revised this sentence: The user request arrival process can be modelled as a NAME_1 process with a rate $s_1$. Considering the fact that the majority of the user request duration is limited, and only a small fraction is of a long time \cite{10.1145/1028788.1028795}, an exponential distribution with parameter $\lambda_t$ is used to describe the request duration. Then the PDF of the request duration is obtained by