ADSL: Long Range, Moderate Speeds
ADSL (Asymmetric Digital Subscriber Line) was the dominant residential broadband technology through the 2000s and early 2010s, and remains widely deployed in areas that have not yet received fiber or cable upgrades. ADSL uses frequencies between roughly 25 kHz and 1.1 MHz for ADSL, extending to 2.2 MHz in the ADSL2+ standard. This relatively low frequency range attenuates slowly over copper wire, allowing ADSL to function over copper runs of up to 5–6 km from the DSLAM at the telephone exchange.
The tradeoff for this reach is speed. ADSL2+ has a theoretical maximum of 24 Mbps downstream and 3.5 Mbps upstream — figures that are only achievable within a few hundred meters of the exchange. Most ADSL subscribers at typical suburban distances of 1–3 km receive 5–15 Mbps downstream and 0.5–1 Mbps upstream. The "asymmetric" in the name reflects the heavily lopsided allocation of bandwidth: much more spectrum is devoted to downstream than upstream, matching typical residential usage where people download far more than they upload.
VDSL: Faster but Distance-Limited
VDSL (Very High Speed Digital Subscriber Line) achieves dramatically higher speeds by operating at much higher frequencies. VDSL2 Profile 17a uses frequencies up to 17 MHz; Profile 35b (also called Supervectoring) extends to 35 MHz. At these frequencies, the available bandwidth per unit of spectrum is enormous, enabling downstream speeds of up to 100 Mbps on Profile 17a and up to 300 Mbps on Profile 35b under ideal conditions.
The problem is that high-frequency signals attenuate far more rapidly over copper than the lower frequencies ADSL uses. VDSL2 at 17 MHz attenuates roughly ten times faster per meter than ADSL at 1 MHz. The practical result is that VDSL2 only achieves its headline speeds within approximately 300 meters of the DSLAM. At 1 km, speeds typically fall to 40–60 Mbps. Beyond 1.5 km, VDSL2 speeds drop into ADSL2+ territory. At 3 km and beyond, VDSL offers no meaningful advantage over ADSL.
FTTC: How Telecoms Make VDSL Work
The short range of VDSL would make it useless as a mass-market technology if DSLAMs remained inside telephone exchanges, which can be several kilometers from many homes. The solution is FTTC — Fiber to the Cabinet. In an FTTC deployment, the ISP runs fiber from the telephone exchange to a street-level cabinet located close to a cluster of homes, typically within 300–500 meters. A VDSL2 DSLAM is installed inside the cabinet, and the final connection from cabinet to home uses the existing copper pair.
This architecture lets VDSL2 operate in its high-speed range for most customers while avoiding the cost of running fiber all the way to each individual home. FTTC VDSL2 has been the primary superfast broadband strategy in the UK (deployed by BT Openreach), Australia (as part of the NBN), and numerous European countries. Customers close to the cabinet get 70–80 Mbps; those further away get progressively less, reverting to ADSL-class speeds at the far end of the copper run.
VDSL2 Vectoring: Cancelling Crosstalk
One of the most impactful improvements in VDSL2 deployments is vectoring, standardized as ITU-T G.993.5. When many copper pairs from different homes run through the same cable bundle, they create electromagnetic interference in each other — a phenomenon called crosstalk. At VDSL2's high frequencies, crosstalk can be severe enough to halve the effective throughput on affected lines.
Vectoring addresses crosstalk with a form of coordinated noise cancellation. The DSLAM continuously measures the crosstalk that each line creates in its neighbors, then generates a pre-cancellation signal that is superimposed on the transmitted data. By the time the signal reaches the far end, the crosstalk has been largely cancelled out. In real-world deployments, vectoring typically improves speeds by 20–50% on lines in dense cable bundles. VDSL2 vectoring is now standard in most modern FTTC deployments.
G.fast: The Next Step in DSL
G.fast, defined in ITU-T G.9700/G.9701, pushes DSL frequencies up to 106 MHz or 212 MHz — far beyond VDSL2's ceiling. This enables theoretical speeds of up to 1 Gbps downstream, approaching fiber-like performance. However, the attenuation at these extreme frequencies limits G.fast to a range of roughly 100 meters from the distribution point, making it suitable only for the very last segment of the copper network.
G.fast is deployed in FTTDP (Fiber to the Distribution Point) scenarios — typically apartment buildings where fiber runs to a node in the basement or utility room, and G.fast then covers the existing in-building copper wiring to each apartment. It is also used in dense street deployments with distribution points in small cabinets every 50–100 meters. G.fast is not a replacement for ADSL or VDSL in typical suburban deployments; it occupies a specific niche where the copper run is very short but running fiber to each individual unit is impractical.
Which Should You Choose?
In most cases, you will not choose between ADSL and VDSL — your ISP determines which technology is available at your address based on your distance from the nearest DSLAM and the type of infrastructure deployed in your area. If you are within range of an FTTC cabinet, your ISP will provision you on VDSL2 automatically when you order a superfast broadband plan. If only a telephone exchange-based DSLAM serves your address, ADSL2+ will be your DSL option.
If you have a choice — for example, if your ISP offers both a standard DSL plan (ADSL) and a superfast plan (VDSL) and you are wondering whether to pay more for VDSL — use your distance from the nearest cabinet or exchange as the deciding factor. If you are within 500 meters, VDSL will make a large difference. If you are 2 km or more away, the speed improvement may be negligible.
ADSL vs VDSL2 vs G.fast Comparison
| Feature | ADSL2+ | VDSL2 (Profile 17a) | G.fast |
|---|---|---|---|
| Max downstream | 24 Mbps | 100 Mbps | Up to 1 Gbps |
| Max upstream | 3.5 Mbps | 50 Mbps | Up to 500 Mbps |
| Max effective distance | 5 – 6 km | 1 – 1.5 km at useful speeds | ~100 m |
| Frequency range | Up to 2.2 MHz | Up to 17 MHz | Up to 212 MHz |
| Symmetric capable | No | Partially (with bonding) | Yes |
| Typical deployment | Exchange-based DSLAM | FTTC street cabinet | FTTDP building node |
Frequently Asked Questions
What is the maximum speed of ADSL2+?
ADSL2+ has a theoretical maximum downstream speed of 24 Mbps and a maximum upstream speed of 3.5 Mbps. These figures are only achievable very close to the telephone exchange or DSLAM — typically within 300–500 meters. At 1.5 km, typical downstream speeds fall to around 10–15 Mbps. At 3 km or more, speeds often drop below 5 Mbps. In real-world deployments, most ADSL2+ subscribers receive between 3 and 15 Mbps depending on their distance from the exchange and the quality of the copper pair.
How far can VDSL work at full speed?
VDSL2 achieves its maximum speed of around 100 Mbps downstream only within approximately 300 meters of the DSLAM. Between 300 and 1,000 meters, speeds typically fall to 40–70 Mbps. Beyond 1,500 meters, VDSL2 speeds drop into ADSL territory — below 20 Mbps. At around 3 km, VDSL2 provides no meaningful advantage over ADSL2+. This short effective range is why VDSL2 is almost always deployed in FTTC configurations, where fiber runs from the exchange to a street cabinet close to homes, and VDSL2 covers only the short final copper segment.
What is VDSL2 vectoring?
VDSL2 vectoring is a noise cancellation technique that reduces crosstalk — interference between adjacent copper pairs in the same telephone cable bundle. Vectoring works by having the DSLAM measure the crosstalk from each line and generate a cancellation signal that is added to the transmitted data, effectively removing the interference before it corrupts the signal. Vectoring can improve VDSL2 speeds by 30–50% on lines that are severely affected by crosstalk, and is standard in modern FTTC VDSL2 deployments.
Is VDSL better than ADSL?
VDSL is faster than ADSL for customers who are close to the DSLAM — within about 1 km. If you are within 300 meters of a VDSL2 cabinet, you may receive 80–100 Mbps compared to the 20 Mbps ceiling of ADSL2+. However, if you are more than 1.5 km from the DSLAM, VDSL2 provides little to no speed advantage over ADSL2+. The technology that is better for you depends entirely on your distance from the nearest DSLAM, not on a fixed technical superiority of one standard.
What is G.fast DSL?
G.fast is the latest generation of DSL technology, defined in ITU-T G.9700/G.9701. It uses frequencies up to 212 MHz — far above VDSL2's 35 MHz — to achieve theoretical downstream speeds of up to 1 Gbps. The tradeoff is extremely short range: G.fast only works within about 100 meters of the distribution point. G.fast is primarily used in FTTDP deployments in apartment buildings and dense urban areas, where fiber runs to a node in the basement or a street cabinet and G.fast covers the last few tens of meters over existing in-building copper wiring.
Why does my VDSL speed decrease over distance?
VDSL operates at much higher frequencies than ADSL — up to 17 MHz for VDSL2 Profile 17a, or 35 MHz for Profile 35b. These high frequencies allow VDSL to carry enormous amounts of data per unit of bandwidth, but they also attenuate very rapidly over copper wire. The attenuation rate at 17 MHz is roughly ten times greater per unit length than at the 1 MHz frequencies ADSL uses. This means that the high-frequency channels that carry VDSL's fastest data become unusable just a few hundred meters from the DSLAM, forcing the modem to fall back to lower-frequency, lower-bandwidth channels — which produces speeds comparable to ADSL.