hrough network The simplest method of out of service testing is a simple full bandwidth Bit Error Ratio Test (BERT) using, typically, a long pseudo-random pattern at 2.048Mbit/s, with no framing, to qualify the performance of transmission media. G.821 performance statis- tics can be applied at the primary rate, even though they are intended for use at 64kbit/s, provided that the system can be out of service for long enough to collect valid results. Time related events, or quality In-service test degradation periods, can be isolated by using time stamped stored or printed reports, and result histograms. Drop and Monitor insert Alternatively the frames, with or without CRC4, can be generated by the tester for transmission to terminal, multiplexing or network equipment. In this case the tester can be set to corrupt the alignment Protected signals in a pre-defined manner, in order to check the ability of the monitoring point equipment to detect and recover from sync loss, according to the correct criteria. Testers can be set to replace the normal frame and Figure 61.18 Connections for ISDN Primary Rate testing multiframe alignment signal by programmable words, to exercise the facilities controlled by each word bit, for example distant alarm. At the same time a selection of the data fields (timeslots 1 to 15 and Intermittent, or time related, faults can be identified, using a tester 17 to 31) can be filled with a pseudo-random sequence, performing a BERT test to another similar tester, and CRC4 can be generated with a printer facility to produce time stamped reports in response and checked to test CRC performance. to error conditions. By comparing physical layer parameters, for example bipolar errors (infringements of the bipolar encoding By substituting test equipment for network elements, or by using scheme), with supervisory functions such as long term CRC4 per- testers connected to individual transmission equipments in isola- formance, it is possible to differentiate between transmission and tion, problems of inco