at the system is ready to operate correctly all network operations. Monthly Tasks An online incremental backup is performed daily. In addition, what is called 'cold backup' exists and is performed offline. It consists of freezing the network and storing all the parameters and settings on external devices. This integral backup is time consuming and is done on a monthly basis. Given the crucial importance of correct functioning of the NMS, an overall audit can be done system- atically to check whether the daily, weekly and monthly tasks are performed well and according to the process. Alarms Reporting in the NMS Alarms monitoring constitutes the main source of information on the fault status of the site. It can be separated into different types, as explained previously. Not all alarms are critical or require clearing. Impact of Transmission and NSS Faults The end user experiences bad quality of service regardless of the problem location on the network. In this chapter, radio optimisation has been discussed but quality degradation may come from the NSS or from the transmission lines. Incompatible timer values between the NSS and BSS can impact on the quality. Synchronisation faults on the transmission network also degrade the quality. The next two chapters handle in detail all optimisation aspects of the transmission and core networks, respectively. Multivendor Interoperability The standardisation organism made some interfaces open in the GSM network. This means that the manufacturers of the two end equipments of such interfaces should follow the same protocol. Operators can then use different vendors to supply their infrastructure. A multivendor environment introduces verification of different equipments from different vendors that can interoperate with each other. For that, main vendors follow interoperability tests (IOTs) between their latest versions of products and software. The IOT is a running program as vendors produce new hardware and software continuously. From the operator's perspective, this operation is