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

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)