[HN Gopher] Why 56k is the fastest dial up modem speed
___________________________________________________________________
Why 56k is the fastest dial up modem speed
Author : marcodiego
Score : 199 points
Date : 2023-11-25 14:28 UTC (8 hours ago)
(HTM) web link (www.10stripe.com)
(TXT) w3m dump (www.10stripe.com)
| TonyTrapp wrote:
| TIL: Phone networks already carried digital signals in the 60s. I
| absolutely did not expect that.
| p3n1s wrote:
| "Fast-forward to 1962. Ma Bell begins using T-1s to connect its
| switching centers. T-1s are a digital communications link, not
| analog. The entire phone network goes digital. In a modern
| phone network, the only analog communication is between an
| individual house's phone line and the phone company's box."
|
| This is misleading. The phone network used analog links
| (typically microwave relay) for long distance almost
| exclusively for many years (into the 80s) after the
| introduction of T1. T1 itself is not terribly well suited to
| very long distance links though and can only be pressed into it
| with repeaters every mile or so.
| throwaway167 wrote:
| Hence towers with dishes on the outside. Long perplexed why
| they had canvas stretched across, and didn't point up: https:
| //en.m.wikipedia.org/wiki/Microwave#/media/File%3AFraz...
| jjjjmoney wrote:
| Our Charter Spectrum "cable" connection goes over a
| microwave link like this.
|
| How do I know? Well, we had a pretty big wildfire a couple
| years ago that burned a microwave tower a couple hours
| away. The official response was that they couldn't get
| internet back in town until they rebuilt the tower. Our
| internet was down for a couple weeks until it was safe
| enough for them to get back to the tower site.
| Sharlin wrote:
| Microwave relay links predate spaceflight and even when
| comms satellites became a thing, it made (and still makes)
| little sense to beam regional TV/radio/phone up to
| geosynchronous and back, due to cost and latency. There was
| no good replacement to terrestrial microwave until fiber
| optic became a thing.
|
| In a very real sense microwave relays are the direct
| descendant of the optical telegraph (aka "clacks" in
| Discworld lingo).
| ac29 wrote:
| > Long perplexed why they had canvas stretched across
|
| Its a radome, used to protect the antenna from weather.
| h2odragon wrote:
| 130VDC on the copper, apparently.
|
| Had "the second longest T1 in the service area" for a while
| there. BellSouth declined to renew the contract in 2008, the
| business folded, etc. In 2013 I was digging through the old
| building, and the T1 line card in the wiring closet was
| _still powered_.
|
| Nowadays the relay boxes all the way into town are mostly
| hanging open or have been repeatedly run over. I wonder how
| much of the rest of that infrastructure they just abandoned
| in place.
| Aloha wrote:
| the 130V was to power either repeaters or the smart jack.
| dylan604 wrote:
| >The phone network used analog links (typically microwave
| relay) for long distance almost exclusively for many years
| (into the 80s) after the introduction of T1
|
| The reason should be obvious. Digging long distance trenches
| and all of the property buying/leasing and right of
| ways/easements and all of the other fun stuff makes it a
| total nightmare. Radio broadcast just makes much more
| financial sense as well as logistically.
| brookst wrote:
| Were the microwave links really analog signaling?
| NovemberWhiskey wrote:
| The Long Lines network used frequency modulation and
| frequency division multiplexing and was completely analog.
| It was really only advancements in semiconductor lasers and
| optical fiber manufacturing that drove its abandonment.
| the_third_wave wrote:
| > The phone network used analog links (typically microwave
| relay) for long distance
|
| Those microwave links represent layer 0 in the connection
| which is by definition always analogue, the same is/was true
| for T-1 which used alternate mark inversion [1] signalling
| over a 4-wire circuit. It is the type of signal carried over
| the link which decides whether it is considered "digital" or
| "analogue". T-1 carried 24 time-divided PCM channels
| ("digital"), microwave links could and can carry both
| frequency-divided multiplexed analogue channels ("analogue")
| as well as time-divided PCM or packet-switched channels
| ("digital").
|
| [1] https://en.wikipedia.org/wiki/Bipolar_encoding#Alternate_
| mar...
| fragmede wrote:
| Computers are _old_. The future is here, it 's just unevenly
| distributed. I take a self-driving car multiple times a week
| yet anyone outside my bubble hasn't ever ridden one and thinks
| that technology doesn't work yet. I have conversations with an
| LLM, but people on the other side of the world don't even have
| laptops or reliable Internet access. Military people have
| jetpacks to move around on. Inequality is as old as dirt,
| unfortunately.
| flyinghamster wrote:
| It also strikes me how much the old phone network, and its
| cultural aspects, have faded into the past - particularly
| when I drive past some derelict bit of telecom
| infrastructure. Even 30 years ago, it was amusing to watch
| kids try to deal with a rotary-dial phone. Now even landlines
| are an endangered species, and area codes are barely
| relevant. Finding a working telco pay phone these days would
| be like finding a unicorn.
|
| Meanwhile, on a random roadtrip through rural Illinois
| yesterday, I was seeing "5G UC" on my cell phone, out in the
| middle of nowhere. Unevenly distributed, indeed!
| dylan604 wrote:
| >Finding a working telco pay phone these days would be like
| finding a unicorn.
|
| Even if I found one, I would only be able to make a collect
| call. I never carry loose change on me, and I haven't had a
| calling card in decades.
| actionfromafar wrote:
| There used to be a way to pay with credit card IIRC?
| dylan604 wrote:
| Not that I was ever exposed to. Calling cards were just
| account numbers with a monthly statement. If you used a
| credit card for that, then maybe, but to me paying with a
| credit card suggests a per call transaction. That I have
| not experienced. But the world is a bigger place than
| just me, so maybe it was available in other areas?
| the_third_wave wrote:
| > Inequality is as old as dirt
|
| Yes
|
| > unfortunately
|
| No. Perfect equality means that nobody can excel in anything,
| anywhere and any time. With perfect equality comes
| stagnation. If you happen to know Rush' song "The Trees"
| you'll know what I'm talking about:
|
| _Now there 's no more oak oppression For they passed a noble
| law And the trees are all kept equal By hatchet, axe, and
| saw_
|
| It is not so much _inequality_ which can be a problem but the
| fact that the lower bounds can be too low for an acceptable
| quality of life. As long as the lower bounds are high enough
| - above the "poverty line" - there can be inequality without
| that being a problem in and of itself.
|
| Another thing which can be a problem is the _reason_ for
| there being inequality, e.g. when some rule or custom creates
| artificial limits for some people but not for others (
| "discrimination").
|
| For a society to thrive you want people to make the best of
| their possibilities and to use their capabilities to the
| fullest extent. With that you automatically get inequality
| because not everyone has the same possibilities and
| capabilities, e.g. a farmer on a rock-infested piece of
| glacial detritus will not be able to produce bumper crops
| even if he is at least as capable as the farmer in the river
| delta who can not leave his wooden shoes outside without them
| sprouting roots and growing leaves. On that same river delta
| a capable farmer will be able to more reliably produce good
| crops than one who has never heard of crop rotation. If you
| take the most productive developer in your company and
| provide him with a fully equipped tool shop he will not be
| able to produce as good a widget as would the most
| experienced mechanic who in his turn would not be as
| productive a developer. Both can do their best to reach their
| local optimum but the developer will most likely see that
| result in higher earnings than the mechanic will.
|
| Inequality is not a problem. Equity - forced equality as
| personified by the "they" in the lyrics to "The Trees" - _is_
| a problem.
| fragmede wrote:
| Great point. The distinction between equality and equity is
| an important one, with the canonical image for it for me
| being the one with the fence*. Where inequality today looks
| like billionaires throwing parties and doing drugs on their
| own private island at one end, and people living in tents
| and going hungry, also doing drugs, but with no hope at the
| other end, it's not the inequality itself that's the
| problem, it's that the lowest end of the spectrum is so
| utterly bleak that people have to steal and commit crimes
| to survive that's the problem.
|
| * https://www.researchgate.net/profile/Shrehan-
| Lynch/publicati...
| retrac wrote:
| Telecommunications were the first application for nearly every
| principle and technology now used in computers.
|
| In the 1930s, Claude Shannon was trying to optimize the phone
| network's automatic switching fabric built out of millions and
| millions of relays when he, more or less by accident, proved
| that switching circuits are equivalent to Boolean algebra, in a
| deep sense, and so the appropriate arrangement of switches, can
| compute anything that can be computed.
|
| The first major commercial application for the vacuum tube was
| not in radio, but in amplifying long-distance telephone links.
| They would quickly find use in frequency multiplexing telephone
| lines, too. [1]
|
| Similarly, one of the first applications of vacuum tubes as a
| high-speed electronic switch was for multiplexing telegraph
| lines, and much of the initial research on high-reliability
| low-power tubes able to switched fully on without damage began,
| with the telephone and telegraph companies.
|
| The first computers were built out of relays and vacuum tubes
| specially designed for telecommunications.
|
| The transistor was invented at Bell Labs. The planned
| application was the telephone network. The very same year, the
| theory behind modern error correcting codes was developed at
| Bell Labs too. These find extensive use today in data storage
| in computers, but the original goal was arbitrarily accurate
| transfer of telegraph data over noisy links.
|
| Digital transmission of photographs dates to the late 1920s
| too; they wanted to send images over extremely noisy long-
| distance telegraph links. So record the intensity of light over
| each spot of the image on paper tape. Then send the tape over
| the wire. And reverse on the other end. [2]
|
| I've come to understand the computer revolution as really the
| story of the telecommunication revolution. Computers
| serendipitously came about with advances in telecommunications,
| enabled by the same technologies, and indeed, often driven by
| the needs of telecommunications. (One of the very first
| applications for a computer in an embedded context - dating to
| the late 1950s in experiment - the telephone exchange, of
| course.)
|
| [1] https://long-lines.net/tech-equip/misc/Graybar103-0693.jpg
|
| [2] http://www.hffax.de/history/html/bartlane.html
| Merrill wrote:
| At one time teletype switching was done in centers where the
| incoming line was connected to a tape punch. The operator
| tore the tape off the punch, read the destination address
| punched at the beginning of the tape, and put the tape on a
| tape reader connected to an outgoing line to another center
| closer to the destination.
| https://en.wikipedia.org/wiki/Tape_relay
|
| IP routers now do this faster.
| trealira wrote:
| I'd argue that they were also invented specifically to do
| specific mathematical calculations. Many predecessors of the
| modern computer were designed to do math. Some examples:
|
| The ENIAC was invented to do mathematical calculations for
| the army.
|
| Blaise Pascal invented a mechanical calculator to do
| arithmetic in 1642. Leibniz improved on his design and made
| his own version in 1673. However, not many of either version
| were made, as it was still cheaper to just do the addition,
| subtraction, multiplication, and division yourself.
|
| Herman Hollerith invented a punch card tabulating machine and
| patented it in 1884. It was used to process the 1890 US
| Census. It could increment counters to do addition and
| multiplication, but its main purpose was to assist with
| processing large amounts of data using punch cards, the way
| SQL databases are used today.
|
| In 1930, Vannevar Bush invented a machine called the
| Differential Analyzer, which was an analogue computer that
| could solve differential equations by integration. You could
| store functions in it by adjusting gears. The purpose was to
| be able to automatically calculate things like the position
| of a projectile moving through the air after _n_ seconds
| without having to calculate it by hand.
|
| Most of this I got from the book _ENIAC, The Triumphs and
| Tragedies of the World 's First Computer_, which touches on
| all these predecessors to the computer.
| queuebert wrote:
| Isn't Morse code technically a digital signal, albeit with an
| extremely low bit rate?
| dreamcompiler wrote:
| Yes. It's sloppy PWM with a non-ASCII character code. It's
| optimized for human encoding/decoding rather than machine;
| the pulse widths are not globally uniform like they would be
| in a machine-oriented code, which is what I mean by "sloppy."
| bwann wrote:
| To nitpick, the 53.3k limit only affected US Robotics' pre-V.90
| standard called X2, due to how much power they would require,
| which violated FCC rules. K56Flex (the competing 56k protocol),
| and later the ratified V.90 standard didn't have this problem and
| you could achieve carrier speeds of 56,000 bps.
|
| This distinction was getting lost to time even back then with
| everyone just assuming it was some arbitrary FCC limit. This post
| from NANOG in 1998 explains it:
| https://archive.nanog.org/mailinglist/mailarchives/old_archi...
| qingcharles wrote:
| What's crazy is how badly I was waiting to upgrade from X2 to
| K56Flex just to get the 3kbps, which was like a life-altering
| speed difference back then!
| philistine wrote:
| I'll go to my grave thinking _wow 3.2 kilobytes is a good
| connection!_
| guidedlight wrote:
| 3.2kbps is the maximum rate of symbols (i.e. 3200 baud). So
| K56Flex squeezes one extra bit per symbol.
| rascul wrote:
| I don't know how it worked but I never saw better than 53/54k
| with v.90 or v.92. Maybe there was more going on.
| Aloha wrote:
| It really depended a lot of how clean your pair was, and
| sometimes even what kind of switch you were homed on.
|
| Like channel banks prior to the D4 and a 1AESS would very
| likely have a marked difference to your speeds.
| MBCook wrote:
| Once, for a short while, I was getting around 110kbps
| reported from my modem (obviously it was doing compression).
|
| It couldn't do it for binary, but it still was noticeable
| with HTML and email.
|
| It didn't last long. Whatever accidental phone line + ISP
| magic enabled it was quickly "fixed".
| tibbydudeza wrote:
| Ahh the 56K modem wars - each group swore either by USR or
| K56Flex and were incompatible with each other until V90
| standard came along - some modems could be firmware upgraded
| (USR) others not that used discrete components.
| hypercube33 wrote:
| I'm pretty sure my external modem had a firmware update that
| took it from 53kbps to 56k flex
| dylan604 wrote:
| Growing up, I lived in the boonies. Even with a 56k modem, the
| fastest it could go was 33.6k. I got to learn all of the info
| from the TFA when I was a trying to find out why. That's also
| when I learned that some companies had full T1 lines all to
| themselves for a whopping ~1.5Mbps. Until some makes a call or
| sends a fax when your top speed drops 64k per phone line being
| used. It's also why ISDN didn't connect at 128k but at 112k since
| it just used two "bonded" lines of the T1
| reactordev wrote:
| Twin bonded ISDN lines were my only option for anything faster
| than 33.6k. "You too could have had 128k for the low monthly
| price of $250 + cost of two phone lines + cost of equipment +
| labor". When I finally moved to somewhere that had cable (and
| cable internet at 1.5mbps) I vowed never to return to the
| sticks.
|
| Then Elon came along... Starlink is probably one of the most
| useful inventions next to the lightbulb, DC current, Solar
| Cells, Toilets, desalination/ro and the WWW. I've gotten 100mb+
| out in places where you have no cell service, no humans, no
| life, and no way of contacting anyone if in trouble. Starlink
| kept the connection no problem.
|
| I'm excited for the future if we can just get over our
| differences. XR, Starlink, EV's, Highspeed rail, Electric
| planes, Mars...
| Aloha wrote:
| I just dont see how starlink makes money in the long term,
| without massive government subsidy like Iridium gets. Elon's
| seeming unwillingness to play nice with the federal
| government in regards to this harms Starlink's ongoing
| viability.
|
| Technically, it's a pretty neat trick - architecturally, it
| works just like any cellular network, except the power is
| moving, not the subscriber.
| charcircuit wrote:
| The government has been trying to subsidize ISPs to expand
| broadband to more rural areas, but that has been a failure
| compared to Starlink which actually exists.
| toast0 wrote:
| > You too could have had 128k for the low monthly price of
| $250 + cost of two phone lines + cost of equipment + labor".
|
| + per minute billing from your telco and your ISP.
| marcus0x62 wrote:
| > Until some makes a call or sends a fax when your top speed
| drops 64k per phone line being used.
|
| That wasn't how it worked. You could have a T1 with certain
| channels configured for voice and others for data, but the
| configuration was static -- you didn't get to use the bandwidth
| from the voice channels when they were idle.
|
| > It's also why ISDN didn't connect at 128k but at 112k since
| it just used two "bonded" lines of the T1
|
| ISDN bearer channels were 8-bit clean. If you bonded both
| channels of a BRI, you had a 128kbps data path. What you
| typically couldn't use was the 16kbps D channel, which was used
| for signaling.
| InvaderFizz wrote:
| Correct. And IIRC, IDSL was just ISDN with all three channels
| dedicated to data. Yielding 144kbps at phenomenal distances
| from the CO.
| Aloha wrote:
| IDSL was an ISDN bearer with Frame Relay running on top, it
| basically didn't use the channelization scheme at all - and
| IIRC had different framing than a real BRI line.
| marcus0x62 wrote:
| IDSL used HDLC natively as a frame format. You could run
| Frame Relay on top of that (or PPP.) Line coding was
| standard 2B1Q in North America (same as a "regular" ISDN
| BRI.)
| tbyehl wrote:
| Some ISDN TAs supported data-over-voice for 56kbps per
| channel. Useful in areas where data calls has per-minute
| charges but unlimited local "voice" calls were included in
| the monthly charges.
| marcus0x62 wrote:
| Right, and some TAs supported data notifications over the D
| channel. There was also 144kbps IDSL which made use of the
| D channel bandwidth. And a bunch of other variations.
|
| But, the fundamental channel width was 64kbps and ISDN data
| calls were not routinely connecting at 56kbps due to
| robbed-bit signaling, which the GP comment claimed.
| jeffbee wrote:
| Seems like the article failed to address the architectural reason
| why any of these details were relevant. A modem is a point-to-
| point communicating device and the signal has to be carried
| between two arbitrary points on the telephone network. DSL isn't
| really fundamentally different from a 56k, but the architecture
| is superior: DSL only goes to the other end of a wire, then the
| encapsulated IP traffic is forwarded elsewhere. 56k hit a wall
| because circuit switching has fundamental disadvantages. Packet
| switching allowed us to get past that.
| p3n1s wrote:
| 56k was not relevant for point to point communicating between
| arbitrary points on the network though. 56kbps dial-up worked
| in an asymmetric fashion with a fully digitally connected modem
| on one end (eg the ISP). Station to station analog modem
| standards capped out at 33.6k.
|
| > DSL isn't really fundamentally different from a 56k
|
| There's a great deal of additional integrated digital signal
| processing to make DSL work that was too costly for consumer
| modems at the time. I wouldn't agree with this assessment. And
| other than operating on phone lines for last mile it is
| completely different on all the layers and uses completely
| different equipment.
|
| > 56k hit a wall because circuit switching has fundamental
| disadvantages. Packet switching allowed us to get past that.
|
| There were circuit switched networks far faster than 56k, so
| not sure this had much to do with any inherent limitation.
|
| Packet switching has obvious advantages for network utilization
| and was a necessary evolution but I don't think it is relevant
| to any 56k limit or the revolution in DSP that make multi-
| megabit over copper pairs possible.
| jeffbee wrote:
| As you said, the DSP revolution was not scientific it was
| economic. They always knew how to do it, for decades, but it
| was unholy expensive. VLSI economically enabled DSL but it
| was a thing that they knew they wanted to do, and knew how to
| do, for decades. DSL technology was perfected before the 56K
| modem was even invented.
| p3n1s wrote:
| > They always knew how to do it, for decades
|
| Unless we're limiting to the purely theoretical this is
| quite the hyperbole and in that case applies equally well
| to the 56k modem.
|
| Size and heat are also factors. The process nodes did not
| exist in the 70s and early 80s to produce the integrated
| circuits necessary. VLSI _technically_ enabled DSL.
|
| The idea that somehow the difference between 1960s
| integration and 1990s is "purely economic" is pretty silly.
| If only they threw trillions of dollars more into research
| in the 60s would we have had the Intel Pentium then rather
| than 30 years later, maybe, but that is a useless line of
| reasoning. Someone has to actually do the "science".
|
| You need to actually design and produce a suitable line
| card equivalent for every subscriber that will support DSL
| speeds without taking up the space of an airport for it and
| the cooling. That didn't exist for decades before DSL was
| rolled out (which started barely a decade after it was
| patented).
|
| > DSL technology was perfected before the 56K modem was
| even invented.
|
| Considering that DSL standards continued to evolve after
| the 56K modem was obsolete this is also hyperbole.
|
| The other thing not yet mentioned which is pretty
| fundamental is that DSL eschews the 3.3khz bandwidth of
| POTS.
| Syzygies wrote:
| I mentioned this article to my retired AT&T engineer wife, and
| she blurted out "Robbed-bit signaling!"
|
| https://en.wikipedia.org/wiki/Robbed-bit_signaling
| hlandau wrote:
| Unrelated, but I highly recommend anyone visiting Seattle check
| out the Connections Museum, open on Sundays only. It has
| several different genuine analogue and early digital telephone
| exchange frames saved from being scrapped from genuine
| telephone exchanges. They turn them on and you can actually
| call from one phone to another inside the museum. The museum
| itself actually shares a building with a real telephone
| exchange. Lots of old books, technical journals and schematics
| too.
|
| I just thought to post this since when I visited in September,
| some actual retired telecoms engineers were part of the tour
| group. One had been involved in the development of submarine
| cable technology.
| jonathankoren wrote:
| I recently discovered their YouTube channel, and it is
| absolutely fascinating.
|
| https://youtube.com/@ConnectionsMuseum
| Aloha wrote:
| Technically only the not yet functional DMS-10 is digital,
| everything else is analog. The 3ESS is a stored program,
| computer controlled analog switch - it uses the same remreed
| technology the 1AESS used to set up call links.
|
| Out of all of those however, panel is my favorite however, it
| sounds amazing to listen to, both the equipment and what the
| line sounds like. A call between a panel and the number 1
| xbar sounds amazing, nothing quite sounds like revertive
| pulse signaling.
|
| Interestingly enough, there were three switches made that
| used RP internally - Panel, 7A Rotary and 1XBAR - and if you
| go find video of the 7A Rotary, you'll see it sounds almost
| exactly like panel, because the unique sounds of the rotary
| sequence switches both exchanges use.
| Solvency wrote:
| Is the fact that the museum isn't available 85% or the week
| intentional irony or something?
| MBCook wrote:
| That was great. Never knew exactly where the 56k number came
| from.
|
| When 56k modems first came out I was able to get full speed, and
| it was a great upgrade. Then we moved and I could never pass 28.8
| or so.
|
| Turns out in their infinite wisdom in the new development we
| moved to the phone company divided things up even more so they
| could get two lines out of what should have been one. 28 + 28 =
| 56.
|
| I don't know if they were using some sort of compression or
| something so that it wasn't audible on voice calls but it meant a
| modem was capped forever.
|
| The wait for cable internet was interminable (it came before
| DSL).
| bwann wrote:
| Usually this meant the new neighborhood was put behind a
| subscriber loop carrier (SLC) which added an extra
| analog/digital conversion. Voice calls were getting digitized
| locally, sent across a T1 (only 4 wires needed, instead of
| 96-192), and back to analog to the normal frame at the CO. The
| effect of this was the telltale sign of carrier speeds only
| hitting 26,400 bps.
| MBCook wrote:
| That's basically the kind of thing I suspected. Thanks!
|
| I wonder if that config screwed them when it came time to try
| to sell DSL. It still wasn't available years later when I
| moved out.
| redm wrote:
| This brings back a lot of memories, but I think what I lament the
| most is that this was really the end of American soup-to-nuts
| engineering in telecom. At the end of the 56k production and the
| start of G.Lite DSL, all this engineering started going to China
| to save money. Long gone are the days of Radio Shack and
| companies EE's designing, prototyping, testing, and manufacturing
| electronics. Sad.
| rasz wrote:
| Usenet was a fantastic period correct source of information. Here
| an archive of insightful posts about phone networks
| https://yarchive.net/phone/
| flyinghamster wrote:
| A great source of archived information - and TELECOM Digest is
| still functioning.
|
| http://telecom-digest.org/
|
| http://www.telecom-digest.org/latest-issue.html
| pjmlp wrote:
| No mention of dual speed models like Diamond, which would use two
| 56k lines simultaneously?
| ryan-c wrote:
| That could even be done with two completely separate modems.
| pjmlp wrote:
| Not really, as with using something like the diamond that was
| hidden by the driver.
| don-code wrote:
| Indeed, PPP has a multilink feature which an bond packets
| from multiple modems into a single bitstream. Both the client
| (your machine - Windows 98 could do it) and your ISP needed
| to support it.
|
| I remember trying this a few times while at my parents'
| store. My dad would tell me which phone line I could use that
| day - either the dedicated line used to process credit cards
| (if they were closed), or a spare line off the PBX. But
| occasionally, I was allowed the use of both - and luckily had
| a pair of PCMCIA modems with the dongles in different places
| - and could get speeds well above 56k.
| NikkiA wrote:
| Diamond called theirs 'shotgun technology' iirc. I used a pair
| of USR external modems cobbled into a shotgun configuration via
| a linux proxy to manage our internet connection, but we got DSL
| shortly after (and surprisingly early for the rural location we
| were in).
| marcus0x62 wrote:
| This article glances at, then misses the fundamental technical
| reason why modems topped out at ~ 56kbps: that's really all the
| bandwidth that is available on a POTS line, due to the bandpass
| filter, the ADC on the other end, companding, and engineering
| practices that led the Bells to maintain their lines to the
| standards of an analog voice conversation.
|
| The robbed-bit signaling thing is really a red herring: it was a
| problem, but not THE problem. At the time analog 56kbps modems
| were popular, it was already possible to negotiate 64kbps/8-bit
| clean DS0 channels over T1 -- you just needed a PRI or an inter-
| machine trunk to do so.
|
| In fact, if you were an ISP at the time, you generally needed
| those modems connected to T1 PRIs or IMTs _anyway_ to support
| your ISDN customers economically. (My memory may be getting fuzzy
| on this, but I also remember that certain access concentrators
| with k56 /v.90 digital modems would only negotiate the higher
| analog speeds on a PRI or IMT bearer channel.)
|
| What there was absolutely no appetite to do, on the part of the
| voice carriers, was beef up the bandwidth available on POTS lines
| by developing new ADCs, getting rid of companding, or engaging in
| labor-intensive "facilities grooming"[0] for a POTS subscriber,
| in the way they routinely did for T1[1] circuits.
|
| 0 - Telcos call their lines and equipment "facilities." Grooming,
| in the context of outside plant (telco terminology for the
| physical cabling that goes to subscribers) means to select wire
| pairs that exhibit good noise and echo characteristics and to
| remove elements, like poor splices and bridge taps, that impede
| performance.
|
| 1 - T1s, unless delivered to a customer via an optical transport
| network, use the same copper lines as analog POTS lines.
| Originally, T1s required two pairs (four wires). This eventually
| was replaced with HDSL4 (two pairs, but better distance and
| repeater characteristics,) then HDSL2 (single pair.) As the names
| suggest, the encoding used was a DSL variant.
| Sesse__ wrote:
| And even for 56kbps, you need to have the ISP end talk
| digitally to the telco, so that you don't get the noise
| (including from the DAC) of the local loop on the ISP end.
| That's really what pushes the SNR far enough down to make it
| work (and why the uplink is only 33.6kbps; I guess they didn't
| want to try to make a separate modulation for that direction).
| marcus0x62 wrote:
| V.92 supported up to 48kbps uplinks, but that standard caught
| on about as well as you might imagine an analog modem
| standard from the year 2000/2001 to.
|
| https://en.m.wikipedia.org/wiki/ITU-T_V.92
| Sembiance wrote:
| The article mentioned using an 8bit ADC. Could a 16bit ADC be
| used on BOTH sides and achieve 112kbps?
| dreamcompiler wrote:
| No because you still have to modulate the signal to get it
| to/from a house, and modulation schemes are where you squeeze
| maximum bits per second into available analog bandwidth
| within the limits imposed by Claude Shannon.
|
| The article didn't talk about modulation schemes and I wish
| it had.
| marcus0x62 wrote:
| Sure, with other changes. You would still have to:
|
| * Select (or repair) "good" pairs that didn't have poor echo,
| impedance, noise, etc. characteristics.
|
| * Not compand the resulting PCM sample with mu/alaw
| companding.
|
| * Provide an atomic bearer channel from the telephone switch
| to the ISP that was at least 112kbps.
|
| * Develop a modem encoding scheme that could take advantage
| of the extra bandwidth.
|
| At this point, you've reinvented a really crappy version of
| DSL that could be user-signaled to call different networks
| (i.e., phone numbers.)
|
| The other option, the one that people selected, was to use
| the copper facilities but bypass the phone switch and its
| associated audio and in-band signaling shenanigans. That
| was/is xDSL.
| 13of40 wrote:
| When I was in highschool I lived in a rural town of about 7000
| people that had an analog telephone exchange. I read about the
| bandwidth limitations, and a friend of mine and I decided to
| test them over a local and long distance call. The long
| distance one cut out pretty much exactly at the frequency the
| spec said it should, but the local one could transmit and
| receive much higher frequencies, presumably because it had its
| own circuit and wasn't multiplexed with other ones. (That led
| to a lot of unrealized fantasies about creating a homemade
| super modem for local connections.)
| marcus0x62 wrote:
| Assuming the local call wasn't on a party line, your lines
| were probably served by an analog switch or maybe on the same
| line card of something like a DMS-100. What you described
| wouldn't work on "modern" digital switch.
| kazinator wrote:
| Here is a much shorter explanation. PSTN voice calls go over 64
| kbps digital links (e.g. G.711 uLaw codec, 8 kHz sample rate).
|
| 56 kbps pushes close to the theoretically available channel
| bandwidth.
|
| That's all there is to it: information coding theory.
|
| To get more than around 56 kbps from modems, you would need more
| fidelity from the underlying lines: a wider-band, lower noise
| pure copper end-to-end connection, or higher bitrate codecs.
| xcv123 wrote:
| Shorter but missing an important detail.
|
| The article explains that every 8th bit is lost to allow for
| control signalling that steals the 8th LSB on every 6th frame.
| Hence 7/8 * 64 = 56.
|
| https://en.wikipedia.org/wiki/Robbed-bit_signaling
| chriscjcj wrote:
| I was never able to exceed 24kb/s with my "56k" modem. I came to
| find out that the phone company (in Austin TX) was using a single
| pair to service more than one customer in my apartment complex.
| Naturally, whatever piece of equipment they used to accomplish
| this stymied faster modem speeds.
|
| I finally installed an ISDN. That was delightful. Not only did it
| allow me to have two lines - one for voice and one for data - the
| netgear RT-338 ISDN router automatically dialed my ISP
| instantaneously. It felt like I was always connected because I
| didn't have to wait for a modem handshake. Speeds were more than
| twice as fast as what I could get with my analog modem. However,
| by far the MOST IMPORTANT thing (and mostly the reason I
| installed the ISDN in the first place) is that my latency took a
| nosedive allowing me to become an LPB with a 50ms ping time on
| QuakeWorld. >:-D
|
| (Shoutout to Charlie S. from OuterNet, Austin's best ISP in the
| 90's)
| queuebert wrote:
| What is an LPB?
| zain wrote:
| Low Ping Bastard
| hypercube33 wrote:
| Back in the day LPB was someone with low pings (remember
| dial up was 150 to 300ms sometimes) and they'd have an
| upper hand on games before they were "unlagged" with
| prediction code.
| oh_sigh wrote:
| A common lament among the dead 15 years ago in real time
| online gaming...low ping bastard/bitch, basically just saying
| the only reason you got the kill is because your ping is 60ms
| and mine is 250ms.
| bcrl wrote:
| I worked on ISDN drivers and firmware at a job back in the late
| 1990s. One of the optimizations I implemented eliminated a
| memory copy by the 68302 which improved ping times from ~30ms
| to ~16ms. The good old 68000 is not exactly the fastest at
| memory copies...
| mips_r4300i wrote:
| Seems like an oversight that the extra stuff the 68302 added
| wouldn't have been a DMA block copy or something.
| Aloha wrote:
| You likely had something called a 'pair gain' installed - which
| worked much like DSL, by using a high frequency FM carrier over
| the audio frequencies to provide a second phone line on the
| same pair. These things were largely gone by the mid 90's - and
| were not that common in the first place.
| IntelMiner wrote:
| They plagued Australian homes up through the late 2000's
| unfortunately
|
| Fucking Telstra
| throwaheyy wrote:
| Yup, we built a new house in Australia in 2001 and thought
| we were being so forward thinking by getting a 2nd phone
| line for the modem/fax. Of course it ended up being done
| using a pair gain and we were stuck at 33.6k.
| bigtex wrote:
| Back in the late 90s friend had ISDN installed at his apartment
| and it was glorious. Compared to dialup it was like high speed
| internet. We used some Mac app that basically gave you access
| to all the cracked software you could have ever wanted.
| tibbydudeza wrote:
| You could also bond the two channels together for a 128K
| connection - installed Linux to get the bonding to work on my
| ISDN card as Windows 3.1 For Workgroups knew nothing about it.
| fanf2 wrote:
| My understanding from discussions about modems in the mid 1990s
| was that the bandwidth limit for POTS-to-POTS modems was about
| 36kbit/s, because the signal has to go DAC (modem) -> ADC (telco)
| -> DAC (telco) -> ADC (modem), and the modems at either end are
| not synchronized with the telco's digital path.
|
| The thing that allowed speeds to increase to 56kbit/s was digital
| connections at the ISP end: typically the ISP would get a T1 or
| E1 circuit fed into a box that would act as 24 modems, but purely
| digital and synchronized with the telco. So the signal now went
| Ascend MAX (isp) -> T1 (telco) -> DAC (telco) -> ADC (modem),
| providing much less quantization noise.
| rollcat wrote:
| I loved the article, and when I love an article the first thing I
| do is look for the website's news feed.
|
| From <https://www.10stripe.com/10blog/>:
|
| > The blog has been discontinued due to Wordpress security
| concerns.
|
| Keep this tragedy in mind next time you pick your tech stack.
| toomuchtodo wrote:
| https://web.archive.org/web/20111202070238/https://www.10str...
| theoldlove wrote:
| Boils down to the fact that a phone line to a house was built
| with a a certain amount of bandwidth for a human voice,
| implemented with a lowpass filter of 4 kHz. But the article
| doesn't explain or show why the magic 4 kHz number was chosen.
| rasz wrote:
| There are no filters and cable is not BW limited, otherwise
| ADSL wouldnt work on same wiring.
| quercusa wrote:
| See the section _Long, long ago_
| Arubis wrote:
| Ohhhh, is _this_ where the 7-bit transfer settings came from?
| That always seemed a weird number to me (unless a bit was instead
| repurposed as a parity bit).
| Arubis wrote:
| On second though, probably not; the same 7/8 bit settings plus
| parity and stop are available on non-POTS-dependent serial
| ports as well.
| aYsY4dDQ2NrcNzA wrote:
| https://en.wikipedia.org/wiki/SupraFAXModem_14400
| RecycledEle wrote:
| TL/DR The standard for digital phone lines is (was) 7 bit samples
| multiplied at 8,000 samples per second.
|
| 7 * 8000 = 56,000 = 56k
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