gital network error performance can be measured over a complete Out of service measurements involve removing live traffic from end-to-end connection called a path, or over parts of the network the link and replacing it with a known test signal, usually a pseudo- termed lines and sections. These network layers are shown in Figure random binary sequence (p.r.b.s.). These tests are disruptive if 61.8. Path measurements indicate the overall quality of service to applied to working networks, but do give very exact performance the customer. Line and section measurements are used for trouble- measurements as every bit is checked for error. shooting, installation and maintenance, and for assuring trans- mission performance objectives are met. 61.3.3 The error performance tester (out of 61.3.2 In service and out of service service) measurements The bit error rate tester (BERT), shown in Figures 61.9 and 61.10, In service error performance measurements rely on checking known consists of a pattern generator and error detector, often combined in bit patterns in an otherwise random data stream of live traffic. As a single instrument though sometimes separate. The p.r.b.s. is gener- Regenerators Regeneration section layer Line layer Path layer Path layer LTE: Line Terminating Equipment PTE: Path Terminating Equipment Figure 61.8 Illustration of lines, paths and sections 61/8 Telecommunication system measurements ated using a feedback shift register which is driven by a very stable clock source, either internally or derived externally from a fre- quency synthesiser. The feedback connections on the shift register determine the p.r.b.s. characteristics, and are defined in CCITT Recommendation 0.151, 0.152, and 0.153. The raw data from the shift register is usually passed through an interface circuit to generate the correct code format and output level (G.703). At the receive side the same type of interface circuit strips off the code and recovers a clock. This clock drives a reference p.r.b.s. generator whose output is compared w