Why DSL and VDSL have a slow upload and what speed depends on

DSL and VDSL internet runs over the old telephone line and has two traits that confuse people: the upload tends to be much slower than the download, and two neighbors can have completely different speeds. Let us explain why that is, what DSL speed really depends on, and how to check it yourself.

What DSL and VDSL are

DSL is internet over a metallic telephone line, that is over the copper pair that runs to the house. The older and slower variant is ADSL, the newer and faster one is VDSL. The internet runs alongside the ordinary telephone on higher frequencies, so both work at the same time. While fiber carries data with light, DSL is bound by the physical limits of copper.

Why the upload is slower than the download

This is not a fault, but a deliberate design. The letter A in ADSL stands for asymmetric. The available frequency band on the line is divided unevenly: a larger part is reserved for downloading and a smaller part for uploading. It comes from the fact that an ordinary user downloads (web, videos) far more than they upload. That is why you see plans like 50 Mbit down and only 5 Mbit up. VDSL is asymmetric too, just with higher numbers.

What a slow upload means in practice

A slow upload is not just a number in a speed test, you feel it during everyday tasks. Backing up photos and documents to the cloud, see our guide on cloud backups, takes disproportionately long with a weak upload, and a larger batch of data sometimes will not finish sending even overnight. During video calls, whether a work meeting or catching up with family through an app, a low upload causes the picture to freeze and the sound to drop even when the download is fine, because it is your side that is sending video through a narrow channel. Streaming on platforms like Twitch or YouTube Live needs a stable upload of several Mbit/s or more, which many DSL plans simply cannot provide. The same applies to sending large files, say photos from an event to a client or footage from a security camera to remote storage, where a slow upload shows up as long waits or interrupted transfers.

What DSL speed depends on

DSL speed is not given only by the plan, but mainly by the physics of the line:

• Distance from the exchange or cabinet. This is the most important factor. A signal over copper weakens with length (this is called attenuation), so the further you are, the lower the speed. VDSL uses higher frequencies that weaken even faster, so it is very sensitive to distance.
• The quality and condition of the line. Old, oxidized or badly joined wires lower the speed.
• Interference from adjacent lines. In one cable bundle the lines interfere with each other (crosstalk).
• The technology used. ADSL, VDSL and VDSL2 with vectoring differ significantly in maximum speed.

Why two neighbors have different speeds

Precisely because the length and quality of the specific line to the exchange decide. Even two neighbors can be connected by lines of different lengths or different ages, so one has full speed and the other significantly lower, even though they have the same plan.

Comparing typical upload speeds

The table below shows indicative speed ranges by technology, so you know what to realistically expect from your connection.

These figures are indicative, the real speed depends on the specific plan, the distance from the exchange and the technology used by your particular provider.

How to measure and diagnose your real upload speed

If you suspect your connection is not reaching what you are paying for, work through it step by step:

1. Connect with a cable directly. Run the test from a device wired straight into the router, not over Wi-Fi, so you rule out any loss caused by the wireless signal.
2. Turn off other activity. Close downloads, background backups and any other connected devices that could be loading the line at the same time.
3. Measure the speed several times. Run a speed test repeatedly, ideally in the morning, during the day and in the evening, to see whether the values change with network load.
4. Compare with the plan in your contract. A realistic result should reach roughly 70 to 80 percent of the advertised “up to” figure, a small amount of fluctuation is normal and not a reason to worry.
5. Check with more than one server. If the difference is large, say you get 2 Mbit instead of 10 Mbit, repeat the test on a different testing server to rule out a single faulty measurement.
6. Judge whether it is a fault. DSL speed fluctuates slightly with network load and even weather, moisture can raise attenuation on older lines. A difference of a few percent is normal. If the measured value stays well below half of the plan over time, it is likely a fault or a line problem.

Common mistakes and myths about a slow upload

• “A new router will speed up my upload.” Router settings can only adjust a small part of performance, the real limit on upload comes from the physical line and the exchange, not from the equipment in your home.
• “Wi-Fi is to blame for my slow upload.” Wi-Fi can lower speed between a device and the router, but the DSL upload limit is set earlier, at the connection to the exchange. A test over a cable reliably shows where the real cause lies.
• “My provider is lying about the speed.” Plans are usually advertised as “up to”, meaning the maximum theoretical value under ideal conditions. The real speed depends on the length and condition of the specific line, it is not necessarily deception.
• “Priority settings (QoS) in the router will fix a slow upload.” QoS can only redistribute the existing capacity among devices in the household, it cannot create extra capacity that is not there.

Troubleshooting and when to contact your provider

If measured values consistently and repeatedly fall well short of the plan, first try restarting the modem or router and repeat the test a few hours later. If nothing improves, contact your provider and ask them to measure the attenuation on the line directly, providers can usually read this remotely from the exchange. Keep the results of several measurements with timestamps ready, it makes diagnosing the problem much easier. If the cause is physical distance from the exchange or an old line, the only lasting fix is usually switching to a different technology, such as fiber.

If you need a higher upload, for example for cloud backups, video calls, working from home or sending footage from cameras, DSL will keep limiting you in the long run. The real solution is a fiber connection, which tends to be symmetric (the same download and upload). Check whether fiber is available in your area, or consider LTE or Starlink. You will find more on speeds in the article internet speed.

This article is part of our Computer networks overview.