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Measuring network throughput with iperf3 and speed tests

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tagsglossar, netzwerk, werkzeug

3 min read

Measuring network throughput with iperf3 and speed tests

In short: iperf3 measures the transfer rate between two computers on your own network and shows whether cable, switch or Wi-Fi slows things down; online speed tests, by contrast, measure the route to the internet.

In more detail: “The network is slow” is not a diagnosis. With measurements you separate internet connection, Wi-Fi, cable/switch and disks/application.

In Depth

iperf3 in your own network

One computer is the server, one the client:

iperf3 -s                       # server (port 5201)
iperf3 -c 192.168.1.20          # client: TCP test 10 s
iperf3 -c 192.168.1.20 -R       # reverse direction (server sends)
iperf3 -c 192.168.1.20 -P 4     # 4 parallel streams
iperf3 -c 192.168.1.20 -u -b 100M   # UDP at 100 Mbit/s (packet loss, jitter)
iperf3 -c 192.168.1.20 -t 30 -i 5   # 30 s, interim values

Expectation: gigabit LAN ≈ 940 Mbit/s (≈ 110 MB/s), 2.5 GbE ≈ 2.35 Gbit/s, Wi-Fi depending on standard/distance 100–1500 Mbit/s with fluctuation. Much less → cable (Cat 5e/6, kink), port negotiated at 100 Mbit, switch/router load, drivers, CPU limit, duplex errors. Bit vs. byte: Mbit/s ÷ 8 = MB/s.

Speed tests for the internet

  • Speedtest (Ookla), fast.com, Cloudflare Speed Test, speedtest.net, Breitbandmessung of the Federal Network Agency (official measurement with report, important for underperformance versus the contract).
  • Measure by cable at the router, other devices off, several times/servers.
  • Download, upload, ping/latency, jitter, packet loss (important for gaming/video calls).
  • Bufferbloat (high latency under load): test at waveform.com/tools/bufferbloat, remedy SQM/smart queue in the router.
  • Results depend on remote side, time of day, Wi-Fi and VPN.

Finding typical bottlenecks

  1. Own device: CPU/disk (HDD ≈ 100–150 MB/s!), antivirus, drivers.
  2. Cable/port: ethtool eth0 (speed/duplex), Windows adapter status; test another cable/port.
  3. Switch/router: load, firmware, powerline/mesh bridge limits.
  4. Wi-Fi: band (2.4 vs. 5/6 GHz), channel, distance, interference (see Wi-Fi channels).
  5. Internet: provider, tariff, DSL line, time of day, DNS (slow loading ≠ low throughput).
  6. Remote side/server.
  • Method: change one factor, repeat, note results; with iperf3 end-to-end vs. sections (e.g. router ↔ PC).

More tools

  • ping/mtr (latency, loss), traceroute, ss/nload/iftop/bmon (real-time load), Wireshark (packet level), NetFlow/sFlow (traffic analysis in companies), iPerf3 as container, fio (storage), LibreSpeed (self-hosted speed test), nuttcp, netperf.
  • Documentation (date, device, tariff) helps with complaints to the provider and verification after changes.
  • Exam: units (bit/byte), latency vs. throughput, measurement errors (overhead, duplex).
  • Legal: measure only in your own network/with permission; internet speed tests use data volume (mind mobile data).

See also: Ping, Wireshark, Link aggregation

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