@INPROCEEDINGS{Melikov95, author = "Melikov, A. Z. and Ponomarenko, L. A.", title = "On the approach to optimal control of queuing systems with multiple classes of customers", booktitle = "Proc.\ International Conf.\ on Syst.\ Sci.\ XII.", address = "Wroclaw (Poland)", year = 1995, pages = "507-515" }