ot established a channel to the target side Failure in the target channel Failure in the source channel for instance MS, BTS, BSC and MSC. Many signalling charts exist, each one corresponding to a certain phase in call handling. The next subsection on call phase signalling charts should be referred to for signalling charts. A counter will be updated at a certain point from exchanged messages. This is what is called 'the triggering point'. As millions of signalling messages exist, many counters may also exist. However, the number of available counters is very dependent on the equipment's supplier and his/her strategy. Each BSS vendor has in fact its own technology and infrastructure products. The counter's behaviour from one vendor does not match exactly the same triggering points of the other. In the absence of a GSM standardised definition, different BSS vendors have their own interpretation and hence different formula definitions result. Due to the large number of counters and the basic level of their triggering, it is hard to make an objective analysis through them. They bring an idea to a specific event, the number of calls for example. In practice, however, they have no relevance as such in interpreting the quality of the network. The same counter can be updated after several messages in various call phases, and vice versa; one event can increase several counters. Different individual signalling messages that update one counter can be called 'causes'. A cause describes in detail how the transactions have run. Table 2.22 presents an example of several causes that may affect the SDCCH drop counter. Radio Network Optimisation Triggering The supplier of each equipment has his/her own philosophy in conceiving counters. Therefore, when benchmarking is done, careful analysis is needed in order to highlight the difference of each vendor in reference to the GSM standard. As triggering points differ from one supplier to another, comparing different counters does not constitute an apple-to-apple comparison. Key Perform