V. N. Rama Devi, Y. Saritha, K. Chandan,. “M/EK/1 Queueing System With Vacation, Two Types of Repair Facilities and Server Timeout”. International Journal of Advanced Science and Technology, Vol. 29, no. 8s, Apr. 2020, pp. 846 -55, http://sersc.org/journals/index.php/IJAST/article/view/10832.