Fiber vs DSL: why copper stops at 100 Mbps.
DSL runs internet over the same telephone copper laid down for voice in the 20th century. The physics caps it: signal weakens sharply with distance, upload was an afterthought, and every splice oxidizes. Fiber doesn't play the same game — and both AT&T and BT Openreach are actively retiring copper to replace it.
Side by side.
| Metric | Fiber (FTTH) | DSL (ADSL2+ / VDSL2) |
|---|---|---|
| Max download (typical) | 1–10 Gbps | 24 Mbps (ADSL2+) / 100 Mbps (VDSL2) |
| Max upload (typical) | 1–10 Gbps (symmetric) | 3.5 Mbps (ADSL2+) / 40 Mbps (VDSL2) |
| Practical range | 20+ km from OLT | <2 km useful for ADSL2+; <1 km for VDSL2 |
| Medium | Single-mode glass | 22–26 AWG twisted copper pair |
| Idle latency | <2 ms first hop | 15–40 ms |
| Loaded latency | Essentially unchanged | 100–500+ ms (bufferbloat) |
| Distance penalty | None (passive splitters) | Severe — speed drops with cable length |
| Weather sensitivity | None (glass is dielectric) | High — water in splices, oxidation |
| Modulation | NRZ-OOK / coherent DWDM | DMT with vectoring |
| Future-proof | 25G-PON, 50G-PON on roadmap | Being actively retired |
| Provider status | Net adds across US | AT&T retiring by 2029; BT by 2030s |
Sources: ITU-T G.992 · G.993 · G.9700/9701 · FCC IAS June 2024 · AT&T copper-retirement filing (Mar 2025)
Copper forgets the high notes first.
Imagine playing a song through a long pipe. Bass notes travel fine. The high notes — the cymbals, the violins — fade out before they reach the other end. Copper telephone wire does the same thing to DSL signals.
DSL doesn't send one big signal; it sends hundreds of small ones at different frequencies, modulated to carry data in parallel. The highest-frequency channels — which carry the most bits per second — die first as the cable gets longer. By 2 km you've lost them all. By 4 km you've lost most of the medium ones too.
AT&T spends ~$6B/year maintaining copper that's going away.
The copper telephone plant in the US is being decommissioned. AT&T stopped selling DSL in 2020. In 2025 it filed plans to retire copper across 1,300 central offices by end of year and finish the rest by 2029. BT Openreach in the UK is on a similar trajectory — “stop sell” already covers seven million premises.
The replacement is mostly fiber (FTTH) and partly 5G fixed wireless. The federal BEAD program — $42.45 billion authorized in 2021 — is explicitly the DSL-replacement vehicle: 63% of state final proposals choose fiber as the primary technology.
DSL fell to 6.6% of US broadband by the end of 2024 — and lost 3.1 million connections in a single year. It is collapsing in real time.
Fiber vs DSL — common questions.
Why is fiber so much faster than DSL?
Two reasons: medium and architecture. DSL pushes digital signals up the same copper telephone wire designed in the 20th century for voice; copper attenuates rapidly at high frequencies, so the further you are from the equipment, the slower you go. Fiber carries light through glass that barely loses signal over kilometers, and uses its full available spectrum for data — no analog carrier, no distance-vs-speed tradeoff.
What is DSL?
DSL stands for Digital Subscriber Line. It's a family of technologies that piggyback high-frequency digital signals on top of the same copper telephone wires used for analog voice. ADSL2+ tops out at 24/3.5 Mbps within about 1 km of the central office. VDSL2 reaches ~100/40 Mbps within 300 m of a fiber-fed street cabinet. G.fast hits ~1 Gbps but only when fiber reaches within 250 m of the home.
Why does DSL speed depend on distance from the central office?
Copper attenuates signal in proportion to frequency and length. DSL uses Discrete Multitone modulation — hundreds of narrow sub-carriers, each modulated independently. As cable length grows, the highest-frequency tones drop out first (they get too weak), then the next-highest, until only the lowest tones survive. That's why a house 500 m from the cabinet might get 80 Mbps and a house 1.5 km away might get 12 Mbps on the same plan.
Is AT&T shutting down DSL?
Yes. AT&T stopped selling DSL to new customers on October 1, 2020. In March 2025 it filed a plan to retire copper plant across roughly 1,300 wire centers by end-2025, with full retirement nationwide by 2029 (California has exemptions). Existing DSL customers remain connected until copper retirement reaches their area.
Why is upload so bad on DSL?
DSL's DMT spectrum was carved up asymmetrically by design — the early-2000s assumption was that users mostly downloaded. ADSL upstream gets just ~25–138 kHz of spectrum vs the much wider downstream band. Newer VDSL2 35b and G.fast can theoretically run symmetric profiles, but ISPs almost always configure them asymmetric.
What is FTTC (fiber to the cabinet)?
FTTC is a hybrid: fiber from the central office reaches a powered street cabinet near homes, and copper VDSL2 runs the last few hundred meters. This is what BT Openreach (UK) calls FTTC and what most VDSL2 deployments actually are. It's a transitional architecture; the long-term replacement is FTTH (fiber to the home).
Does DSL still make sense anywhere in 2026?
In rural areas where no other wired option exists, DSL remains the cheapest available 'broadband.' That's also exactly where the federal BEAD program ($42.45B) is investing to replace it — mostly with fiber, partly with LEO satellite or fixed wireless. For most US households, DSL is being actively phased out by both ISPs and government policy.
Why does DSL feel slow even at 'broadband' speeds?
Latency. A clean DSL line typically idles at 15–40 ms ping with interleaving enabled (the standard configuration); under upload load it can balloon to 200+ ms. Video calls degrade, modern web pages stall on round-trips, cloud apps feel sticky. Even a 25 Mbps DSL line and a 25 Mbps fiber line are very different user experiences.