mpatibility or specification interpretation can terminal equipment faults. It is also possible to intercept the be resolved. The electrical characteristics of the transmission or frames, and either extract single, or multiple, timeslots for analysis, terminal equipment, its characteristic impedance, pulse shape and or to drop out timeslots and replace them with data inserted from a encoding can be measured. When testing into the network, the tester source. In this way selected channels of a live system can be transmitter of the tester is set to use, and to measure the frequency tested without disturbing the remaining channels, thereby proving and jitter of, the clock recovered from the received signal. When the quality of capacity before it is brought into service. testing terminal equipment, the tester has to look like the network, SO it uses its own crystal controlled clock to generate the framed signal. The different connection methods are illustrated in Figure 61.18. 61.5.7 Protocol testing The level of testing that covers layers 2 and 3 of the OSI stack is commonly known as protocol testing. Protocol testing in ISDN is 61.5.6.3 In service Primary Rate tests linked to the testing of the 'D' channel signalling protocol. Protocol testing at layers 2 and 3 as well as the layers above can be split into In service testing is ideally carried out unobtrusively with minimum a number of different areas such as conformance testing, perfor- disruption to the traffic. Its objective is to assess the quality of the mance testing, inter-operability testing and trouble shooting. transmission and, where defects are found, to pinpoint the source of Protocol testing is normally carried out by dedicated protocol the problem. analysers with the appropriate ISDN interfaces fitted, although In service testing can monitor a live network signal for correct protocol analysers based on cards for use in PCs are becoming more frame alignment, condition of the distant alarm signals, CRC4 and popular. Some ISDN testers such as those that s