Queuing and buffer overflow in routers

Open NetSim and Select Examples > Internetworks >Queuing and buffer overflow in routers then click on the tile in the middle panel to load the example as shown in below Figure.

Figure4-9

The following network diagram illustrates, what the NetSim UI displays when you open the example configuration file as shown below Figure.

Figure4-10

Network Settings

  1. Click on the Application icon present in the top ribbon/toolbar and set Transport Protocol to UDP
  2. Generation rate = 10Mbps for each application (Packet Size: 1460, Inter Arrival Time: 1168µs)
    a. Generation Rate (Mbps) = (Packet size (bytes) * 8) / Inter arrival time (µs))
  3. The traffic generation rate can be modified by changing application properties. Note that the generation rate should be less than or equal to service rate for steady-state simulation, where the service rate is defined as the data rate supported by the Bottle-neck link. In this case, there is no bottle neck link since all links support up to 100 Mbps
  4. Plots and Packet Trace is Enabled
  5. Simulate for 100s and note down the throughput
  6. Go back to the scenario and change the link speed (both Uplink and Downlink Speed) between Router_5 and Wired_Node_4 from the default 100 Mbps to 25 Mbps. In this case, the link between Router_5 and Wired_Node_4 becomes a Bottle-neck link, since the link rate (i.e., service rate) is less than the generation rate of 30 Mbps (10 * 3).

Discussion

Bottleneck link 100Mbps: In this scenario, router receives packets from three links at the rate of 10 Mbps each, a total of 30 Mbps. And the router-node link supports 100 Mbps. Hence there is no queuing / packet drop in the Router. The application throughput would be approximately equal to the generation rate.

Figure4-11

Bottleneck link 25Mbps: In this case, the bottleneck link supports only 25 Mbps. Due to this, packets get accumulated in the router's buffer, which overflows after reaching its limit and hence router starts dropping the packets. Application throughput would be approximately equal to the bottle neck link capacity.

Figure4-12