a different number of users. In a lightly loaded
scenario, i.e. where the number of sources in the
FTP models is small, it is very rare that more
than one simultaneous session is observed. In
that region, it is expected that the throughput is
increasing linearly with the number of users. In
Figure 13 the observed throughput is plotted as
a function of the number of users and compared
with a linear extrapolation of the increase
observed in the lightly loaded region. When the
load increases several simultaneous downloads
will occur and resource conflict, packet loss and
reduced throughput are observed. With simul-
taneous sessions the average download time
per page/file will increase. In Figure 13 it is
observed that when the download time increases,
due to simultaneous sessions or network conges-
tion, the throughput increase is less than linear.
From the linear extrapolation it is observed that
at approximately 5320 users the average offered
traffic is > 1. The TCP window mechanism is
activated and starts to regulate the rate for a
much lower load level and number of users than
this. To identify the exact point where the TCP
window mechanism is activated, it is necessary
to study e.g. the packet loss ratio.
The plot in Figure 13 shows that the throughput
decreases when the number of users is greater
than 8000. This is an indication that too many
threads are generated by the GenSyn process and
the processor capacity and memory size con-
straints will prevent the creation of new threads
with fewer downloads as the result. This is con-
firmed by the average download time that is
reduced in the same region which indicates that
there is less network congestion, and by looking
at the number of downloaded files during the
experiment period – it is increasing up to 8000
and then decreasing.
Finally, observe that a single instance of GenSyn
was able to model 8000 FTP clients that gener-
ated (received) an average TCP load of 61
Mbit/s through a 100 Mbit/s interface.
The number of users modelled by one GenSyn
process running the FTP model is constrained
by:
- Size of memoryfor temporal storage of down-
loaded files, and handling simultaneous
threads; - Interface cardand network equipment
(hub/switch/router) that will cause traffic
congestion and increase download times
and decrease throughput;- Processing timefor managing the dynamics
of the finite state machine; - Transmission capacitybetween FTP client
and server; - Server performancewhere the download files
are located.
On a moderate PC configuration (600 MHz Pen-
tium III, 512 Mbytes RAM, Fast Ethernet inter-
face) the TCP throughput is constrained by the
network or interface card. Hence, the TCP win-
dow mechanism reduces the throughput due to
congestion (the offered load is greater than the
capacity) before the handling of multi-threads
becomes a problem.2.3.1.2 UDP Traffic – the Packet
Generation Constraints
The processing involved with downloading the
web pages and files are moderate compared with
transmitting UDP packets as required by the
CBR, MPEG, and VoIP interface modules.
Several experiments have been conducted to get
more insight into how the memory size and pro-
cessing capacity constrain the performance of
GenSyn. The network capacity is not an issue
for UDP models because GenSyn transmits
packets to a specific IP address without consid-
ering the congestion situation inside the net-
work, or in fact it does not even require a route
to the host2).The results from a few experiments conducted
by a student [And01] are given in Figure 143).
The number of packets sent per second (pps) is
plotted for a different number of users in a Gen-
Syn model running on a single machine. For all
three model examples it is observed that the
packet rate grows linearly as the number of users
increases up to a certain point where the han-
dling of threads in the Java runtime environment
becomes too heavy for the processor (Pentium
III 600 MHz), see Figure 14 for details. The size
of the memory (512 Mbytes) is sufficient and
will not limit the number of parallel threads. The
unconstrained curves are determined by finding
the expected number of users in the Connect
state, and calculating the packet stream rate
generated by each interface modules.2)Some problems have been observed with ICMP messages from the destination machine stating “port
unreachable”. These messages were ignored in Windows, but had to be filtered out in Linux.
3)Each point in the plots is from a single experiment. However, replicated experiments showed that
the variance was very small.