DSL: The Connection That Outlived Its Welcome (Sometimes)

Getting the Internet Right: Part 3 of 12

DSL stands for Digital Subscriber Line, and if you've been in business long enough, there's a decent chance you've used it at some point. It was the first widely available "always on" internet connection for most of the country, replacing dial up in the early 2000s and giving businesses their first taste of internet that didn't tie up the phone line. Twenty plus years later, DSL is still available in a lot of areas, and some businesses are still running on it, either because nothing better is available or because nobody has revisited the decision since the connection was first installed.

Whether DSL still makes sense for your business depends entirely on your specific situation. Let's look at how it works and what the real world implications are.

How DSL actually works

DSL uses the same copper telephone wires that have been running to buildings since the mid twentieth century. The technology works by splitting the frequency range of the copper pair into channels: the lower frequencies carry your traditional voice calls, and the higher frequencies carry data. This is why DSL originally didn't interfere with your phone service. The two could coexist on the same wire.

The data signal runs from your building through the copper pair to a piece of equipment at your phone company's central office called a DSLAM, which stands for Digital Subscriber Line Access Multiplexer. The DSLAM aggregates all the DSL connections in your area and connects them to the phone company's backbone network. From there, your traffic reaches the broader internet.

The important thing about this setup is that your copper pair, the actual wire between your building and the central office, is yours. Nobody else is sharing that specific physical pair. This makes DSL a dedicated connection at the last mile level, which is one of its advantages over cable internet. Your neighbor's usage doesn't directly affect your speeds.

However, the capacity of DSL is limited by physics, and this is where the problems start.

The distance problem

Copper wire is not a great medium for high frequency data signals. The signal degrades as it travels along the wire, and it degrades faster at higher frequencies. This means that the further your building is from the central office, the slower your DSL connection will be. This isn't a "might be slower" situation. It's a mathematical certainty based on the physical properties of copper.

A business located a few thousand feet from the central office might get 20 to 40 Mbps on a good DSL technology like VDSL2. A business three miles away might get 3 to 5 Mbps. A business five miles away might not be able to get DSL at all, or might be stuck at speeds so low they're barely functional.

The phone company can tell you how far you are from the central office, but many don't volunteer this information unless you specifically ask. And because the wire doesn't run in a straight line, the actual cable distance is often significantly longer than what you'd measure on a map. The wire follows streets, goes through junction boxes, and may take a circuitous route that adds considerable length.

This distance sensitivity is DSL's defining limitation. Unlike cable or fiber, where the technology can deliver similar speeds to most customers in a service area, DSL performance varies enormously from building to building within the same neighborhood. Two offices on the same street can have very different experiences if one is closer to the central office than the other.

The speed ceiling

Even under ideal conditions, with a short copper run and modern equipment on both ends, DSL tops out at speeds that feel increasingly inadequate for business use. Standard ADSL maxes out around 8 Mbps download, and ADSL2+ pushes that to around 20 to 24 Mbps download and maybe 1 to 3 Mbps upload. VDSL2 can reach higher, up to 50 or even 100 Mbps at very short distances, but those speeds fall off quickly as distance increases.

More importantly, look at those upload numbers. Even VDSL2 is typically asymmetric, meaning the upload speed is a fraction of the download. As we discussed in How Internet Connections Actually Work, upload speed matters a lot for business use. Phone calls, video conferencing, cloud backups, and file sharing all need upload capacity. A DSL connection giving you 20 Mbps down but only 2 Mbps up might support four or five simultaneous VoIP calls before the upload starts getting congested. Our Bandwidth Calculator can help you estimate the exact threshold for your office.

That might be fine for a three person office. It's probably not fine for a fifteen person office, especially if some of those people are on video calls while others are pushing files to the cloud.

The reliability question

DSL connections tend to be reasonably reliable in terms of uptime. The copper infrastructure has been in place for decades, and the phone companies generally maintain it to a functional level. But "functional" for voice calls is a very low bar. A copper pair can have minor faults, corroded connections at junction boxes, or water intrusion at splice points that cause no noticeable issue for a voice call but create enough signal noise to cause errors on the data side.

These intermittent issues are particularly frustrating because they can come and go. Your connection might work perfectly fine for two weeks, then have a bad day where latency spikes and packet loss increases, then go back to normal. The copper is more susceptible to interference when it's wet, so these problems often correlate with rainy weather. If you've ever noticed that your call quality seems worse on rainy days, this might be why, and it's not your imagination.

The age of the copper plant varies significantly by area. In some places, the wiring has been updated relatively recently. In others, you're sending data over copper that was installed during the Carter administration. The phone company isn't typically investing in upgrading this infrastructure, because they're focused on building out fiber where it makes economic sense and maintaining the copper only where they're required to. In fact, the copper is now on borrowed time: AT&T begins decommissioning copper in hundreds of wire centers starting mid-2026, and DSL rides the same copper plant as the POTS lines being retired.

Where DSL still makes sense

Despite its limitations, there are situations where DSL is a reasonable choice.

If you're a small office in an area where DSL is the best available option, meaning no cable, no fiber, and no fixed wireless, then a DSL connection that delivers consistent speeds at a short distance from the central office can work fine. If your building happens to be close to the central office and you only have a handful of employees, DSL can handle your phone system and basic internet needs adequately.

DSL can also make sense as a backup connection. Because it runs over the copper telephone network, which is physically separate from the cable and fiber networks, a DSL line can provide redundancy against an outage on your primary connection. If your main connection is cable or fiber and it goes down, a DSL backup can keep your phones running and your basic internet accessible until the primary is restored. We'll talk more about this in the Redundancy and Failover post later in the series.

Where DSL doesn't make sense

If you have more than a handful of people in the office, or if you're relying on cloud applications for significant parts of your workflow, DSL is probably not giving you what you need. The upload constraints alone will limit how many simultaneous calls and cloud connections you can maintain without quality degradation.

If your building is more than about two miles from the central office by cable distance, DSL speeds are likely to be disappointing regardless of what the provider's marketing says. The "up to" speeds that DSL providers advertise are best case numbers achieved at close range. They bear little resemblance to what most customers actually get.

If you're experiencing call quality problems on a DSL connection, running a VoIP quality test will show you whether the connection itself is introducing the jitter and packet loss that cause audio problems. The connection should be near the top of your troubleshooting list. The combination of limited upload bandwidth, potential copper degradation, and susceptibility to environmental interference makes DSL one of the more common root causes of VoIP issues, especially in offices that have grown since the connection was originally installed. What worked for a two person office five years ago may not work for an eight person office today.

The honest take

DSL was a great technology for its time. It gave millions of businesses their first usable internet connection and served that purpose well for years. But the demands on business internet have changed dramatically. Cloud computing, VoIP phones, video conferencing, and software as a service applications have all increased what's expected of a connection in ways that DSL mostly can't keep up with.

If you're currently on DSL, it's worth finding out what other options might be available at your address. The landscape has changed a lot in the last few years, with fiber buildouts expanding into areas that previously had no options beyond DSL and cable. You might be surprised at what's available now that wasn't available when your current connection was installed.

If DSL is genuinely your only option, the best you can do is make sure you're on the best DSL technology available to your location, that your copper is in good shape (the phone company can run line quality tests), and that you're managing your usage to stay within the connection's limits. Prioritizing voice traffic on your router, if your router supports it, can help ensure your calls get the bandwidth they need even when other things are competing for the limited upload.

Next up: Cable Internet: Fast Until the Neighborhood Wakes Up, why the speeds look great on paper and the reality is more complicated.