Troubleshooting network related issues can be challenging at times. One great tool for on-the-spot testing of network bandwidth is Iperf. It’s a simple, yet powerful tool that can provide visibility on network performance metrics such as bandwidth, latency (delay), jitter, and packet loss. If you are not comfortable with the command prompt, the program is available in a GUI version called Jperf too.

Iperf  measures network performance in terms of bandwidth, but its worth noting that the unit of measurement for file size and transfer rate are different. Files are typically stated in bytes, while transfer speeds are expressed in bits, so remember that!

When testing bandwidth performance with Iperf, what we’re actually testing is maximum TCP bandwidth at the transport layer (L4). Lots of applications use TCP as the transport protocol, include HTTP, SMTP, FTP, etc. Unlike UDP (another commonly used L4 protocol) TCP is a reliable, connection-oriented protocol with built-in mechanisms for connection established, acknowledgement, and termination.This connection management is why TCP is used (and not UDP) to test bandwidth. If UDP was being used, any lost packet(s) would go missing from the traffic flow and not be resent. Simply put, TCP has a means to detect and retransmit any lost segments.  If your network is configured for QoS or CoS, you must make sure that the IPs or ports for these tests are configured for the highest priority QoS.  Otherwise, your results will not be accurate.

The following examples use a combination of Microsoft Windows 7 (64-bit) and XP (32-bit). Also, the default TCP window size is used but keep in mind you can adjust window size and effect bandwidth results. To use the program, open the command prompt and navigate to the directory where Iperf.exe is located (Start>Run and type cmd, then press enter). Once in the command prompt you can change directories via the cd command. As you can see I am using Iperf version 1.7.0.

Iperf command line

Before we get started I want to quickly discuss Iperf’s architecture. It uses a client/server model, where traffic is initiated from the client and traverses the network (LAN and/or WAN) to the server. So when testing bandwidth in both directions we’ll need to run the test twice, once in each direction. Also, by default the server will listen for the client on TCP port 5001 but like most options this can be changed. Note the



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