56k modems' upstream speeds were limited to 33.6kbps due to analog-to-digital conversion at the phone company. However, downloads could reach 56kbps because they leveraged a mostly-digital path from the telco's server to the user's modem. This asymmetry existed because the phone company's infrastructure used digital signals internally, even for analog phone calls. The digital audio was converted to analog only at the last mile, at the user's local central office. This meant a 56k modem downloading data was essentially receiving a slightly-modified digital signal, bypassing much of the analog conversion process and thus achieving higher throughput. Uploads, originating from the analog modem, had to be fully digitized at the central office, resulting in the lower speed.
The Hackaday article, "Why 56k Modems Relied on Digital Phone Lines You Didn't Know We Had," elucidates the somewhat obscure technical underpinnings of how 56k modems achieved their then-remarkable speeds, a feat often misunderstood by the general public. It meticulously details how these modems, far from operating purely on the analog signals traditionally associated with telephone lines, actually leveraged the inherent digital nature of much of the Public Switched Telephone Network (PSTN). The article begins by establishing a fundamental understanding of pulse-code modulation (PCM), the process by which analog audio signals, like voice, are digitally encoded for transmission across the phone network. It explains how this digital encoding involves sampling the analog waveform at regular intervals and quantizing the amplitude of each sample into a discrete digital value, which is then represented by a series of bits.
The key insight, the article argues, lies in the realization that a significant portion of a typical long-distance phone call's journey traverses digital segments of the PSTN. When a user with a 56k modem connected to an Internet Service Provider (ISP), the "last mile" from the ISP to the user's home was often the only analog portion of the connection. Since the ISP itself was connected to the digital backbone of the telephone network, a clever trick could be employed. Instead of converting the digital data from the internet into an analog signal for transmission over the entire phone line, the ISP could, under optimal conditions, maintain the digital signal all the way to the user’s modem. The modem, in turn, could interpret this digital signal directly, bypassing the limitations imposed by analog-to-digital and digital-to-analog conversions. This direct digital connection allowed for a higher data transfer rate, reaching a theoretical maximum of 56 kilobits per second (kbps), significantly faster than the previous generation of modems.
The article further explains why the upstream speed (data sent from the user to the ISP) remained limited to a lower rate, as this direction still involved a full analog-to-digital conversion at the user's end. It also elaborates on the technical limitations and conditions that prevented 56k modems from consistently reaching their theoretical maximum speed. These include the presence of echo cancellation equipment, noise on the line, and the requirement for a clean digital signal path between the ISP and the user. Finally, the article concludes by highlighting the eventual obsolescence of 56k modems with the rise of broadband technologies like DSL and cable internet, which offered substantially higher bandwidths and eliminated the dependence on the aging PSTN infrastructure.
Summary of Comments ( 95 )
https://news.ycombinator.com/item?id=43282668
Several Hacker News commenters pointed out that the article's title is misleading. They clarified that 56k modems didn't rely on digital phone lines in the way the title implies. Instead, they exploited the fact that the trunk lines between central offices were digital, while the "last mile" to the user's home remained analog. This allowed the modem to receive data digitally at the CO's end and convert it to analog for the final leg, maximizing the speed within the constraints of the analog local loop. Some users also shared anecdotal memories of early modem technology and discussed the limitations imposed by analog lines. One commenter noted the importance of echo cancellation in achieving these higher speeds. A few commenters discussed related topics like the technical reasons behind the asymmetry of upload and download speeds and the different standards used for upstream communication.
The Hacker News post "56k modems relied on digital trunk lines" has generated a moderate number of comments, mostly focusing on clarifying technical details and sharing personal anecdotes related to dial-up modem technology.
Several commenters delve into the specifics of how PCM (Pulse Code Modulation) encoding is used in the phone system and how this relates to the asymmetric speeds of 56k modems. They explain that the upload speed was limited by the analog-to-digital conversion process at the user's end, while the download speed could take advantage of the digital signal already present in the trunk lines. This discussion includes nuances like the use of µ-law and A-law companding in different regions, affecting the achievable bitrates.
Some comments offer corrections or expansions on the original article's points. For example, one commenter clarifies that not all calls were digitized end-to-end, especially for international calls, and that the digital sections were typically within the telco's network rather than extending all the way to the user's home. Another points out the role of echo cancellation in enabling full-duplex communication. There's also discussion about the limitations imposed by regulations on the maximum power output of modems, a factor that contributed to the speed cap of 56k.
A few comments offer personal recollections of working with or experiencing dial-up technology. These anecdotes add a human element to the technical discussion, highlighting the frustrations and limitations of the technology, such as the susceptibility to noise and the difficulty of achieving the theoretical maximum speed. One user even recalls the specific model of modem they used.
A couple of comments touch on related topics like the use of ISDN and the evolution of DSL technology. While these are not central to the main discussion, they provide additional context about the broader landscape of data communication technologies during that era.
While there isn't one single "most compelling" comment, the collection of comments provides a valuable supplement to the original article, offering greater technical depth and personal perspectives on the intricacies of 56k modem technology.