he conforming cells of an ATM connection, over time-scales that are long relative to those for which the PCR is defined. Maximum burst size (MBS). This is the maximum number of cells that can be transmitted on the peak cell rate. Minimum cell rate (MCR). This is the rate negotiated between the end systems and the network(s), such that the actual cell rate sent by the end system on the ABR connection need never be less than the The performance of the ATM layers can be measured by monitoring certain parameters. These pa- rameters can be divided into two groups: negotiable and non-negotiable. Parameters that are negotiated peak-to-peak cell delay variation (CDV); maximum cell transfer delay (maximum CTD); mean cell transfer delay (mean CTD); cell Loss Ratio (CLR). Transmission Network Planning and Optimisation Cell Delay Variation The CDV parameter describes variability in the pattern of cell arrival (entry or exit) events at an MP (measurement point) with reference to the negotiated peak cell rate 1/T (see Recommendation I.371). It includes variability present at the cell source (customer equipment) and the cumulative effects of variability introduced (or removed) in all connection portions between the cell source and the specified MP. It can be related to cell conformance at the MP and to network queues. It can also be related to the buffering procedures that might be used in AAL1 to compensate for cell delay variations. Cell Transfer Delay Cell transfer delay (CTD) is the time, t2 t1, between the occurrence of two corresponding cell transfer events, CRE1 (cell reference event 1) at time t1 and CRE2 at time t2, where t2 > t1 and < Tmax. The value of Tmax is for further study, but should be larger than the largest practically conceivable cell transfer delay. Mean Cell Transfer Delay The mean cell transfer delay is the arithmetic average of a specified number of cell transfer delays. Cell Loss Ratio The cell loss ratio (CLR) is the ratio of total lost cells to total transmitted cells in the population of interest. L