the basic introduction about queueing theory:

The purpose of this site is to teach the user basic Queueing Theory. There are pages outlining the way in which single queues work and also explanations and illustrations of queueing networks. It is suggested that the user looks first at M/M/1 queues and then works their way through  http://www.dcs.ed.ac.uk/home/jeh/Simjava/queueing/index.html ( more details here)