K. V. Vijayashree , K. Ambika. “Transient Analysis of an M/M/1 Queueing Model With Two Kinds of Differentiated Working Vacation”. International Journal of Control and Automation, Vol. 13, no. 02, June 2020, pp. 1071 -89, http://sersc.org/journals/index.php/IJCA/article/view/26885.