A node in NS is compound object made of address and port classifiers (described in a later section).
We’ll look at how to create topologies, how to get the traces and then how to use NAM to visualise our network.
Our major section is devoted to this part, which is how to use the simulator.
Read Installing NS2 is non-trivial on the cs machines because of various dependencies, so its been already done for us.Apart from having a trace file, it comes with a Network animator tool that allows visualization of nodes (much like the diagrams that we see in text books).The picture shows an example trace file: - + shows that a packet entered the queue - shows that a packet left the queue - r shows the packet was received time, type of packet etc.NS2 lets us create topologies with different nodes, different types of links in betweeb them.We are going to now look at a particular simulator – the NS2 simulator Its open source – available at source forge.Start out with a numerical model – a scheme of mathematical equations Maintain abstractions, but try to grab general idea using simulation Emulation – you try to model the exact behaviour of an intented system Small scale deployment.Simulation lets you separate the design bugs from the implementation bugs (for example – a server farm - have not got enough provisioning) – estimate etc.Simulation is repeatable – When studying flash crowds, its hard to create flash crowds again and again, but its possible to simulate them as many times as needed More importantly, Simulation can let you abstract out some of the complexity. We should go ahead and observe tha actual system if possible, but we can’t do so for a variety of reasons.Properties, so that we can observe the relevant ones We should be able to distiguish between the important and the non-imp. We want to model the system so that we can engineer better systems - Its clear that simulation becomes harder as we have large systems and varied inputs. Our “exogenous” behaviour” can be, for exampe – how users are using the system – for example growth of online streaming video with youtube. We want to study the behaviour of system under a given set of external factors - For Internet, our system is the way we have placed nodes, routers, our protocols running on them (HTTP, TCP) etc.we’ll be looking at the following in particulat - fundamentals of event simulation – what are events? How are they tracke? - details of a particular popular network simulator other - other examples are opnet, qualnet etc - The goal for us is to understand how networks work by simulating them.