What is ADSL?

Short for Asymmetric Digital Subscriber Line, ADSL uses copper wire telephony infrastructure to provide fast internet access to home and business premises. It’s the oldest and slowest broadband standard used in the UK.

First trialled in the 1990s in the UK, ADSL uses the old copper fibre Public Switched Telephone Network (PSTN) to send data between computers. ADSL can provide download speeds of up to 8Mbit/s while the more recent ADSL2 standard has a theoretical maximum download speed of 24 Mbit/s. Most UK ADSL customers – over 95% – are on the ADSL2 standard.

Upload speeds are significantly slower for ADSL than for most other options, maxing out at 1.5Mbit/s. This is the asymmetric part of this. ADSL is designed to provide faster download than upload speeds, based on the assumption that most consumers will download more than they could legitimately upload. Needless to say, this is rather outdated in the era of video conferencing, live service games and streaming content creators.

In the UK, in 2021, average ADSL download speeds were just 10.7 Mbit/s, compared to averages of 42.6 Mbit/s for Fibre To The Cabinet (FTTC) VDSL, 111.1 Mbit/s for cable broadband and 145.4 Mbit/s for full Fibre To The Premises (FTTP).

According to Ofcom, three-quarters of residential broadband lines had an average download speed of 30 Mbit/s or higher in 2021, compared to that 10.7% among ADSL subscribers. Similarly, ADSL upload speeds in the UK averaged 1Mbit/s in 2021, while average UK upload speeds across all fixed connection types were just under 10 Mbit/s.

Unfortunately, ADSL remains the only option for internet connectivity for some households, but it’s now a minority option. Only 1% of lines had an advertised speed of less than 10Mbit/s, while 8% provided a 24-hour average download speed of under 10Mbit/s.

A 10Mbit/s download speed is the government-defined minimum for an acceptable broadband connection, although in practice that’s still much too slow for may common online activities.

Since March 2020, consumers who cannot get a download speed of 10 Mbit/s and an upload speed of 1 Mbit/s have been able to request an upgraded connection under the broadband universal service obligation (USO).

See Ofcom’s Your right to request a decent broadband service page for full information on the eligibility criteria, then contact BT or, if you live in Hull, KCOM to check your eligibility and apply for assistance in getting connected.

As the UK is planning to fully switch to VoIP (Voice over Internet Protocol) telephony carried over high-speed fibre broadband networks by 2025, ADSL, like the legacy PSTN infrastructure it uses, is set to become obsolete in this country.

Data in this explainer is taken from Ofcom’s 2021 Online Nation Report and Broadband Speeds Research.