uplink quality, which indicates a greater probability of a higher drop call and lack of setup success. TRX Quality TRX quality counters give an idea of the percentage of samples having quality 0 to 7 in the uplink and downlink directions. Quality scale 0 is the best. The more samples are in Q-0 the better is the performance of the cell. Low samples in Q-0 to Q-4 indicate an interference or hardware problem on the TRX. SDCCH Drop A high SDCCH drop rate explains the high values in CSF. It can be due to a bad radio link perfor- mance or any other hardware problem. High SDCCH traffic does not necessarily mean high TCH traffic. The SDCCH is used for other services that do not generate TCH traffic. KPIs for monitoring SDCCH performance in the SMS and location update also exist. Handover Distribution Handover distribution gives the percentage of outgoing handovers depending on the cause (UL/DL quality, UL/DL interference, UL/DL level, power budget, slow moving mobile station, etc.). For example, on a cell that does more handovers due to the uplink level, a problem of path unbalance (weak uplink signal) can be assumed. More details on handover performance are presented later. Call Phase Signalling Charts Physical Layer The GSM communication protocols follow the open system interconnection (OSI) model with certain adaptations and additions due to the special characteristics of cellular networks. Therefore, some functions may involve more than one layer. This raises the necessity of shaping new protocol functions. The physical layer is similar to layer 1 in the OSI model, except for the fact that it can be directly accessed by layer 3 for power control, handover control and roaming tasks where there is a need for estimating the signal level, C/I, error rate - to mention only a few. Figure 2.59 shows how the other layers can access layer 1. Logical channels used on the air interface can be divided into two classes: common control channels that include paging, access grant and random access and dedicated channels containing 