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My local internet service providers can provide ADSL (asymmetric digital subscriber line) service at various speeds, for example, 50 M bits per second. This service is in Canada.

Is this typically provided on the same twisted pair of copper wires that in the 1990s was formerly used to provide dial-up internet service?

In other words, I want to know if the last mile of copper wires was not the data rate limiting facility in the old days of dial-up internet service or has the last mile been upgraded in the years since?

Also offered in my area is fiber to the home but that ends at a separate wall outlet so it seems to be a parallel system. If it is a parallel system it would seem that the copper wires are still in use and they might be the same copper wires that have long existed.

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    \$\begingroup\$ For the history of that copper wire : Plain old telephone service (POTS) \$\endgroup\$
    – J...
    Jul 26, 2021 at 12:19
  • \$\begingroup\$ 50 Mbits on twisted-copper ADSL?? wow. that does not work in my universe. \$\endgroup\$
    – PcMan
    Jul 28, 2021 at 8:14

5 Answers 5

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Is this...the same ...pair of copper wires that...was formerly used to provide dial-up internet service?

Same wires, but those wires weren't put there for dial-up, they were put there for voice. And, the whole telephone switching system was put there for voice, and that includes the trunk lines that run between cities, and between exchanges within a city.

In order to make the best use of those trunk lines, the voice circuits were multiplexed. And, the multiplexing scheme allowed just enough bandwidth (typically, 4kHz) for each voice channel to transmit intelligible speech.

It was the arbitrary bandwidth of those virtual voice circuits, and not the physical characteristics of the "last mile" wiring that prevented dial-up from ever going faster than 56k.

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    \$\begingroup\$ And 56k was only possible over digital networks because ISPs had direct digital connections. When going from analog to analog the limit was 33.6k. en.wikipedia.org/wiki/Modem#Evolution_of_dial-up_speeds \$\endgroup\$ Jul 26, 2021 at 7:21
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    \$\begingroup\$ And 56k (plus some framing and backchannel information) was all that would fit into a 64k slot in a digital E1 or T1 link, allocated to a single voice line when the backhaul went digital. So even if ADSL modems could have been developed earlier, the backhaul would have throttled each subscriber to 56k. \$\endgroup\$ Jul 26, 2021 at 11:50
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    \$\begingroup\$ It's worth noting that the bandwidth of DSL is broken up into 4kHz channels. You just get quite a few of them! \$\endgroup\$
    – Cort Ammon
    Jul 27, 2021 at 5:29
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It's primarily that the equipment has moved closer to you.

Originally, the ISP had a bank of modems, connected to regular phone lines. You dialled, and a analogue end to end-connection was set up; audio was transmitted from your modem to the ISP's modem. This was limited to 33600bps, due to the bandwidth of the channel (3kHz).

Then the audio connection was moved; the modem was not at a regular phone line in the other end anymore, but rather in telco's system. This allowed for a bit greater speed, 57600bps. But it was still encoded as audio and sent.

DSL effectively moves the equipment one step closer to you: the DSL terminal is physically at the other end of the two copper wires coming out of your wall. This means it doesn't have to care about a lot of infrastructure, it can use the full bandwidth that your physical wires can transport - which will depend on type of cable, distance and age of cable.

This was partially made possible by the fact that more people subscribed to the Internet, so that suddenly the majority of phone customers also wanted internet, partially by the decreasing cost of electronics, and partially by general development of technology.

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    \$\begingroup\$ The trick with 56k modems was not that the modem was closer. It was made possible by playing nasty games with the digital end nodes that replaced the direct connect analog line. \$\endgroup\$
    – JRE
    Jul 26, 2021 at 14:23
  • \$\begingroup\$ Thanks for the correction! \$\endgroup\$
    – vidarlo
    Jul 26, 2021 at 14:28
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    \$\begingroup\$ Probably worth mentioning that a DSLAM is on every street, which is a lot closer than the telco central office building \$\endgroup\$
    – Nayuki
    Jul 26, 2021 at 16:35
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Yes the same voice lines are used. But I’m not sure how much BW is enhanced on ADSL2+ SLIC’s and modems but the channel is split into (I think) 3kHz consecutive bands each yielding 64kbps. Thus 50Mbps computes to 2.34MHz BW needed probably shared by 2 copper pairs. A powerful DSP equalizes the gain and phase in each sub-band to be flat by training signals somewhat like the old 56/64kb modems did. This limits the max baudrate to lines < 1km from the hub. As the S/N ratio must be high and attenuation is very sensitive to line length.

Splitting into sub-bands is a common way for all high speed modems to equalize the group delay, phase and amplitude with DSP’s to then aggregate the bit rate in parallel.

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Your question presents a false dichotomy, as the answer is "both".

Solomon Slow's answer covers one side. The last mile lines were predominantly built to exchange voice to a central office and onward around the world, so "dial-up" was limited to what could be done with existing equipment in the context of long distance analog voice circuits. Throw in monopolistic practices by companies like Bell and the system becomes very constrained as you were not permitted to connect anything except a Bell approved phone. Pre-dating your 1990s time frame you would have had an acoustic coupler linking speakers and microphones, with no physical connection to the wires. Key is that these were long distance analog signals.

However voice was not the only option. If you ran a large enough site you more than likely ran other systems over the same last mile twisted pair wires. Serial protocols connecting dumb terminals on private networks over short distances easily transmitted at higher rates as they had none of the restrictions of long-distance voice circuits. A large enough institution would have dedicated circuits connecting to other institutions. While still slow by modern standards, this was far more capable than voice. The biggest difference is that these were typically shorter distance digital signals on the wire.

By your '90s time frame Bell had lost the fight and you could hook up a modem directly; at the central office equipment had improved the quality of the voice circuits; and the wire itself in many cases had improved. As examples many rural and other longer distance "last miles" had load coils installed which allowed analog voice circuits to be run farther, but were not friendly to digital signals. By the '90s load coils were getting removed, central office equipment was more capable and often encoded analog voice into digital backbones, and smaller equipment was getting located closer to customers. Old steel lines were switched to copper. Voice equipment and modems were getting better, but it was still fundamentally a long distance analog voice system.

However voice was still not the only option. For a price you could switch the analog voice line to a digital system. While modems were getting 2400-19200bps, ISDN BRI could run 2 x 64Kbps bonded channels on the same pair of wires, while ISDN PRI, T1, E3, and other protocols allowed much higher speeds on the same copper lines (though 4 or more wires rather than the 2 required for voice). Today I still occasionally come across T1 1.5Mbps circuits running on 2 pair (4 wire) phone lines in rural areas where this is the best connection available, often with bonding applied to net a 6, 9, or 12Mbps connection on "every phone wire coming into the building". The biggest difference was still that these were typically digital signals on the wire over short distances (with your provider routing to the wider world).

This trend continues of replacing the voice equipment in the central office and on the customer premise with digital endpoints while optimizing the copper lines by moving more central office equipment toward the customer. Today the last mile on copper VDSL2+ is often short to a DSLAM in a relatively close cabinet where the traffic moves to fibre. Interestingly, the wires no longer carry one or the other between voice or digital but rather carry both with filters separating voice in lower frequencies from digital at higher frequencies on the same wire.

Most of this comes from my own experience (also in Canada), so I'd gladly take improvements. I ran 300bps in the '80s, '70s era campus serial connections on mainframe in the early '90s, ISDN BRI in mid '90s, and pretty much everything else mentioned personally or professionally over the years up to my current VSDL2 bonded pair (which sucks, I used to have FTTH).

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Here's my simple explanation:

Dial-up modulates the data as sound, so it can be carried on infrastructure that was never designed for data in the first place. The connection is not simply wires and switches. There are various filters and other things involved and these are designed to pass voice specifically.

DSL on the other hand, is designed for data. The last mile to your house might be the same, but the rest of the system is totally different.

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