If you came of age in the 1990s, you’ll remember the unmistakable auditory handshake of an analog modem negotiating its connection via the plain old telephone system. That cacophony of screeches and hisses was the result of careful engineering. They allowed digital data to travel down phone lines that were only ever built to carry audio—and pretty crummy audio, at that.
Speeds crept up over the years, eventually reaching 33.6 kbps—thought to be the practical limit for audio modems running over the telephone network. Yet, hindsight tells us that 56k modems eventually became the norm! It was all thanks to some lateral thinking which made the most of the what the 1990s phone network had to offer.
Breaking the Sound Barrier
The V.34 standard enabled transmission at up to 33.6 kbps, though many modems topped out at the lower level of 28.8 kpbs in the mid-1990s. Credit: Raimond Spekking, CC BY-SA 4.0
When traditional dial-up modems communicate, they encode digital bits as screechy analog tones that would then be carried over phone lines originally designed for human voices. It’s an imperfect way of doing things, but it was the most practical way of networking computers in the olden days. There was already a telephone line in just about every house and business, so it made sense to use them as a conduit to get computers online.
For years, speeds ticked up as modem manufacturers ratified new, faster modulation schemes. Speeds eventually reached 33.6 kbps which was believed to be near the theoretical maximum speed possible over standard telephone lines. This largely came down to the Shannon limit of typical phone lines—basically, with the amount of noise on a given line, and viable error correcting methods, there was a maximum speed at which data could reliably be transferred.
In the late 1990s, though, everything changed. 56 kbps modems started flooding the market as rival manufacturers vied to have the fastest, most capable product on offer. The speed limits had been smashed. The answer lay not in breaking Shannon’s Law, but in exploiting a fundamental change that had quietly transformed the telephone network without the public ever noticing.
Multiplexing Madness
Linecards in phone exchanges were responsible for turning analog signals into digital signals for further transmission through the phone network. Credit: Pdesousa359, CC BY-SA 3.0
In the late 1990s, most home users still connected to the telephone network through analog phone lines that used simple copper wires running to their houses, serving as the critical “last mile” connection. However, by this time, the rest of the telephone network had undergone a massive digital transformation. Telephone companies had replaced most of their long-distance trunks and switching equipment with digital technology. Once a home user’s phone line hit a central office, it was usually immediately turned into a digital signal for easier handling and long-distance transmission. Using the Digital Signal 0 (DS0) encoding, phone calls became digital with an 8 kHz sample rate using 8-bit pulse code modulation, working out to a maximum data rate of 64 kbps per phone line.
Traditionally, your ISP would communicate over the phone network much like you. Their modems would turn digital signals into analog audio, and pipe them into a regular phone line. That analog audio would then get converted to a DS0 digital signal again as it moved around the back-end of the phone network, and then back to analog for the last mile to the customer. Finally, the customer’s modem would take the analog signal and turn it back into digital data for the attached computer.
This fell apart at higher speeds. Modem manufacturers couldn’t find a way to modulate digital data into audio at 56 kbps in a way that would survive the DS0 encoding. It had largely been designed to transmit human voices successfully, and relied on non-linear encoding schemes that weren’t friendly to digital signals.
The breakthrough came when modem manufacturers realized that ISPs could operate differently from end users. By virtue of their position, they could work with telephone companies to directly access the phone network in a digital manner. Thus, the ISP would simply pipe a digital data directly into the phone network, rather than modulating it into audio first. The signal remained digital all the way until it reached the local exchange, where it would be converted into audio and sent down the phone line into the customer’s home. This eliminated a whole set of digital-to-analog and analog-to-digital conversions which were capping speeds, and let ISPs shoot data straight at customers at up to 56 kbps.
The basic concept behind 56 kbps operation. So-called “digital modems” on the ISP side would squirt digital signals directly into the digital part of the phone network. These would then be modulated to analog just once at the exchange level to travel the last mile over the customer’s copper phone line. Credit: ITU, V.90 standard
This technique only worked in one direction, however. End users still had to use regular modems, which would have their analog audio output converted through DS0 at some point on its way back to the ISP. This kept upload speeds limited to 33.6 kbps.
USRobotics was one of the innovators in the 56k modem space. Note the x2 branding on this SPORTSTER modem, denoing the company’s proprietary modulation method. Credit: Xiaowei, CC BY 3.0
The race to exploit this insight led to a minor format war. US Robotics developed its x2 standard, so named for being double the speed of 28k modems. Rival manufacturer Rockwell soon dropped the K56Flex standard, which levied the same trick to up speeds. ISPs quickly began upgrading to work with the faster modems, but consumers were confused with the competing standards.
The standoff ended in 1998 when the International Telecommnication Union (ITU) stepped in to create the V.90 standard. It was incompatible with both x2 and K56Flex, but soon became the industry norm.. This standardization finally allowed for interoperable 56K communications across vendors and ISPs. It was soon supplanted by the updated V.92 standard in 2000, which increased upload speeds to 48 kbps with some special upstream encoding tricks, while also adding new call-waiting and quick-connect features.
Final Hurrah
Despite the theoretical 56 kbps limit, actual connection speeds rarely reached such heights. Line quality and a user’s distance from the central office could degrade performance, and power limits mandated by government regulations made 53 kbps a more realistic peak speed in practice. The connection negotiation process users experienced – that distinctive modem “handshake” – often involved the modems testing line conditions and stepping down to the highest reliable speed. Despite the limitations, 56k modems soon became the norm as customers hoped to achieve a healthy speed boost over the older 33.6k and 28k modems of years past.
The 56K modem represents an elegant solution for a brief period in telecommunications history, when analog modems still ruled and broadband was still obscure and expensive. It was a technology born when modem manufacturers realized the phone network they were now working with was not the one they started with so many decades before. The average consumer may never have appreciated the nifty tricks that made the 56k modem work, but it was a smart piece of engineering that made the Internet ever so slightly more usable in those final years before DSL and cable began to dominate all.
This articles is written by : Nermeen Nabil Khear Abdelmalak
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By Nermeen Nabil Khear 
