This measurement series was performed to get an
overview of the maximum throughput reachable with
GPRS. The File Transfer Protocol (FTP) was used to download
one single file (size: 2.7 MByte) from the Internet.
Considering the high round-trip times of GPRS (see Section
III), the delay imposed by the Internet could be neglected.
Regarding the maximum number of three downlink
timeslots and the maximum throughput of 31.5 kbit/s,
a maximum of 10.5 kbit/s per PDCH could be seen as the
performance limit (see Figure 4). Therefore, a maximum
of around 42 kbit/s can be assumed for the network at this
time, if 4 downlink slots can be used in parallel, which is the
current status for newer GPRS terminals. Figure 5 shows
the average download times using GPRS and fixed access
networks. The GPRS performance with 3 downlink slots is
comparable to the performance of an 28k analog modem.
Since the regarded cell was not isolated, some cell reselections
occurred with more than one GPRS terminal active.
Although they could be pinpointed by counters within
the BSC, this fact limited the maximum number of GPRS
terminals to three for this experiment. With four terminals,
the number of re-selections increased dramatically, as
shown in Figure 6. It is obvious that the re-selection performed
by one terminal instantly affects the performance of
all other terminals within that cell, leaving more resources
to the remaining stations.
For proper interpretation of the results, samples measured
during time periods, where one or more terminals
were connected to other cells were removed from the measurement
data.