How to Introduce Jitter in Netsim Emulations

How to introduce Jitter in NetSim Emulations

Introduction:

Jitter is defined as a variation in the delay of received packets.

Let us suppose at the sending side, packets are sent in a continuous stream with the packets spaced evenly apart. Due to network congestion, improper queuing, or configuration errors, this steady stream can become lumpy, or the delay between each packet can vary instead of remaining constant. This variation in delay is ‘jitter’.

While there are many ways of measuring this variation, in NetSim ‘jitter’ is measured as the statistical variance of delay. Variance is defined as the square of deviation from the mean.

Introducing Jitter using Background traffic

Background traffic can be used to test the performance of applications when link bandwidth is consumed by other traffic. It can also be used to induce jitter for testing real-time applications.

The Background traffic in NetSim can be modelled as a Poisson process in which bursts of data of a fixed sized are transmitted at an average rate such that the link will be occupied at the specified link utilization rate. Because it is a random process, over short periods the actual background traffic link utilization rate may vary from the configured value.

The rate of arrival of background traffic frames affects the jitter. Larger number of background packets induce greater jitter in competing traffic. In NetSim, the way to increase the number of background packets arriving is to reduce the inter-arrival time of that application (as explained below).

Settings done in the system where NetSim Emulation server is running:

Open Command promptand ping any system, it displays thewindow shown below. For example,

ping 192.168.0.192 –t

Create a network scenario with 1 router and 2 wired nodes as per the following:

Network Settings (without background application)

Drag and drop devices / links as per the image above and set

a)Link speed: 10Mbps

b)Link BER: 0

c)Link Propagation Delay: 3ms

d)Nodes – Transport Layer - TCP: disable

e)Drop an application and set Application Type: EMULATION

f)EMULATION Application settings: Inside the application set

1. Source_Real_IP: give IP address of the system where netsim emulation server is running
2. Destination_Real_IP: give the IP address of the system which is being pinged

Enable Event Trace and simulate the network for 100s. Open Event trace file available in the metrics window and calculate jitter which is explained below in the section “Steps to calculate Jitter (Packet Delay variance)”. You would note that jitter value got would be close to Zero.

Modelling Back Ground Traffic to introduce jitter

Next we introduce background traffic which is modelled as a Poisson process. This is known as an M/D/1 queuewhere

• Arrivals are a Poisson process and hence notated as M
• Service time(s) is deterministic or constant and hence notated as D

To get Poisson arrivals we set the inter arrival time in the application properties as an exponential distribution. And, to get a constant service rate we set the packet size in the application properties as a Constant distribution.

Network Settings (with background application)

Keep the same network settings as above and add a second Background Application. This can be done by adding an application inside application properties

a)2nd Application type - CUSTOM

1. Packet Size (constant distribution): 1440B &
2. Inter arrival time (exponential distribution): 19200μs
3. Application generation rate: 600 Kbps

Enable Event Trace and simulate the network for 100s. Similarly do the samples by varying the Application generation rates as 1200kbps, 1800kbps of application 2 (by reducing the interarrival times to 9,600 μs and 6,400 μs respectively) and calculate jitter for application1.

Output:

Note:Since we are running a real application (ping), there would be a slight variation in the results between different PCs.

Steps to calculate Jitter (Packet Delay variance):

1. Open Event Trace file which is available in NetSim metrics window
2. Click on Pivot Table in INSERT tab

1. Then a window named Create Pivot Table pops up which automatically selects the entire table, then click on OK button. In case the entire table isn’t selected please enter the range such that all rows are selected.

1. A blank PivotTable and Field List will appear on a new worksheet.

1. Drag and drop the Event_Type, Protocol_Name Fields into FILTERS, Packet_Id into ROWS, Event Time into VALUES and Device_Id into COLUMNS.
2. After dropping, the Pivot table Fields would look like

1. In the Event_Type select APPLICATION_IN and APPLICATION_OUT, Protocol_Name select 0, APPLICATION and in Column Labels select the Source_Id and Destination_Id. In our example source node ID is 2 and destination node ID is 3

1. Then select any cell of Destination node column and right click and select Show Values As option and then select Difference From option.

1. Then it opens a window named Show Values As, in that select Base Field as Device_Id and Base Item as the Source_Id (2 in this case) and click OK.

1. Then the Pivot Table would look like

1. Select any empty cell and enter =VAR.P(range of delay) and press enter. In this case the range is c6:c110. This will give variance or jitter

Note:

1. If more than 1 application is configured in network, then drag and drop APPLICATION_ID into FILTERS field and then filter the application to which the jitter need to be calculated
2. This calculation is valid only when there is no packet segmentation