s on the one hand and perform test functions such as handle these processes as it is not just a question of simulation, these error injection, logging etc. on the other. processes have to be monitored, and under certain circumstances logged, so that any errors can be analysed at a later date. 61.11 Data Link Layer (Layer 2) 61.13 A.F. and speech codec The second layer of the OSI model deals with the correct trans- measurements mission of well defined data blocks or telegrams. The trigger and synchronisation facilities are the most important features of test The test procedures for acceptance testing make a distinction be- equipment for dealing at this layer. Special attention must be paid tween a.f. measurements and r.f. measurements. The result is a new to skew problems which arise when a number of instruments are kind of measurement procedure, requiring acoustic and digital in- triggered externally. To solve problems of this kind at an acceptable terfaces. It seems very likely that new definitions will emerge for outlay, it is necessary to design multi function test equipment which testing the overall function of the transmit and receive path is controlled via a common trigger. (r.f./acoustics). This kind of measurement will play an important Layer 2 of GSM is indeed a highly intelligent processor unit which role in the maintenance and repair of telephone handsets, and is must make a large number of decisions in real time. Equipment that illustrated in Figures 61.34 and 61.35. can test these functions satisfactorily also requires a high degree of Test set built in intelligence and automatic real time behaviour. Here too, modern processors can provide a suitable solution. To simulate Digital input/output situations where errors occur, both layer 2 'error model' which can be modified by the user and an automatic model of correct beha- viour are required. Stimulation Measurements 61.12 Network Layer (Layer 3) Acoustic input/ The network level of OSI model also specifies an intelligent ex- output change of datagram