I’m trying to understand how an ISP could handle multiple concurrent calls on a single phone number? And did they need a physical modem for each connection? How did this scale to medium to large cities with millions of people? How did the call get transferred from copper wiring between the modem at home, to a modem in an exchange somewhere? What was that modem connected to? Did it vary between countries?
My naive understanding is that there is a direct connection (circuit) between the two modems, but I don’t understand how you could support two concurrent calls on one number.
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> how an ISP could handle multiple concurrent calls on a single phone number? And did they need a physical modem for each connection?
Not exactly.
Individual houses had (and often still have!) only a single analog line. The analog line is converted to a digital audio signal at the Phone Company’s central office.
If you were calling another house, it would be sent digitally to the central office near that house, then converted back out to an analog signal.
ISPs would generally skip the second half of that.
On their end, the ISP would have digital lines like T-1 or ISDN-PRI from the central office that could carry a dozens of phone line signals still in digital forms. Those lines fed into racks of special digital multi-line modems connected to the ISP’s data network.
https://www.patton.com/products/product_detail.asp?id=22
When your modem dialed into their main modem number, the phone switch would run through its list of virtual lines tied to that number and connect you to the first one that wasn’t busy. That’s called “Line hunting.”
This is the same sort of setup that is still used for 1-800 sales or tech support lines; dozens or hundreds of people can call into the same number and all be connected with agents at the same time. Though today a lot of it is done virtually using VoIP instead of relying on a phone switch.
> How did this scale to medium to large cities with millions of people?
A large ISP serving a large area would need racks of equipment in a point of presence somwhere in town, or would have to pay a Phone Company to carry the phone traffic over long distance lines to someplace where they did. (The latter option was very expensive in the ’90s, so I doubt many ISPs did it. It would be very cheap to do today…but of course nobody uses modems anymore today.)
> Did it vary between countries?
Probably! The setup I’ve described is for a generic country with a modern phone system like the US or Western Europe.
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