Реферат: Evaluating the GPRS Radio Interface for Different Quality of Service Profiles

In an additional experiment, we study the performance loss in the GSM voice

service due to the introduction of GPRS. Figure 5 plots the carried voice traffic and

voice blocking probability for different numbers of reserved PDCHs. The results are

valid for both channel allocation schemes because of the priority of GSM voice

service over GPRS. The presented curves indicate that the decrease in channel

capacity and, thus, the increase of the blocking probability of the GSM voice service

is negligible compared to the benefit of reserving additional PDCHs for GPRS users.

Figure 6 shows carried data traffic and packet loss probabilities for the dynamic

channel allocation scheme and different packet priorities. For GPRS 1 PDCH is

reserved. Weights for packets with priority 1 (high), 2 (medium), and 3 (low) and

percentages of GPRS users utilizing these priorities are given in Table 1. We observe

that for low traffic in the considered cell most channels are covered by packets of low

priority. This is due to the high portion of low priority packets (60%) among all

packets sharing the radio interface. With increasing load medium priority packets and

at last high priority packets suppress packets of lower priority and therefore the

utilization of PDCHs for low and medium priority packets decreases. For a call arrival

rate of up to 2 calls per second the loss probability of high priority packets is still less

than 10-5 and therefore the corresponding curve is omitted in Figure 6.

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