AbstractDue to the high cost of obtaining and maintaining a large scale computing centre , time sharing of computer resources remains a reality for many businesses and institutions. While data has been available for some time , as to the reliabilty performance of various computer systems,for users there seems to be discrepancies between the levels of computersystem reliability performance and the actual computing time available to them. It is theaim of this paper to explain this discrepancy by simulation and graphical techniques and in this way provide a basis for determining optimal terminal service availability from a user's a perspective.