ost cells and transmitted cells in severely errored cell blocks are excluded from the calculation of the cell loss ratio. QoS parameters that are not negotiated are: cell error ratio (CER); severely errored cell block ratio (SECBR); cell misinsertion rate (CMR). Cell Error Ratio The cell error ratio (CER) is the ratio of total errored cells to the total of successfully transferred cells, plus tagged cells, plus errored cells in a population of interest. Successfully transferred cells, tagged cells and errored cells contained in severely errored cell blocks are excluded from the calculation of the cell error ratio. Severely Errored Cell Block Ratio The severely errored cell block ratio (SECBR) is the ratio of total severely errored cell blocks to total cell blocks in the population of interest. Cell Misinsertion Rate The cell misinsertion rate (CMR) is the total number of misinserted cells observed during a specified time interval divided by the time interval duration 2 (equivalently, the number of misinserted cells per connection second). Misinserted cells and time intervals associated with severely errored cell blocks are excluded from the calculation of the cell misinsertion rate. Some other parameters of interest may be the availability ratio and the mean time between outage. Availability Ratio The availability ratio (AR) applies to ATM semi-permanent connection portions. The service AR is defined as the proportion of time that the connection portion is in the available state over an observation period. The service AR is calculated by dividing the total service available time during the observation period by the duration of the observation period. Digital Hierarchies - PDH and SDH Table 3.8 Service class related parameters Service class rt-VBR nrt-VBR Traffic parameters Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified Specified QoS parameters Specified Specified Specified Specified Unspecified Maximum CTD Maximum CTD Mea