Tutorial Stuff

Basics

The basic idea is that we have a series of nodes (which can be alikened to eth0) connected by links (like a cable). We create a 'node' object by:

set n0 [$ns node]

and a duplex-link like this:

$ns duplex-link <$from_node> <$to_node> <$pipe_thickness> <$delay> <$type>

usually, $type is of DropTail.

Then for each node, we must assign an 'Agent'. In the examples of the tutorials, we use UDP datagrams. This is done via,

set <source_name> [new Agent/UDP]

and to assign the Agent to a node we define,

$ns attach-agent <$node> <$source>

for each type of agent, there are various parameters such as packetSize_ etc. which can be defined.

We must also assign the type of traffic that will flow on the link using,

set <traffic_type> [new Application/Traffic/<type>]

and setting it to the <source_name>

<$traffic_type> attach-agent <source_name>

As the definition language for the Agents, Nodes and Links are written in oTcl, we can use for and if loops to define a mesh of objects.

Jumping a few tutorials, i Implemented a simple network like this:

In this case, nodes {0,1,2} generate exponential UDP traffic of Agent type Application/Traffic/Exponential, with rates of {100k, 200k, 300k}, all towards node 4. We want to monitor the outcome of the traffic at node 4. The Tcl file is here.

To monitor a sink, we have to assign a [new Agent/Loss Monitor] to each. eg.

set <sink> [new Agent/LossMonitor]

We also need a record procedure that outputs the results of each sink to a file, each time resetting the total amount of data held inside each LossMonitor Agent. We then have to respawn this procedure from within so that it takes readinds every $time seconds.

we write to a file using:

puts <$file_handle> "text"

In order to do anything, we have to define start and stop points for ns, and we do this through:

$ns at <time> "<$source_traffic> {start|stop}"

And finally run the simulation with

$ns run

Results

with 0.1 second readings:

with 0.5 second readings:

with 1.0 second readings:

The main difference's is the resolution of the results.

Another Example

Set a queue length of n between two nodes,

set queue-limit <$node1> <$node2> <n>

Thu, 24 January, 2002 17:57