Traffic Generator

• How can you translate the load of the system to the rate of the agent(s) you want to generate?

  The load of the system depending on the limited resource of the system. In the ns simulation, the link capacity is the limited resource in most cases. The capacity that equals to the total capacity of the limited resource represents 1.0 load. Therefore when you calculate of the load of the system, you need to find the proportion of total rate of your traffic to the capacity of the bottleneck link

    

  • What is the rate of the generated traffic for 0.1 load of 10 Mb/s link?
    • 0.1*10 = 1 Mb/s

     

• Once you know the rate of the traffic that your agent(s) need to generate, how can you translate that rate to the implementation? TCP agent rate? UDP agent rate?

  TCP

  • For TCP, you assume that once a TCP agent starts, it will use all available bandwidth. The only way you can limit the rate of the agent is to turn it on and off.
  • From the above answer, you have to generate 1 Mb/s traffic on the 10 Mb/s link. So you have to turn it on for the proportion of 1/10 of time and turn it off 9/10 of time. If you have turn it on 1 second, you need to turn it off 9 second in average.   
  • Now you know the numbers! Yet, you cannot just use those numbers directly because you need to know the behavior of the system in all aspects not just the specific aspect acquired from the fixed interval. In the practical implementation, you need the poison process to generate number of arrivals for you. Poison <--> Exponential. More specifically, you need exponential inter-arrival time.
  • Ex. If the TCP agent is turned on for 1 second, you need to generate the next arrival time by using exponential(10)+current time. This can interpreted that the TCP agent will generate the traffic at time Y (X+current time), where X is exponentially distributed with average of 10 s.
  • You can generate the exponential random variable in tcl script. Check the random variable in mathematical support section in ns manual

  UDP

  • For UDP, it is pretty much straight forward. You can specify the rate and the packet size at the application level. The total rate of the UDP agents is calculated from all the average bit rates from all UDP applications. Check out how to specify these parameters in Traffic Generator section in the ns manual.
  • However, to apply with the project, you have to design the scenario that these applications will come and leave at some times. You need to implement admission control. Therefore, the total rate of all UDP applications will be calculated from not only the rate from each application but also the arrival rate of each application and the period that they stay on.