[ns] RTP questions using Agent/RTP and Application/Traffic/CBR

[Kevin Yuen <knyuen@cse.cuhk.edu.hk>]

  Dear all,

	I am a newbies using ns to do RTP simulation for VoIP
	(Voice over IP).
	
	I have some problems with my simulation.

	I have created two nodes such that RTP data are sent from
	one node to another node. In the sender node, I use
	Agent/RTP to send data generated from Application/Traffic/CBR.
	
	That is:

	set rtp1 [new Agent/RTP]
	set cbr1 [new Application/Traffic/CBR]
	$cbr1 attach-agent $rtp1

	Then I start the simulation using

	$ns at 0.50 "$cbr1 start"
	$ns at 0.50 "$rtp1 start"

	I found that if I comment out the line starting $cbr1, 
	the number of packets sent by this RTP stream will be halved.

	Why the number of packets is havled?


	To conclude, what I am trying to do is:

        Application/Traffic/CBR -> Agent/RTP -> network

        But it seems that the simulator is doing this for me:

        Application/Traffic/CBR -> network
        Agent/RTP               -> network

        The two objects are sending data to the network in parallel!

        Why this happens?

   
        Attached with this email is my simulation tcl script. 
        You may try to comment/uncomment the "$cbr1 start" line
        and you can see the number of packets is halved/doubled.


        Please help me, thank you very much!


  Regards,
  Kevin
  M.Phil Year 2
  The Chinese University of Hong Kong


========================================================================
# voip.tcl
#
# Description: The simulation script for VoIP
#
# Written by: Kevin (knyuen@cse)
#
# v1.2: Generate graphs for the link bandwidth utilization
# v1.1: Use Agent/RTP as lower class, 
#       then use Application/Traffic/CBR to provide VoIP traffic
#       for the RTP packets
# v1.0: initial code

# Setup simulator object
set ns [new Simulator -multicast on]

# Enable multicast
$ns multicast

# Open file for simulation
set nf [open crm.nam w]
$ns namtrace-all $nf

# Open file for plotting graphs
set f1 [open out1.tr w]
set f3 [open temp.tr w]

proc finish {} {
    global ns nf f1 f3
    $ns flush-trace
    
    # Close files
    close $nf    
    close $f1 
    close $f3
    
    exec xgraph out1.tr -t "Simulation 1" -0 "Stream 1" -geometry 800x400 &
    exec xgraph temp.tr -t "Packet arrivals" -geometry 800x400 &
    exec nam crm.nam
    exit 0
}

# Setup network topology: nodes
for {set i 0} {$i < 6} {incr i} {
    set n($i) [$ns node]
}

# Color
$ns color 1  red
$ns color 30 purple
$ns color 31 bisque
$ns color 32 green

# Setup network topology: links
for {set i 1} {$i <= 5} {incr i} {
    $ns duplex-link $n($i) $n(0) 100Mb 10ms DropTail
}

$ns duplex-link-op $n(0) $n(1) orient left-up
$ns duplex-link-op $n(0) $n(2) orient right-up
$ns duplex-link-op $n(0) $n(3) orient right
$ns duplex-link-op $n(0) $n(4) orient right-down
$ns duplex-link-op $n(0) $n(5) orient left-down

# Set multicast parameters
set mproto DM
set mrthandle [$ns mrtproto $mproto {}]

# Setup RTP agents
set rtp1 [new Agent/RTP]
set rtp3 [new Agent/LossMonitor]

$ns attach-agent $n(1) $rtp1
$ns attach-agent $n(3) $rtp3

$rtp1 set packetSize_ 99
$rtp1 set interval_ 60ms

# Setup CBR on RTP
set cbr1 [new Application/Traffic/CBR]
$cbr1 attach-agent $rtp1
$cbr1 set packet_size_ 99
$cbr1 set interval_ 60ms

$ns connect $rtp1 $rtp3

# Specify the record function
proc record {} {
    global f1 f2 f3 rtp3 
    
    # Initialize variables
    set ns [Simulator instance]
    set time 0.5
    set now [$ns now]
        
    set bw1 [$rtp3 set bytes_]
    
    set bw3 [$rtp3 set npkts_]
       
    puts $f1 "$now [expr $bw1/$time*8]"
    
    puts $f3 "$now $bw3"
            
    $rtp3 set bytes_ 0
    $rtp3 set npkts_ 0
    
    # Schedule next execution
    $ns at [expr $now+$time] "record"
}

# *** Initialize ***
$ns at 0.00 "record"

# *** Run ***
# Strange... uncomment the following line will double the bandwidth!
#$ns at 0.50 "$cbr1 start"  
$ns at 0.50 "$rtp1 start"


# *** Finish ***
$ns at 5.00 "finish"

$ns run