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 does a corporation or call center handle multiple concurrent calls with a single phone number?
The answer is they use a megalink, which is a digital phone line that can handle multiple simultaneous calls.
A standard one is called a T1 (1.5mb/s) and can support 24 simultaneous calls.
The number call is sent down the link digitally and includes a header that defines the source and destination phone numbers.
The hardware on the company side is a phone system or router that is designed to work with a T1 link as an input and has logic to identify and negotiate incoming calls on different channels on the T1 line, as figures out where to send those calls internally.
Today T1s are falling out of favor in exchange for pure digital links like SIP trunks. This work over the internet but while the protocols are very different they serve the same purpose.
>I’m trying to understand how an ISP could handle multiple concurrent calls on a single phone number?
Phone companies offer a service to businesses that you can have one number for multiple lines. Nothing weird. Normal stuff. You actually will own like 2 or 10 or 10000 lines, but they all connect to the same number, and it “rolls over” to the next connection if one is in use. This is old and part of the normal phone system. Very large systems would often have a specific high capacity telephone line directly to them… or lots of these lines.
>did they need a physical modem for each connection? How did this scale to medium to large cities with millions of people?
In the earliest days, yes. There would be racks and racks of modem — however this changed with technology and ISPs were able to transition to some more complex software and hardware versions that didn’t require individual modems and was much much easier to scale.
> 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?
It was done via the phone lines, later the phone lines connected to more complex networking centers which would allow more connections and easier access like above
Multiple phone lines. In the old days (90’s and before), they’d actually pull bundles of cables instead of a single pair for a single phone line. Each pair would be connected to a modem. There would be banks of modems in racks to do just that. The modems would be connected to a network router that connected to the internet via a T1 or similar. The phone number is just what’s assigned at the phone company. It has a switch that routes calls to a single phone number to the next open line in the bundle that they send to the customer.
But yes, at it’s core, the phone copper goes from the customer house all the way to the ISP’s modem. It went through several switches that physically connect one set of copper wires to another. A modem is just a device that converts an analog signal to a digital signal. Once at the ISP’s modem, the digital signal goes through the ISP’s router to the internet.
Source: I was a sysadmin at an ISP in the early 2000’s.
> 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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https://www.wikiwand.com/en/Dial-up_Internet_access#Media/File:Modem-bank-1.jpg here’s what it might have looked like.
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