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
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
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.
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.
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.
Thank you! I have wondered how a 56k modem work with a 64k digital system without any way to synchronize the samples.
What about 115 kbps HIS by Ericsson?
That’s also why nowadays if you talk on a landline you may not be able to talk at the same time, you won’t hear the person on the other end while you talk as a result of the digital compression, makes it cheaper for the phone company
Cell phone to cell phone may not have that issue
Back in the realtek ac’97 days, needed a catch a call dongle attached in series with the computer and phone Line
Because either of someone called while you’re on the Internet either you got disconnected, or the phone won’t ring, they hear this ear rape screeching of the modem, or possibly a busy signal
i used the heck out of my 14.4kbps modem, and by enlarge it always connected at 14.4. there was some variety whether it would use v32 or v42bis, which i think had more to do with the modem on the other end. and i certainly got to know the handshake sounds and what kind of speeds they foretold. but mostly, it was consistent. and man! it was a big step up over the 2400bps that came with our 286!
i used a 56k modem only briefly, and that thing was different every time. sometimes it would be fast, sometimes it would be slow. sometimes it wouldn’t connect at all. so in my mind, 14.4k is still the max…anything above that is flying too close to the sun for our actual existing POTS system, apparently. by the time 56k modems proliferated, hardly anyone was using them. a last gasp of a technological dead end
Distance and line quality is all, I was 3.1 miles from the exchange according to the TDR the tech used, I got reliable connects at full speed on most of the ‘real’ V34 modems I had (some of the crappy Winmodems were really poor) and when I got 56K modems, I almost always got connects over 50K.
They were fairly quickly outpaced by ADSL but in the UK at least, 56K modems were around for a good few years and most ISPs offered 56K service for quite a while after ADSL became available because it wasn’t cheap.
I was in Serbia at that time (in the early 2000s) and usually had a reliable connection at 53.3 kbps, though sometimes, due to weather or bad luck, it dropped to 33.6 kbps. I’m not sure, but I think I even saw 56 kbps few times. However, that was a long time ago, so maybe I just imagined it.
I think you mean by and large
One thing that lots, and lots, and lots of people did not know or realize, is that you needed to ground your computer system properly, if you were using an internal modem. I had lots of problems like you too, when I had a 56K modem. For a while I just thought it was bad luck, too far from the exchange or something. Until one day I had ran out of sockets and took an extension cord and plugged my computer into one of the kitchen wall sockets. And lo and behold, I got 55K the first time. No disconnects, no retrains, nothing, just worked.
So I investigated. It turned out the the socket under my desk and the socket in the kitchen were in the same group (they were both in the kitchen divider wall). In the kitchen, ground was properly connected in the socket (mandatory for a “wet space” as we call it). But in the socket under my desk, on the other side of the divider wall had no ground wire.
As both sockets were connected to the same group, I just ran a ground wire from the kitchen socket to the socket under my desk (about 1 meter, I don’t understand why it wasn’t done in the first place).
And after that, I had such a good connection that I actually learned how to properly kick butt in Quake 2. :P
Many people suffered from this grounding issue. I ‘fixed’ this problem for many of my friends, just by telling them to run an extension cord from their kitchen. ;)
It was only a problem when you were using an internal modem. I think the noise/ground issue happened because most PC power supplies had the bare minimum of line noise filtering (single line single phase I think it’s called). Without a ground connection, the line filter could not properly do its work. Or maybe it was causing ground buzz, I don;t know. I didn’t have the knowledge or even an oscilloscope to properly figure it out at the time.
If you used an external modem, you had no issues. They were self-powered. Only, you should make sure to plug the modem into the same socket/group as the computer, otherwise you could have issues again.
Soon after, I switched to ISDN though. Fully digital, and it didn’t suffer the same ground problem as the 56K modems. And my ISP (XS4All) supported channel bundling for free too. So I had 128KBps to kick ass with!
When I had my hot-rod 286 with 4MB RAM, sound card, CD drive and Windows 3.1,
the 2400 Baud modems were already in the museum. Like acoustic couplers and C64s were.
The slowest modems that could be bought used were 14k4 data/fax modems.
I had such a modem, I think. Or was it an 28k8 model?
Anyway, it was okay for visiting mailboxes or T-Online or CompuServe.
Other users had special BTX modems running at 1200/75 Baud or 1200/1200 Baud depending on model.
But these were single-purpose modems, not usable for anything else other than T-Online Classic (BTX).
Users with brains (not me) had opted for an Fritz! ISDN card for ISA bus.
Browsing data banks and mailboxes that way must have been a breeze.
ISDN was incredible reliable. Slow as as a digital connection in retrospect, maybe, but way more reliable than DSL!
All these decades later I still have the dialup sound burned into my head with with perfect clarity.
This was the time when ISDN with 64 KBit/s sold very well in my country.
Some users used channel combining and had 128 KBit/s,
at the expense off loosing the second telephone line while surfing.