ISP Diagnostics
Bandwidth Saturation Test: Behavior Under Heavy Load
Speed tests usually finish in under 20 seconds, missing problems that only appear under sustained load. The Bandwidth Saturation Test runs an extended download to characterize how your connection behaves once buffers fill, congestion control settles, and any ISP traffic shaping kicks in.
What It Measures
This tool measures sustained download throughput, latency under load, and throughput stability over an extended window. It detects burst limits (where ISPs allow brief peaks), shaping kicks (where speed drops after a threshold), and buffer growth (where latency climbs steadily).
How It Works
- Initiates a long download (typically 60 to 120 seconds) from the nearest edge server
- Samples throughput and latency at 1 second intervals
- Detects throughput drops, shaping events, and bufferbloat progression
- Reports peak burst speed, sustained speed, and a saturation behavior profile
Why It Matters
Many ISPs allow brief speed bursts then throttle back to a lower sustained rate. Some apply traffic shaping after a few hundred MB. Cable modem buffers can fill and cause latency to balloon during long transfers. Short speed tests miss all of this. Saturation testing reveals the speed your connection can actually maintain.
Understanding Your Results
Sustained speed should be within 10% of your peak speed throughout the test. Latency under load should stay within 30ms of idle latency. Throughput dropping more than 20% after the first 10 seconds suggests ISP traffic shaping. Latency climbing steadily over the test window indicates bufferbloat.
Frequently Asked Questions
Why is sustained speed lower than peak speed?
Some ISPs implement burst pools that let you exceed your nominal speed for short periods, then throttle back. This is common on cable connections marketed with PowerBoost or similar features. The peak makes speed tests look great but real workloads (cloud backups, large downloads) only see the sustained rate.
How does this differ from a regular speed test?
A regular speed test runs for 10 to 20 seconds and reports peak throughput. The saturation test runs for 60 plus seconds and reports both peak and sustained behavior, plus shaping events and latency under load. It measures what your connection actually delivers for long downloads.
What does a steadily climbing latency mean?
Climbing latency under sustained load means your modem or router buffers are filling faster than they drain. This is bufferbloat, and it makes everything else on your network feel slow during the download. Enabling Smart Queue Management (SQM or fq_codel) on your router fixes this if your hardware supports it.
Should I be worried if my ISP shapes traffic?
Some shaping is normal and helps fairness. Aggressive shaping that kicks in after a few hundred megabytes effectively reduces the value of your plan. If you see sharp drops within minutes of starting a download, your ISP may be applying selective shaping. Compare against ISPs in your area to know if it is a competitive concern.
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