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Troubleshooting Client Speed using iPerf

iPerf is a tool which can be used to test LAN and WLAN speeds and throughput. It works by generating traffic from a computer acting as a client which is sent to the IP address of a computer acting as the server. Bandwidth is then measured by TCP tests which are geared to show an actual representation of the network speeds you are receiving. iPerf can also be used to determine the quality of a link or test latency, jitter and datagram loss.
This article explains how to use iPerf to test LAN and WLAN speeds.  

Note: To run this test you will need two networked computers which have access to each other through your local LAN. Each computer will have to have a copy of iPerf software installed.

Running iPerf tests

After downloading iPerf:

1. Open a command prompt and navigate to the folder containing the iperf.exe file.

Example: If the file is on your desktop, then you would use the following in the command prompt:

cd C:\Users\Administrator\Desktop\iperf-2.0.5-2-win32

2. In the command prompt, run this command on the server side. This will tell it to listen over port 5001 (default port) for the client data:

  • Server: iperf -s 

3. In the command prompt on the client side, run this command:

  • Client: iperf -c

As long as the client and server have connectivity on the same network, you should receive a result similar to the following on the server and client side, respectively:







There are also several options that can be ran on the client side to change the type of data that is being sent. 

To measure bandwidth in both directions, use option r.

  • iperf -c -r

To measure bandwidth in both directions at the same time, use option d.

  • iperf -c -d

TCP Window Size

Sending large streams of data over fast connections can generate a lot of CPU usage, especially when testing over 1Gbps links. Specifying a larger window size reduces the amount of processing that needs to be done on both the client and server side computers. The window size sets the amount of data that can be passed before it has to be SYN'd. To specify the window size while using iperf, please use the option w command. The default number is in bytes, but you can also specify K and M for Kilobytes and Megabytes, respectively. 

Client side example:

  • iperf -c -w 2000

-w allows for the option to manually set a window size. This can be set to be between 2 and 65,535.  In the previous example, the window size is set to 2000 Bytes

Server Side example:

  • iperf -s -w 4000K

In the previous example, the window size is set to 4000 Kilobytes

Parallel Streams

iperf also allows for sending parallel streams at the same time.  This sends multiple streams of traffic through the tunnel in order to more fully test the link.

Note: If using 1Gbps links, you want to utilize using window sizing and parallel streams.  Use more parallel streams if trying to completely saturate a high speed link.

Client side example:

  • iperf -c -P 2

This has iPerf run 2 parallel streams

Server Side example:

  • iperf -s

The server side does not have to include the parallel streams options since it is already listening for any of the traffic.

Additional Resources

If you prefer a GUI-based tool similar to iPerf for testing LAN and WLAN speeds, see Troubleshooting client speed and traffic shaping using jPerf.

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