ref-18518 (Evaluating the GPRS Radio Interface for Different Quality of Service Profiles), страница 2

Parameter

Figure 2 presents curves for carried data traffic and packet loss probabilities due to

buffer overflow in the BSC for the static channel allocation scheme and one packet

priority. For GPRS 1, 2, and 4 PDCHs are reserved, respectively. The remaining

channels can be used by GSM calls. With 4 PDCHs the system overloads at an arrival

rate of 0.8 GSM/GPRS users per second. This corresponds to an average of 12 GPRS

users in the cell (see Figure 7). In Figure 3 we present corresponding curves for the

dynamic channel allocation scheme. For GPRS 1, 2, and 4 PDCHs are reserved,

respectively but more PDCHs can be reserved "on demand". That means that

additional PDCHs can be reserved if they are not used for GSM voice service. From

Figure 3 we observe that for low traffic in the considered cell GPRS makes

effectively use of the on demand PDCHs. For example if 1 PDCH is reserved GPRS

utilizes up to 2 PDCHs at an arrival rate of 0.4 GSM/GPRS users per second. But

with increasing load the overall performance of GPRS decreases because of

concurrency among GPRS users, and more important, priority of GSM users over the

radio interface. In comparison with the static channel allocation scheme we conclude

that the combination of reserved PDCHs and on demand PDCH leads to a better

utilization of the scarce radio frequencies. The only advantage of the static channel

allocation scheme is that it can be realized more easily.

Figure 4 presents a comparison of overall channel utilization and average

throughput per GPRS user for the static and dynamic channel allocation scheme. For

the static scheme we reserved 2 and 4 PDCHs respectively and for the dynamic

scheme only 1 PDCH. We observe a higher overall utilization of physical channels by

the dynamic scheme. Comparing the dynamic with the static scheme for 2 PDCHs we

detect a slightly higher throughput for low traffic load for dynamic channel allocation.

This results from the high radio channel capacity available to GPRS users in this case.

They can utilize up to 8 PDCHs for their transfer (in contrast to 2 PDCHs in the static

scheme). When load increases, GSM calls allocate most of the physical channels.

Thus, throughput for GPRS users decreases very fast. In the static scheme (4 PDCHs)

the decrease in throughput is not so fast, because GSM calls do not effect the PDCHs.

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

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.