Analysis of the effects of outages on the quality of service of GPRS network systems

The General Packet Radio Service (GPRS) extends the Global System Mobile Communications (GSM) by addressing packet-oriented traffic. Availability is the most important dependability requirement for such Comunication Systems as GPRS. Focusing on the contention phase, where users compete for channel reservation, this paper analyses the GPRS with the objective to understand its behaviour under critical conditions, as determined by periods of outages, which significantly impact on the resulting dependability. In fact, during outages (service unavailability), users tying to access the service accumulate, leading to an overload of the system. When the system resumes its operations, the accuniulated users determine a higher probability of collisions on resources assignment (and therefore a degradation of the QoS perceived by the users). Our analysis, performed using a simulation approach, allowed to gain insights on the impact of outages on the QoS and of the overload that GPRS systems have to face after outages.