Figure 7 presents curves for average number of GPRS users in the cell and
blocking probabilities of GPRS session requests due to reaching the limit of M active
GPRS sessions. We observe that for 2% GPRS users the maximum number of 20
active GPRS sessions is not reached. Therefore, the blocking probability remains very
low. For 10% GPRS users and increasing call arrival rate, the average number of
sessions approaches its maximum. Thus, some GPRS users will be rejected. It is
important to note that the curves of Figure 7 can be utilized for determining the
average number of GPRS users in the cell for a given call arrival rate. In fact, together
with the curves of Figure 2 and 3, we can provide estimates for the maximum number
of GPRS users that can be managed by the cell without degradation of quality of
service. For example, for 5% GPRS users and 1 PDCHs reserved, in the static
allocation scheme a packet loss probability of 10-3 can be guarantied until the call
arrival rate exceeds 0.4 calls per second, i.e., until there are on the average 6 active
GPRS users in the cell. For the dynamic allocation scheme a packet loss probability of
10-3 can be guarantied until the call arrival rate exceeds 0.6 calls per second
corresponding to 9 active GPRS users in the cell on average. Figure 8 investigates the impact of the maximum number of GPRS user per cell to the performance of GPRS for the dynamic channel allocation scheme with 1 PDCH reserved. Of course, the expected number of GPRS users should be less than the maximum number in order to avoid the rejection of new GPRS sessions. On the other hand, the maximum number of active GPRS sessions must be limited for guaranteeing quality of service for every active GPRS session even under high traffic. The tradeoff between increasing performance for allowing more active GPRS sessions and the
increasing blocking probability for GPRS users is illustrated by the curves of Figure 8.
Conclusions
This paper presented a discrete-event simulator on the IP level for the General Packet Radio Service (GPRS). With the simulator, we provided a comprehensive performance study of the radio resource sharing by circuit switched GSM connections and packet switched GPRS sessions under a static and a dynamic channel allocation
scheme. In the dynamic scheme we assumed a reserved number of physical channels permanently allocated to GPRS and the remaining channels to be on-demand channels that can be used by GSM voice service and GPRS packets. In the static scheme no ondemand channels exist. We investigated the impact of the number of packet data
channels reserved for GPRS users on the performance of the cellular network. Furthermore, three different QoS profiles modeled by a weighted fair queueing scheme were considered. Comparing both channel allocation schemes, we concluded that the dynamic scheme is preferable at all. The only advantage of the static scheme lies in its easy implementation. Next, we studied the impact of introducing GPRS on GSM voice service and observed that the decrease in channel capacity for GSM is negligible compared to the benefit of reserving additional packet data channels for GPRS. With the curves presented we provide estimates for the maximum number of GPRS users that can be managed by the cell without degradation of quality of service. Such results give valuable hints for network designers on how many packet data channels should be allocated for GPRS and how many GPRS session should be allowed for a given amount of traffic in order to guarantee appropriate quality of service.