III. ROUND-TRIP TIMES
The first measurement series is focussed on one of the
crucial parameters for most Internet applications, the roundtrip
time. In these measurements, Internet Control Message
Protocol (ICMP) packets were sent using the Ping command
to the Gateway GPRS Support Node (GGSN), which
is the gateway to the Internet. Once the target server receives
an ICMP packet, it sends it back to the sender immediately
followed by the next Ping command. From the time
between sending and retrieval, the average round-trip time
can be measured. Three different GPRS terminals from two
different manufacturers (MS1, MS2a and MS2b) performed
differently. While the measurements using MS2 showed a
linear relationship between the packet size and the roundtrip
time over the whole range, MS1 behaved different with
packets smaller than 300 byte (see Figure 3). Here, the initial
round-trip time is very high but decreases to a lower
value when reaching a certain packet size. This behavior did
not occur when setting the delay time between two Pings
from zero to a value greater than one second. Since TCP
control segments are about 40 bytes long, this phenomenon
has a significant effect on the TCP connection setup time,
which is a crucial parameter in Web sessions. A reason for
this effect is that an ongoing Temporary Block Flow (TBF)