ptimisation Slip Rate G.811 G.812T G.812L G.81s Figure 3.80 Slip rate such as the high-low concept, radius of a loop not less than 3 km, vertical and horizontal, etc., should be applied. Even after this interference is spotted, the source should be found, whether the source is external (other than an own network) or internal (own network). To reduce interference, some techniques such as cross-polar discrimination or antenna tilt power reduction should be applied. For issues related to synchronisation, the slip frequency limit parameter should be observed from the management system (an example is shown in Figure 3.80). The most common problem associated with a high slip rate will be handover failure within the GSM network. The mobile with the need to hand over cannot find the neighbouring BTS because the frequency has drifted too far. This contributes to the drop call rate. The BTS with high slips should be checked by tracing the synchronisation flow or by checking the timing priority bit in case a loop configuration occurs. 3.14.3 UMTS Transmission Network Optimisation Accessibility First consider accessibility. Poor quality in accessibility means that the consumer is not able to make a call at all or is not able to establish a data connection. The well-known cause of this is poor radio coverage and in most cases radio coverage is also the area where there is most room to improve. Radio interface optimisation is discussed in other chapter but it is good to note that poor radio coverage is something that the consumer can monitor. The cellular phone has a display for field strength, which works in two ways. It gives the consumer the ability to monitor the quality of the radio interface even when not planning to make a call. The measurement device goes with the consumer in the phone. On the other hand, it guides the behaviour of the consumer. It sets consumer expectation, since the consumer can see while planning to start consuming a network provided service what is the field strength and whether the radio field is 