56k Is Nothing: Testing the Limits of Dial-Up Modem Bandwidth

Let’s face it: we’re spoiled today; most of us get irritated if the 4K video we’re streaming needs to take a couple of seconds to buffer. But our modern internet speeds would have been unthinkable in the days of dial-up, when 56kbps was the pinnacle. That does, however, make one wonder what speeds are possible with dial-up modem technology. Are those speeds enough to stream a YouTube video? The folks over on The Serial Port channel decided to find out by pushing dial-up modems to their limits.

Let’s kick things off with a terminology lesson that may surprise some readers: the “k” in the term “56k modem” refers to the speed in kilobits per second. So, a 56k modem can transmit up to 56 thousand bits each second. Notice that I’m using the word “bits” and not “bytes” here, which is what we tend to use to measure file size. That means that internet speeds, measured in bits per second, are eight times slower than what many people expect.

All of that to say that 56k modems were very slow. And, of course, there were lots of dial-up modems with even slower speeds than that. But there is a trick that let The Serial Port team improve the situation and that is Multilink PPP (Point-to-Point Protocol).

Multilink PPP was a system for essentially combining multiple phone lines and dial-up modems to increase the total bandwidth. That wasn’t common in the dial-up era, as it required multiple lines and an ISP that supported the technology. But The Serial Port team was able to really flex Multilink PPP’s capabilities by essentially creating their own ISP. They did that by using a CISCO IAD, which is a voice-over-IP gateway, to generate analog phone lines for several 56k dial-up modems.

The CISCO IAD has T1 network connectivity, which is also very outdated at this point. But it supports 1.544 Mbps (megabits per second), which is enough for this test.

The Serial Port team then connected those phone lines to multiple 56k modems, going to an IBM ThinkCentre A50 running Windows XP. There was some trouble with the serial connections and COM port assignments, but they were able to get Multilink PPP working.

First, they connected two 56k modems and were able to achieve actual speeds close to 100kbps. Then they increased the count to five 56k modems and broke 200kbps — enough to stream a YouTube video at 240p. Finally, they reached a scalding rate of 668kbps by connecting 12 modems. That let them play YouTube at an actually kinda watchable resolution.

In theory, it would be possible to achieve even faster speeds, but that seems almost crass at this point.