A plain-language explanation of what iperf3 measures, how the test is structured, and how to read your results.
iperf3 always needs two endpoints: a server waiting for connections and a client initiating the test. Your iPhone is always the client. The server at 185.185.33.209:443 is permanently running and ready.
Why port 443? Most mobile carriers don't throttle or prioritise HTTPS traffic differently, and port 443 is rarely blocked by firewalls — making it the most representative port for real-world throughput.
The server pushes data to the iPhone. The iPhone measures how fast it can receive. This is reverse mode — the client initiates the connection but the server does the sending. Matches real-world browsing, streaming, and app downloads.
The iPhone pushes data to the server. Measures uplink capacity — relevant for video calls, cloud backups, and sending large files. Typically lower than download on mobile networks.
TCP connections take a few seconds to ramp up to full speed (TCP slow-start). The omit (-O 5) flag discards the first 5 seconds so the final result reflects steady-state throughput, not the ramp-up noise.
Each second of the measured window produces one interval line in the log. The sparkline graph in the run cards shows this per-second throughput — you can see how stable (or spiky) the connection was throughout the test.
A single TCP stream is limited by the TCP congestion window — on high-latency or lossy mobile links it often can't saturate the available bandwidth. Using 10 parallel streams opens 10 simultaneous TCP connections, each probing the network independently. Their combined throughput reflects what the link can actually deliver.
↑ Real data from the sample log. Notice streams vary wildly — the SUM is what matters.
| Metric | Good | Acceptable | Investigate |
|---|---|---|---|
| Download (5G) | > 300 Mbps | 100–300 Mbps | < 100 Mbps |
| Download (4G) | > 50 Mbps | 20–50 Mbps | < 20 Mbps |
| Retransmits / GB | < 300 | 300–800 | > 800 |
| Peak / Avg ratio | < 1.3× | 1.3–2× | > 2× (very spiky) |
| Sparkline shape | Flat / rising | Moderate variation | Huge spikes / drops |
185.185.33.209 and port 443For accurate carrier comparison: always test at the same location and time of day. Mobile throughput varies with tower load, signal strength, and time. Run each carrier back-to-back with the same settings.