Digital subscriber line (DSL) is a family of technologies that deliver broadband internet over the copper telephone wires that phone companies ran to homes and businesses for voice service. It sends data at higher frequencies than voice, so internet and a phone call can share one line. DSL was one of the first widely available broadband services and is still in use, but many carriers are winding it down as they retire copper and build fiber.
At a glance
- DSL runs over an existing copper phone line, usually from the incumbent phone company.
- Speed falls with distance from the carrier’s equipment and with the condition of the copper.
- Most DSL is asymmetric, with upload much slower than download; VDSL2 is faster but only over short distances.
- It is a best-effort service in most cases, typically without the commitments of a dedicated business circuit.
- Many US carriers are retiring copper, so DSL may not be available to order or renew at some addresses.
What problem it solves
When businesses first needed always-on internet, the only wire already in nearly every building was the telephone line. DSL made broadband possible over that copper without new construction, replacing dial-up and offering an alternative to expensive leased lines such as the T1. For many small offices and branches it was the first affordable broadband connection, and in some areas with no fiber or cable it still is.
How it works
Shared line, separate frequencies. Voice uses the low frequencies on a copper pair. DSL uses higher frequencies on the same pair, and filters at the customer site keep the two apart.
Carrier equipment. The copper runs to a phone company central office or to a cabinet closer to the customer. There a DSL access multiplexer (DSLAM) aggregates many customers’ lines and connects them to the provider’s network and the internet.
Customer equipment. A DSL modem, often combined with a router, converts the signal into an Ethernet connection for the site.
Distance and line quality. Higher frequencies fade quickly over copper, so the farther the site from the carrier’s equipment, and the older or noisier the wiring, the lower the speed. That is why two addresses on the same street can see different results.
Variants. Asymmetric DSL (ADSL) gives more download than upload. Symmetric versions exist but have become uncommon. VDSL2 reaches higher speeds over short loops, often fed by fiber to a nearby cabinet. Some providers bond two copper pairs to increase speed.
When it matters for buyers
- Sites with few options. In rural or older areas, DSL may be the only wired service; compare it with fixed wireless, satellite and cellular before committing.
- Copper retirement notices. When the carrier plans to retire the copper, DSL at that site will end, so budget time to move. See copper retirement.
- Cloud and video use. Low upload speeds are often the bottleneck for video calls, backups and file sharing.
- Backup links. DSL can serve as an inexpensive secondary connection, but it may share a path into the building with the phone lines and other copper services.
- Inventory clean-up. Old DSL lines are sometimes still billed at sites that moved to other services.
Questions to ask vendors
- What speed do you expect at this exact address, upload and download, and is it guaranteed or best-effort?
- Is copper retirement planned in this area, and how much notice will we get?
- What is the support and repair commitment for business DSL?
- Is a static IP address available?
- What faster services, such as fiber, cable or fixed wireless, are available at this address, and at what price?
- Is there a contract term, and what happens if the service is discontinued during it?
Compare DSL with other internet access options available at each address before renewing.
How it differs from Ethernet over copper
Both run over phone-company copper, and the underlying technology is related, but they are sold differently. DSL is usually a single-pair, asymmetric, best-effort internet service. Ethernet over copper (EoC) bonds several copper pairs to deliver a business-grade Ethernet service, usually symmetrical, often with a service level agreement, and it can carry dedicated internet or a private connection between sites. EoC costs more than DSL and is also affected by copper retirement. Cable broadband, the other common best-effort option, runs over coaxial cable using DOCSIS rather than phone lines.
Problems this helps with
- DSL is the only wired internet option at a branch
- Our phone company says it is retiring the copper lines our DSL uses
- Internet at a site is slow, especially uploads and video calls
- Need an inexpensive backup connection
