etween cells. If the time interval between two cells is 1 us, then the PCR is 1/(1 X 10 6 s) = 106 cells per second (cps). Sustainable cell rate (SCR): This is defined as the average rate of the cells, when measured over a period of time. It is also known as the mean cell rate. Maximum burst size (MBS): This is defined as the maximum number of cells that can be transmitted at the peak cell rate. Minimum cell rate (MCR): This is defined on the basis of the minimum cell rate that is required by the user. It is a rate that is negotiated by the end-system and the network such that the cell transmission rate never falls below the minimum specified/negotiated value. Cell-delay variation tolerance (CDVT): This is an error margin and defines the acceptable variation in the cell transmission time interval; i.e. it defines the upper bound in the variation in cell delay. Parameters like the above define the type of traffic and are thus called traffic descriptors. Before a connection is made, a traffic contract is negotiated based on factors such as traffic descriptors, CDVT, QoS class, conformance definition, etc. These parameters define the traffic type. However, the quality of the traffic is also an important aspect of ATM. The ATM forum has defined six parameters for this purpose. The first three listed below can be negotiated while last three cannot be negotiated. Cell loss ratio (CLR): This is the ratio of the number of cells lost (during transmission) to the total number of transmitted cells. The cells may be lost through error, congestion or even significant delays in their arrival. 3G TRANSMISSION NETWORK PLANNING AND OPTIMISATION Cell transfer delay (CTD): When a cell is being transferred from the source to destination, delay may occur due to propagation, queuing, etc. CTD can be defined as an average time taken by the cell to travel from source to destination inclusive of delays. Cell delay variance (CDV): Variation in the delay of cell transfer is measured by the CDV parameter. CDV can be defined as the differenc