| Applicable Versions | NetSim Standard | NetSim Pro |
| Applicable Releases | v11 | v12 | v13
|
Consider a simple network scenario with two wired nodes connected to router exchanging CBR traffic as shown below:
CBR application is configured between node 1 and 2 with a packet size of 1460 Bytes and Inter arrival time of 20000 Micro Seconds as shown below:
At Simulation end NetSim results dashboard provides the various performance metrics based on the protocols running in different layers of the network stack. Packet trace and Event trace log files will be available if the respective options are enabled prior to running the simulation.
The Link metrics table in the results dashboard provides information about the overheads transmitted in each link.
The Application Metrics table in the results dashboard provides the average end to end application delay experienced by the application packet during the simulation.
As per the Link and Application metrics tables discussed above, the overhead transmitted and Delay are:
| Overheads Transmitted | 1337792 Bytes |
| Delay | 15879.486 Micro Seconds |
NetSim's packet trace log file consists of abundant information from which lot of useful information can be derived. Overheads may be calculated as the sum of the difference between the Physical layer payload and application layer payload of each data packet. Delay may be calculated as the sum of the difference between the Physical layer end time and the application layer start time of each packet.
However if overheads and delay are calculated from the packet trace log file using excel operations we get the following values which is slightly different from those displayed in the metrics window:
| Overheads Transmitted | 1321732 Bytes |
| Delay | 15886.123 Micro Seconds |
Alternate methods for calculating delay and overheads accurately:
Overhead
As packets flow from the application layer to the Physical layer, extra bytes of format information is added to it as headers by the respective protocol running in each of the layers. These additional bytes are termed as overheads.
The difference in Overhead between the Network metrics table(1337792 Bytes) and the calculation from the Packet trace(1321732 Bytes) is 1606 Bytes.
The reason for this is that the calculation of overheads in NetSim considers the data packets that are errored as overheads.
From the packet trace, upon filtering the Errored packets in the PACKET_STATUS column, we see that there are 11 data packets that have got errored.
Upon adding these 1606 Bytes of the payload of packets that are errored, we get the total overheads as 1337792 Bytes.
Delay
The end to end delay of a packet is the time taken by the packet between APPLICATION_OUT and APPLICATION_IN events in NetSim.
Where,
APPLICATION_OUT is the time at which a packet has left the source node's Application layer
APPLICATION_IN is the time at which a packet has reached the destination node's Application layer
Therefore the average end to end delay needs to be calculated from the Event Trace log file instead of the packet trace.
Pivot tables can be used to quickly calculate the end-to-end delay from the event trace.
After opening the event trace log file, switch to the sheet "Pivot Table(Custom)"
From the PivotTable Fields list, add Event_Type to Rows and Event_Time(US) to columns
Here the sum of APPLICATION_OUT time is 249950000000.00 microseconds and the sum of APPLICATION_IN time is 250029397433.41 microseconds.
The average end to end delay experienced by the total of 5000 Application packets is
(250029397433.41-249950000000.00)/5000 = 15879.486688 microseconds
