vant to a unique BSC-BTS transaction should first be isolated. This means that transiting messages are discarded. Then, different MSs in uplink and different TRXs in downlink use different radio channels. How filtering should be performed on an bis trace has just been presented in order to diagnoses call per call. Now, a discussion follows on how the protocol test equipment can be used to improve network quality. There will also be a focus on most common GSM errors and their roots. Quality of service in telecommunications, as a reminder, means the level of usability and reliability of a network and its services. QoS monitoring implies permanent observation, qualification and adjustment of various network parameters. As explained before, three complementary ways exist to measure and follow QoS. QoS Measurement Techniques Drive Tests These present the advantage of reproducing a real subscriber's perception of the network. However, on another hand, they are very expensive and concentrate on a limited area within a small timeframe. Statistics delivered by the OMC cover the whole network. However, they are abstract and do not really reflect what subscribers encounter. An advantage is that the OMC platform is free of charge since it is part of the GSM system and serves statistics collection and newly tuned parameter implementation at the same time. Protocol Analysers All events can be available for further analysis. Moreover, these are independent tools that do not need any specific vendor system knowledge. However, they have the disadvantage of being difficult and complicated to plug to the entire network all the time. As illustrated, all three alternatives have advantages and disadvantages. That is why only a combination of the three methods allows the level of a network quality of service to be stated. Tools Protocol analysers are devices equipped with certain functionalities to enable them to intercept data signals on different interfaces, store the huge amount of information, display it later on a screen for analy