Network bandwidth and data transfer speed

Key concepts

Network bandwidth is the maximum amount of data that can be transmitted over a network within a given period of time. It is usually measured in bits per second: Mbps (megabits per second) or Gbps (gigabits per second).
Data transfer speed is the actual amount of information transmitted over a network connection per unit of time. The actual speed does not always correspond to the maximum bandwidth of the connection.
For example, a connection with a bandwidth of 1 Gbps may actually transfer data at 500 Mbps due to network load, hardware limitations, packet loss, or routing conditions.
When evaluating network speed, it is important to distinguish between bits and bytes. One byte contains 8 bits, so a speed of 80 Mbps theoretically corresponds to approximately 10 MB/s when transferring files.

Factors affecting data transfer speed

The actual network performance can be affected by various factors:

• network and network equipment utilization;
• performance of the router, server, or computer;
• connection type — wired or Wi-Fi;
• distance and route quality to the remote server;
• packet loss and data retransmissions;
• limitations imposed by the Internet service provider or remote server;
• number of simultaneous connections and network streams.

Therefore, a single speed measurement is often insufficient to determine the actual network performance.

Measuring speed with Speedtest

Speedtest is widely used for a quick check of an Internet connection. It allows you to measure:

• Download — the speed at which data is received;
• Upload — the speed at which data is transmitted;
• Ping/Latency — the delay between the user's device and the test server.

However, the result depends on the selected test server. The server may be overloaded, and the route to it may experience temporary issues. Therefore, testing against only one endpoint may produce a result that does not accurately represent the actual bandwidth of the connection.
A simple test:
speedtest


If you need to select a specific test server, you can first retrieve a list of available servers:
speedtest --servers


The test can then be performed by specifying the server ID:
speedtest --server-id=69740


For a more objective assessment, it is recommended to perform several tests against different servers and compare the results. If the speed is approximately the same across several independent endpoints, the results provide a more accurate indication of the available Internet bandwidth.
When using the command line, it is also important to consider the version of the software. Older or third-party Speedtest CLI implementations may use outdated testing methods and produce results that differ from those obtained using modern official services.

Measuring bandwidth with iPerf3

iPerf3 is commonly used for technical network diagnostics. Unlike Speedtest, it allows you to measure bandwidth directly between two specific devices or servers.
Start the server on the first device:
iperf3 -s
On the second device, start the client, where SERVER_IP is the IP address of the first server:
iperf3 -c SERVER_IP
By default, the test runs for 10 seconds and displays the average transfer speed.
For a longer test, you can specify the test duration:
iperf3 -c SERVER_IP -t 30
To test the connection in the reverse direction, use the -R option:
iperf3 -c SERVER_IP -R
iPerf3 also supports multiple parallel streams. For example:
iperf3 -c SERVER_IP -P 4 -t 30
This can be useful when a single TCP stream is unable to fully utilize the available bandwidth.
To test a UDP connection and packet loss, use:
iperf3 -c SERVER_IP -u -b 100M -t 30
During UDP testing, it is important to pay attention not only to the transfer speed but also to packet loss and jitter — variations in packet delivery latency.

Speedtest vs. iPerf3: key differences

Characteristic	Speedtest	iPerf3
Purpose	Testing Internet access	Testing a connection between two endpoints
Ease of use	High	Requires technical knowledge
Test configuration	Limited	Flexible
TCP and UDP	Depends on the implementation	Supported
Packet loss testing	Limited	Supported for UDP
Network diagnostics	General assessment	Detailed diagnostics
Requires a second server	No	Yes

These tools are not direct competitors.
Speedtest shows the quality of Internet access and the transfer speed to external test servers. iPerf3 allows you to test the performance of a specific network connection between two known endpoints and is better suited for technical diagnostics.

How to interpret the results correctly

If Speedtest shows a low speed while iPerf3 between two servers shows normal bandwidth, the problem may not be directly related to the server or its network interface. It may instead be caused by the route to the test server or by an issue within the provider's network.
If iPerf3 shows low bandwidth directly between two servers, you should additionally check:

• network interface utilization;
• errors and packet loss;
• MTU;
• CPU utilization;
• virtual network adapter configuration;
• virtualization limitations;
• network equipment and the route between the servers.

Thus, combining several testing methods makes it possible to determine which part of the network path is causing the limitation.

Conclusion

Bandwidth represents the potential capacity of a network connection, while data transfer speed reflects its actual performance at a specific point in time.
Actual network speed is affected by many factors, so the results of a single test do not always provide a complete picture.
For a quick assessment of an Internet connection, Speedtest is convenient, especially when measurements are performed against several test servers. For troubleshooting network issues and checking bandwidth between specific devices, iPerf3 is more suitable.
Using both tools provides a more objective assessment of network performance and helps determine whether low speed is related to the Internet connection, a specific route, the server, network equipment, or other limitations.