ction mechanism Counters/Performance Indicators Definition A counter can be defined as an incremental value of a specific repetitive event. In GSM language, an event corresponds to a signalling message. Here, it is necessary to introduce the notion of signalling charts. A voice call is supported through millions of messages exchanged between the MS and the MSC. Therefore a signalling chart illustrates the flow of signalling messages between various network elements, Radio Network Planning and Optimisation Table 2.22 Example of causes for an SDCCH drop Description The Abis interface receives an ASSIGNMENT_FAILURE message from the MS during a Basic Call The Abis interface receives a CONNECTION_FAILURE message The Abis interface receives a CONNECTION_FAILURE message and its cause is 'radio interface failure' The Abis interface receives a CONNECTION_FAILURE message and its cause is 'radio link failure' The Abis interface receives an ERROR_INDICATION message The handover has failed and the MS returns to the source channel. The MS has sent an assignment failure or a handover failure message to the network during the handover The Abis interface receives an HO_DETECT message. The contents of the message have been corrupted or a timer expires while waiting for the message The Abis interface receives a RELEASE_INDICATION message 349 The Abis interface receives a CH_MODE_MODIFY_ACK message and the mode has not been the same as the mode requested The timer expires while waiting for the ASSIGNMENT_COMPLETE message An ERROR_REPORT message is received from the BTS The number of failed configuration changes due to configuration change reject or no answer from the MS The call is released after the distance between the MS and the BTS has reached the maximum value allowed, i.e. the MS has exceeded the planned cell boundaries Signalling with the internal processes has failed during handover signalling The timer t3107, t3103 or t9113 has expired. The assignment complete or handover complete message has not been received. The MS has n