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Via Wikipedia,

Finally, unlike dial-up, broadband does not require exclusive use of a phone line...

Why does dial-up require use of the entire phone line? I know that once the data enters the PSTN, it's usually digitized and sent over a trunk, which either has multiple lines or does something like TDM to share the line.

Why doesn't dial-up share the line?

Edit: Specifically, why doesn't dial-up use TDM for the local loops?

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  • One point I think worth mentioning is that telephone modems were designed for exploiting a low bandwidth voice line (signals got filtered at the end office). If you tried to share that limited bandwidth between your voice and data, it'd be a tight squeeze! DSL requires an unfiltered line, which has a much higher bandwidth (~1MHz instead of 3100Hz according to the book I'm lookin at).
    – Andy
    Jul 4, 2014 at 17:12

2 Answers 2

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The PSTN phone lines are analog and only require 2 wires; the second pair of wires are to ring the bell for the original (1950) style of telephone (it required more voltage and current then the voice signal). In order to use TDM on the phone lines, the carrier would need to replace the equipment in their office (to handle TDM) as well as you replacing your phone (also to handle TDM). It would probably also need another pair of wires to operate efficiently. A major expense for the phone company. DSL provided a solution which was easier (and cheaper) to implement.

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  • POTS analog lines only use two wires out of the 4 in an RJ11 jack (and matching wire). The off-hook/carrier is low-voltage DC, and the ring is AC, both done on the same line(s). See: en.wikipedia.org/wiki/Ringtone Jul 4, 2014 at 17:22
  • @techie007 The ring signal was carried on both the voice line and the bell line. I should have said that only 2 wires are required (instead of 2 lines). My answer has been corrected.
    – LDC3
    Jul 4, 2014 at 17:29
  • Weird, I've never heard of a separate "bell line". Perhaps it's country specific? Here in Canada the other two conductors (wires) do nothing. Mind you I wasn't alive in the 50's, just since the 70's. ;) They were often used as a way to add a second line to a jack, without having to run new wires in the walls. Jul 4, 2014 at 17:32
  • @techie007 Since the 80's, the use of IC chips in the phones (and the lower voltage needed to ring the bell), the bell line became redundent.
    – LDC3
    Jul 4, 2014 at 17:35
  • But it's not a lower voltage since the 80's, its still a 60-90V AC jolt to make it ring. I can still take a non-electronic (no ICs) analog phone and plug it in, and it'll work, including ringing. My buddy had a solid metal desk phone from the late 40's, and it worked fine on his analog line in the late 90's (the hand-set part was like 2-4lbs, hehehe -- it sucked for long conversations). Jul 4, 2014 at 17:41
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Dial up uses the frequencies of sound that exist at the human audible levels. This is where the old dial up sounds come from. If you shared the line you (and the computer) would be trying to talk over eachother.

Once your line reaches the network it is digitized and then can be sent using standard sharing protocols. afaik, most of the phone traffic actually routes via the internet these days, although sometimes on dedicated lines.

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  • Yeah, which is why DSL doesn't require the entire line. But why doesn't dial-up just trade off with who's using the line, much how an OS schedules processes to run? Jul 4, 2014 at 15:33
  • How do you trade off with a human? The way computers do it is so fast it's measured in minute fractions of seconds, we can't operate like that.
    – Aboba
    Jul 4, 2014 at 15:38
  • Then how to telephony systems do it? Is the difference that the PTSN is generally dealing with digital data? Jul 4, 2014 at 15:50
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    Yes, once your line reaches the network it is digitized and then can be sent using standard sharing protocols. afaik, most of the phone traffic actually routes via the internet these days, although sometimes on dedicated lines.
    – Aboba
    Jul 4, 2014 at 15:56
  • Ah, alright. Mind editing your answer to add that in? Jul 4, 2014 at 16:46

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