Copper line basics
Sync and attainable rate, attenuation, SNR margin, CRC and FEC. What each number means and how the distance to your cabinet cuts your speed.
Updated
An ADSL or VDSL line is one pair of copper wires from your router to the street cabinet (VDSL) or the telephone exchange (ADSL). Your router measures it all the time. These numbers explain why your line gets the speed it gets.
Sync and attainable rate
- Sync rate (Sync, Actual Rate, Current Rate). The speed your router and the cabinet agreed on. It caps your connection before any other delay on the network.
- Attainable rate (Attainable, Max Rate). The router’s estimate of what the line could reach right now.
If the sync rate sits well below the attainable rate, your ISP has usually capped the line at your plan’s speed. If the two are close, the line is giving all it can.
Attenuation
Attenuation (in dB) shows how much signal is lost on the way. It grows with the length of the line, so it is the best clue to your distance from the cabinet.
A rough rule for 0.5 mm copper is about 13.8 dB per kilometre. By that rule, 14 dB is about 1 km of cable. The cable’s real route is almost always longer than the straight-line distance. Many VDSL routers report attenuation per frequency band (D1, D2, D3), and the higher bands show much bigger values. Always compare the same number.
SNR margin
SNR margin (Noise Margin, in dB) is the headroom above the noise that the line keeps so it does not make errors. Your ISP sets a target, and the router syncs as fast as that target allows.
| SNR margin | What it usually means |
|---|---|
| below 3 dB | The line is at its limit. Expect errors and drop-outs. |
| 3 to 6 dB | Tight, but often stable. |
| 6 to 12 dB | Normal for most lines. |
| above 12 dB | Plenty of headroom. The line is often capped lower than it could go. |
These are rules of thumb, not limits from the standard.
CRC and FEC
- FEC (Forward Error Correction). Errors the router fixed by itself. Large numbers are normal.
- CRC. Errors that could not be fixed and were lost. You notice them as stalls in video or calls.
The counters reset on every restart or resync. What matters is how fast they climb, not the total. A few CRC errors an hour are fine. Hundreds a minute point to a problem.
How distance cuts speed
On VDSL, the high frequencies carry most of the speed and are the first to fade as the line gets longer. That is why speed drops steeply over the first few hundred metres. 35b, for example, only pays off on lines shorter than about 250 m. For real numbers from your area, check the stats others shared on your cabinet’s page.
Before you call your ISP
- Plug the router straight into the main phone socket, with no extension cable.
- Unplug other devices on the same line (old phones, alarms) and see whether the numbers change.
- Write down sync rate, attenuation, SNR margin and CRC at two different times of day.
- Give your ISP the numbers, not just “it is slow”.