https://hackaday.com/2025/03/06/why-56k-modems-relied-on-digital-phone-lines-you-didnt-know-we-had/ Skip to content Logo Hackaday Primary Menu * Home * Blog * Hackaday.io * Tindie * Hackaday Prize * Submit * About * Search for: [ ] [Search] March 7, 2025 Why 56k Modems Relied On Digital Phone Lines You Didn't Know We Had 35 Comments * by: Lewin Day March 6, 2025 * * * * * Title: [Why 56k Modems Relie] Copy Short Link: [https://hackaday.com] Copy [56kModems] 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 [IBM_PCMCIA_Data-Fax_Modem_V]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 [Northern_Telecom_World_Line_Card_NT6X17BA]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. [Screenshot-2025-02-26-135100]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. [US_Robotics_56K_Modem_Front]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. * [share_face] * [share_twit] * [share_in] * [share_mail] Posted in Featured, History, Original Art, Peripherals HacksTagged 56 kpbs, 56k modem, digital signal 0, modem, modulation, pcm, phone line Post navigation - The Future We Never Got, Running A Future We Got Hacking Digital Calipers For Automated Measurements And Sorta-Micron Accuracy - 35 thoughts on "Why 56k Modems Relied On Digital Phone Lines You Didn't Know We Had" 1. Bill says: March 6, 2025 at 7:26 am Thank you! I have wondered how a 56k modem work with a 64k digital system without any way to synchronize the samples. Report comment Reply 2. Sven Hapsbjorg says: March 6, 2025 at 7:38 am What about 115 kbps HIS by Ericsson? Report comment Reply 1. Cezar says: March 6, 2025 at 10:40 am HIS was purely digital ... Similar to ISDN. Very popular in Poland in 90's , known as SDI - Szybki Dostep do Internetu Report comment Reply 3. Clancydaenlightened says: March 6, 2025 at 7:51 am 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 Report comment Reply 1. Clancydaenlightened says: March 6, 2025 at 7:56 am 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 Report comment Reply 4. Greg A says: March 6, 2025 at 7:59 am 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 Report comment Reply 1. CJay says: March 6, 2025 at 8:25 am 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. Report comment Reply 1. Darko says: March 6, 2025 at 9:48 am 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. Report comment Reply 2. K says: March 6, 2025 at 8:30 am I think you mean by and large Report comment Reply 3. RetepV says: March 6, 2025 at 9:46 am 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 Report comment Reply 1. RetepV says: March 6, 2025 at 10:02 am 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! Report comment Reply 1. Jimbob says: March 7, 2025 at 9:08 am Man i remember XS4ALL from playing counterstrike 1.6 - they hosted a lot of public servers Report comment Reply 2. Greg A says: March 7, 2025 at 8:36 am now you tell me! i always assumed the transformer on the modem was supposed to take care of that but i didn't give it much thought. Report comment Reply 4. Joshua says: March 6, 2025 at 9:48 am 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! Report comment Reply 5. justinB says: March 6, 2025 at 4:21 pm i came the same conclusion too by listening to handshakes all over the country obsessively... i feel like i saw 19.2kbit disproportionally with my hardware :-) in CA bay area. Back in the war driving days. Report comment Reply 5. JayCop says: March 6, 2025 at 9:15 am All these decades later I still have the dialup sound burned into my head with with perfect clarity. Report comment Reply 1. SlowBro904 says: March 7, 2025 at 4:17 am It's my ringtone haha Report comment Reply 6. Joshua says: March 6, 2025 at 9:37 am 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. Report comment Reply 1. RetepV says: March 6, 2025 at 10:36 am Is it really an "expense", when nobody can interrupt you with a phone call while you're playing CounterStrike? :P Report comment Reply 2. Antti says: March 6, 2025 at 9:47 pm Back in the day, one played a lot of Day of Defeat and Action Quake, the nifty isdn cost me over 1k EUR per month thanks to the per minute charge. Naturally, having only one provider meant they were really dragging their feet with introducing ADSL. Ditched them the second there was a stable 4G network. Report comment Reply 7. Bryan Wann says: March 6, 2025 at 10:52 am The 53k limit only applies to USR's x2 protocol. K56Flex and V.90 didn't run awry of power (dB) limits and therefore could eke out slightly higher speeds. Report comment Reply 8. George White says: March 6, 2025 at 11:03 am Check out magviz.ca. I had a ddial running back in the 1980's. 7 modems, 7 phone lines. 300 baud of pure text chat. I did have Zoom 28.8k modem, but the chat system was a marvel for its time. Met my first love on that thing. The modems were SSM Modemcards for the //e. Report comment Reply 9. George says: March 6, 2025 at 11:12 am working with a Credit Card Processing Company they had a way to connect at a lower baud rate (say 1200), perform less hand shaking and deliver their data payload in less then the 6 second time slot that was a billable unit. This meant they cut their phone bills in half compared to connecting at a higher baud rate. The higher baud rate connection took longer to get connected and then deliver data. This company had a 50% advantage in their costing structure. Report comment Reply 1. Bryan Wann says: March 6, 2025 at 2:06 pm I've seen this firsthand. Back in 1997 when I was first accepting credit cards, my processor did support high speed modems but the 28.8k handshake took 10-15 seconds, just to process a few hundred bytes of CC data. Dusted off my old 2400 bps modem, it would call, handshake, send the batch and be done in the same amount of time. Report comment Reply 10. Nick says: March 6, 2025 at 2:56 pm Brief, yes, but the window between v.90 and DSL and DOCSIS that ISDN inhabited was shorter still. Report comment Reply 11. x0rpunk says: March 6, 2025 at 4:59 pm There was also shotgun connections that people did from baud all the way till the last v92 56k service; they broke 100kbps using cross-modulation with two-lines. AT&T still has a POTS service in 2025; it's copper all the way to the local switch. You still see splicing pedestals and trunk boxes in rural parts of the US.. Report comment Reply 12. ilike8bits says: March 6, 2025 at 5:34 pm Fond memories of working for a regional ISP. We only supported K56Flex. Most people couldn't get much beyond about 44k, but once you'd done it for a while you could recognise what speed it would connect at by listening to the rise and fall of the negotiation hissing. We all developed a bunch of strings to do things like change flow control, maximum speed and error checking and to this day I remember the one that worked on about 80% of modems, assuming you wanted to throttle them to 44K AT&F&C1&D2&K3%C0+MS=,,,44000 I used to know what each part did but that has been lost to age, coffee and alcohol. Report comment Reply 13. n3hat says: March 6, 2025 at 7:39 pm The previous residence had POTS. No mobile phones, so we asked the LEC for a 2nd line, said it was for a fax machine, so we could use voice and data simultaneously. Rather than run an additional copper pair to the house, they installed a PairGain unit. It worked, but we never got faster than 28.8 Kb/s. When we moved here 25 years ago, we were part of the LEC's 250-home trial rollout of that new-fangled ADSL service. Some years later, when the remnants of Hurricane Ike came through, the mains power went off (and stayed off for days, as it turned out). We couldn't power the DSL modem, but the dialtone was still there. My wife's laptop had a modem, so we used that to connect via dialup and scope out the situation. Our 14-year-old heard the modem negotiation, and asked "what's that?" -- she had never heard the song of the modem before. The LEC discontinued service over copper a couple of weeks ago, so no more DSL and POTS; it's fiber to the house now. Faster connection, but one more thing I have to worry about battery backup for. Report comment Reply 14. Miles says: March 6, 2025 at 9:44 pm I remember downloading 94MB file using NetZero (they ran a banner ad, and played a tiny commercial video while you were dialing in). I remember them limiting soon after, claiming a few users were taking 94% of their bandwidth. I had heard that it was digital downstream, cool to hear a bit more of that side of the story. Report comment Reply 1. SkowBro904 says: March 7, 2025 at 4:25 am Coming soon: NetZero TV Report comment Reply 15. Bruce says: March 7, 2025 at 4:39 am One of the selling points of many of the 56k modems was that they had "Upgradable ROMs". Did anyone EVER upgrade a modem? I don't believe I ever saw any kind of upgrade actualy offered for these "furure proofed" modems. Report comment Reply 1. Andre says: March 7, 2025 at 6:00 am Yes - had a Courier that started as 33.6, then went to x2 then later to V90. That modem also saw multiple lighting strikes and survived, only to be murdered by water when it went into the cleaners bucket. Report comment Reply 16. Panondorf says: March 7, 2025 at 6:02 am So... it basically it used the digital part of the phone network directly as a form of network and moved the modem from the ISP to wherever the phone company's transition from digital to analog occurred. Right? Does that mean that with a good connection via an old-school analog phone switching system, if home computers and 'fast' modems had co-existed with those in time then people could have had 56k via plain old analog modems? Report comment Reply 1. IIVQ says: March 7, 2025 at 2:04 pm Basically old-fashioned phone lines used frequency multiplexing so only 4000Hz bandwith slots existed, with some filtering I think 3200Hz was available - this differred per county and even technology, which works out to about 36kbps. When everything up to the last mile became digital, there was no need to limit a bandwith slot. Theoretically near-infinite bandwith was now available - shown by those same twisted pair now supporting ADSL with hundreds of megabits per second. But in the analog world bandpass filtering (not necessary intended as such) still existed - as well as the 64kbps bandwith of the digital signal, so that was what limited the actual data to about 56kbps. Report comment Reply 17. A Texan says: March 7, 2025 at 8:53 am I still have my US Robotics 28.8 and another 56k that I used for a few years. I listened to many hours of a podcast along with downloading stuff since that's all I had. Now the last couple years I finally got TMobile wireless in my semi-rural area. Report comment Reply Leave a ReplyCancel reply Please be kind and respectful to help make the comments section excellent. (Comment Policy) This site uses Akismet to reduce spam. Learn how your comment data is processed. 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