2A_{{p}}+A_{{t_{{ACK}}}}}}+{\frac  {1-P_{{F}}}{T_{{o}}+\lfloor {\frac  {N\cdot A_{{t}}}{T_{{o}}}}\rfloor \cdot N\cdot A_{{t}}}})
$$
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