ependent network accessibility is evaluated in terms of the GPRS attach success ratio (sometimes it is also used as the complementary failure ratio), defined as the ratio between the successful GPRS attach attempts and the total GPRS attach attempts in a given measurement period. Service Accessibility The second level of accessibility is that of the service access. In this case, the user has been already successfully accessing the generic network service by means of a successful registration, or GPRS attach, and now the service accessibility will provide an indication of actual accessibility for a given data service. Service based accessibility is defined separately for the different types of mobile services. However, when the service accessibility requires a PDP context activation, the PDP context activation success ratio gives a good indication of the service accessibility. In a more general case, since the PDP activation can be a prerequisite for a certain type of service provisioning (e.g. push email services), there is a need for separated definitions for the different types of mobile services. In the case of SMS over EGPRS, the indicator of service accessibility would be the ratio of successful SMS attempts to the total amount of SMS attempts in a given period. In the case of MMS, separate ratios can be defined in similar terms for mobile originated and mobile terminated MMS respectively. Likewise, for streaming, the accessibility can also be defined as the ratio of the successful stream request attempts to the total Packet Core Network Optimisation stream request attempts during a given period. For browsing services, similar ratios can be established with the content download attempts. Integrity The KPIs describing integrity are related to the actual quality of service, for the given service under consideration, during its use. In the case of mobile data services, this type of KPI is very closely related to those defined as retainability KPIs, but in the case of more traditional voice services it is possib