Jon Brodkin of ArsTechnica published an amusing story about how the DSL went out of service in a 400-resident village in Wales each morning at 7:00 am. It turns out that one of the residents turned on an ancient television that interfered with the DSL signal to the extent that the network collapsed. The ISP finally figured this out by looking around the village in the morning with a spectrum analyzer until they found the source of the interference.
It’s easy to think that the story points out another weakness of old DSL technology, but interference can be a problem for a lot of other technologies.
This same problem is common on cable company hybrid-fiber coaxial networks. The easiest way to understand this is to think back to the old days when we all watched analog TV. Anybody who watched programming on channels 2 through 5 remembers times when the channels got fuzzy or even became unwatchable. It turns out that there are a lot of different devices that interfere with the frequencies used for these channels including things like microwave ovens, certain motors like power tools and lawnmowers, and other devices like blenders. It was a common household occurrence for one of these channels to go fuzzy when somebody in the house, or even in a neighboring home used one of these devices.
This same interference carries forward into cable TV networks. Cable companies originally used the same frequencies for TV channels inside the coaxial wires that were used over the air and the low TV channels sat between the 5 MHz and 42 MHz frequency. It turns out that long stretches of coaxial wires on poles act as a great antenna, so cable systems pick up the same kinds of interference that happens in homes. It was pretty routine for channels 2 and 3, in particular, to be fuzzy in an analog cable network.
You’d think that this interference might have gone away when cable companies converted TV signals to digital. The TV transmissions for channels 2 through 5 got crystal clear because cable companies relocated the digital version of these channels to better frequency. When broadband was added to cable systems the cable companies continue to use the low frequencies. CableLabs elected to use these frequencies for the upload portion of broadband. There is still plenty of interference in cable networks today – probably even more than years ago as coaxial networks have aged and have more points for interference to seep into the wires. Until the pandemic, we didn’t care much about upload bandwidth, but it turns out that one of the major reasons that cable companies struggle to deliver reliable upload speeds is that they are using the noisiest spectrum for the upload function.
The DSL in the village suffered from the same issue since the telephone copper wires also act as a big outdoor antenna. In this village, the frequency emanating from the old TV exactly matched the frequencies used for DSL.
Another common kind of interference is seen in fixed wireless networks in a situation where there are multiple ISPs using the same frequencies in a given rural footprint. I know of counties where there are as many as five or six different wireless ISPs, and most use the same frequencies since most WISPs rely on a handful of channels in the traditional WiFi bandwidth at 2.4 MHz and 5 MHz. I’ve heard of situations where WiFi is so crowded that the performance of all WISPs suffer.
WiFi also suffers from local interference in the home. The WiFi standard says that all devices have an equal chance of using the frequencies. This means that a home WiFi router will cycle through all the signals from all devices trying to make a WiFi connection. When a WiFi router connects with an authorized device inside the home it allows for a burst of data, but then the router disconnects that signal and tries the next signal – cycling through all of the possible sources of WiFi.
This is the same issue that is seen by people using WiFi in a high-rise apartment building or a hotel where many users are trying to connect to WiFi at the same time. Luckily this problem ought to improve. The FCC has authorized the use of 6 GHz spectrum for home broadband which opens up numerous new channels. Interference will only occur between devices trying to share a channel, but that will be far fewer cases of interference than today.
The technology that has no such interference is fiber. Nothing interferes with the light signal between a fiber hub and a customer. However, once customers connect the broadband signal to their home WiFi network, the same interference issues arise. I looked recently and can see over twenty other home WiFi networks from my office – a setup ripe for interference. Before making too much fun of the folks in the Welsh village, there is a good chance that you are subject to significant interference in your home broadband today.