I can't get collisions -- Help Please!!!

[Fernando Cela Diaz <fcela@ce.chalmers.se>]

Hi all,

I have set a simulation where one machine tries to do an ftp to a server
in a very heavily loaded LAN. On the server side, I have set an
application recording the bytes correctly arrived to the TCP Agent, as
described in:
http://www-mash.cs.berkeley.edu/dist/archive/ns-users/9904/0116.html 

...and I can't notice the effect of increasing the load on the network!!!

Before posting this letter to the mailing list I have took a look to the
archives for people with the same problem, for example
http://www-mash.cs.berkeley.edu/dist/archive/ns-users/9810/0081.html 
OK, so NAM is NOT going to show the collisions. No problem, I'm not
interested on that, but what about the rest?

and I have took a look to he source code of the sim (channel.h ...). I'm
not used to dig in the ns code, but there are quite a few lines commented
that are scaring me, for example

//double txstop_;    // end of the last transmission
//double cwstop_;               // end of the contention window
//int numtx_;           // number of transmissions during contention
//Packet* pkt_;         // packet current transmitted on the channel


Can anybody help me? (It's just a moment ... ;-) Thanks in advance!.

Here you have the interesting part of the script:

-------------------

set N_NODES 20

# Create a node list
set nodelist [ list ]
for {set i 0} {$i < $N_NODES} {incr i} {
    set n($i) [$ns node]
    lappend nodelist $n($i)
}

# Connect the nodes to a CSMA LAN
# newLan is used instead of make-lan for more fine-grained parameter
control
#
# NOTE I've used 5Mb/2ms instead of 10Mbps 64us !!! 
set lan [$ns newLan $nodelist 5Mb 2ms] 
$lan llType   LL
$lan macType  Mac/Csma/Cd
$lan chanType DuplexChannel
$lan phyType  Phy/WiredPhy


# Main server
set server [new Agent/LossMonitor]
$ns attach-agent $n(0) $server

# The other server
set test_server [new Agent/TCPSink]
$ns attach-agent $n(1) $test_server

# The trace application
set traceapp [new TraceApp]
$traceapp attach-agent $test_server

# Test traffic
set test_tcp [new Agent/TCP]
$ns attach-agent $n(2) $test_tcp
$ns connect $test_tcp $test_server
set test_traffic [new Application/FTP]
$test_traffic attach-agent $test_tcp


# Background traffic: nodes n(3..) send UDP packets to the server
# in n(0) with the parameters indicated below
for {set i 3} {$i < $N_NODES} {incr i} {
    set udp($i) [new Agent/UDP]
    $ns attach-agent $n($i) $udp($i)    
    set traffic($i) [new Traffic/Expoo]
    $traffic($i) attach-agent $udp($i)
    $traffic($i) set packet-size 1500
    $traffic($i) set burst-time 5s
    $traffic($i) set idle-time 0.0001s
    $traffic($i) set rate 10Mb
    $traffic($i) set fid_ 2 
    $ns connect $udp($i) $server
}


$ns at 0.0 "record"
$ns at 0.2 "$traceapp start"
$ns at 1.0 "$test_traffic start"
$ns at 2.0 "$test_traffic stop"
for {set i 3} {$i < $N_NODES} {incr i} {
    $ns at 3.0 "$traffic($i) start"
}
$ns at 4.0 "$test_traffic start"
$ns at 5.0 "$test_traffic stop"
$ns at 6.0 "finish"


----------------------------------------------------------------------
Fernando Cela
email: fcela@ce.chalmers.se                     phone: +46 31 772 1709