Quality of Service Review A basic performance measure for any digital data communication system is bit error rate (BER). Well-designed fiber-optic links will predominate now and into the foreseeable future. We may expect BERs from such links on the order of 1 X 10-12 and with end- to-end performance better than 5 X 10-10 (Ref. 14). Thus other performance issues may dominate the scene. These may be called ATM unique QoS items, namely: Cell transfer delay Cell delay variation Cell loss ratio Mean cell transfer delay Cell error ratio Severely errored cell block ratio Cell misinsertion rate 20.10.2 Selected QoS Parameter Descriptions 20.10.2.1 Cell Transfer Delay. In addition to the normal delay through network ele- ments and transmission paths, extra delay is added to an ATM network at an ATM switch. The cause of the delay at this point is the statistical asynchronous multiplexing. Because of this, two cells can be directed toward the same output of an ATM switch or cross-connect resulting in output contention. The result is that one cell or more is held in a buffer until the next available opportunity to continue transmission. We can see that the second cell will suffer additional delay. The delay of a cell will depend upon the amount of traffic within a switch and thus the probability of contention. The asynchronous path of each ATM cell also contributes to cell delay. Cells can be delayed one or many cell periods, depending on traffic intensity, switch sizing, and the transmission path taken through the network. 20.10.2.2 Cell Delay Variation (CDV). By definition, ATM traffic is asynchronous, magnifying transmission delay. Delay is also inconsistent across the network. It can be a function of time (i.e., a moment in time), network design/switch design (such as buffer size), and traffic characteristics at that moment in time. The result is cell delay varia- tion (CDV). CDV can have several deleterious effects. The dispersion effect, or spreading out, of cell interarrival times can impact signaling functions or the