to run error- 30.5.2 General factors influencing end to end free for long periods of time with errors coming in unpredictable performance bursts. CCITT attempt to describe the long term error characteristics of To illustrate principles consider a digital network connecting reference connections in Recommendation G.821 using the concept together 2-wire local lines when the structure of the digital network of error free seconds, (EFS) severely errored seconds (SES) and has already been agreed in outline and is shown in Figure 30.8. This degraded minutes (DM). network structure is representative of a significant number of typical Table 30.1 gives the error performance objectives for a long digital networks either in operation or being planned. international reference circuit averaged over a period of one month. If periods of less than one month are used then there will be INTERNATIONAL increased uncertainty in the values quoted. Table 30.1 G.821 objectives for error performance of a 64kbit/s reference connection Performance classification Objective Degraded minutes <10% of 1 minute intervals to DIGITAL have a bit error ratio >1.10-6 TRANSIT CENTRES Severely errored seconds <0.2% of 1 second intervals to have a bit error ratio >1.10-3 Errored seconds <8% of 1 second intervals to have any errors ( >92% to be error free) DIGITAL PRIMARY CENTRE It is clearly difficult therefore to accurately predict the perfor- mance of a switched digital connection with a holding time of a few minutes. The other digital impairment that may be seen on some types of services is slips i.e. the deletion or repetition of one bit, or a number DIGITAL of consecutive bits. Recommendation G.822 (CCITT 1989d) gives LOCAL EXCHANGE slip rate objectives for international connections. 30.5 Meeting end to end requirements The previous section outlined typical objectives for end to end Local Local values of the key transmission parameters. The next step for the transmission planner is to identify the main drivers in this process. End to end performanc