If dialup works over POTS just like a phone call, how was it possible for many people to dial the same number/connect to the same place at once? How didn't they get a busy tone?

  • Its the same way a company has a single main phone number that thousands of people can call and use simultaneously.
    – Ron Maupin
    Nov 26, 2016 at 2:28
  • @Ron but in that case each call goes to a separate phone, whereas in the dialup case, they all go the same route, do they? Nov 26, 2016 at 2:30
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    It's simply POTS call routing. In the case of a person-to-person call, all the calls end up on different telephones, but a modem-to-modem call, all the calls end up on different modems. A POTS call is a POTS call, and POTS routing has been basically the same for over 100 years.
    – Ron Maupin
    Nov 26, 2016 at 2:32
  • @Ron what about the server side? How can it establish connections to many users? Nov 26, 2016 at 2:35
  • That is the same way that millions of people not on a dial-up can connect to Google at the same time. That is a really broad question, and we cannot simply answer that here. Suffice it to say that everyone has a different source address, and a server can tell the difference in connections.
    – Ron Maupin
    Nov 26, 2016 at 2:39

1 Answer 1


Essentially because the dial up server has more than one POTS circuit associated with it. By contrast, a residential line has one circuit, so when A calls B who is on the phone, the circuit is already tied up. The dial up server would not have one phone line (circuit) but many, aggregated into an entity known as a trunk. Different size trunks are possible depending on anticipated traffic patterns. Of course, if you get it wrong, it's still possible for a trunk to fill up, and give you the busy signal.

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