me haps of very different type. The network operator needs to consider interval, in which the transmitted data is delivered error free. Again the effects of these individual links on the overall G.821 objectives it is usually expressed as a percentage. which the customer expects. Outline Recommendation M.550 For modern high bit rate transmission systems a second may be (CCITT, 1988b) attempts to address these matters by defining too long (it will contain many bits) and SO an error-free millisecond, performance objectives for: and error-free microsecond or more specifically, an error free data block (as mentioned above) may become more appropriate 1. The long-term (several days); for bringing a digital link into measures. service. 2. The short-term (several minutes/hours); for removing an unsat- 28.2.5 Performance standards - an example isfactory link for maintenance. The parameters BER, available time and EFS and adaptations of However, this is a complex and challenging topic which remains them are used in the emerging error performance standards for the subject of continuing study, such as (Yamamoto and Wright, digital links (McLintock and Kearsey, 1984). The important CCITT 1989; Kubat and Bollen, 1989). Recommendation G.821 (CCITT, 1988a) serves as an example. It specifies error performance objectives for long distance 64kbit/s 28.2.6 Error performance monitoring data connections (Table 28.2). These are long term objectives, usually measured over many days, For many applications it is considered highly desirable to monitor and apply only to the available time. When the error performance is the occurrence of transmission errors while a system is in service. consistently bad (a BER worse than 10-3 in ten consecutive one-sec- This enables problems to be seen quickly and appropriate action ond intervals is specified) the connection is deemed to be unavail- taken such as reducing the transmission rate to ensure better perfor- able for information transmission. Figure 28.9 illustrates the mance or switching to an al