How does telephone infrastructure change with multiple phones

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I am imagining the typical POTS configuration where one telephone set is connected to modem, which is connected to a phone jack inside a two-story building. The wires from the phone jack are somehow carried up to the telephone poles and travel to a switch inside a local exchange carrier's central office. What changes to the underlying infrastructure would need to occur to accept, say, 100 phones to service 100 phone numbers within the same building? Would the carrier need to extend new wires, from the building to the central office, for each new phone number serviced? Would more modems need to be obtained, as each modem has a limited number of ports? How many phones can a single phone jack typically support, anyway? I pose these questions exclusively in the context of voice communication over PSTN.

Harkut

Posted 2020-01-05T02:49:11.077

Reputation: 3

Question was closed 2020-01-05T08:29:29.260

At this scale, you don't have to be limited to one physical analog line (copper local loop) per number. You could have a T1 line provisioned as an ISDN PRI which gives you 23 voice channels. So if only about a quarter of your employees are ever goung to – Spiff – 2020-01-05T04:09:35.843

Answers

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This question has many answers, depending on which technologies the carrier and the customer are able and willing to support.

  • With the (nearly died out) classic POTS model, 99 new wire pairs would have to be laid from the PBX (in its building) to the site, exh would be connected to one plug.

  • With ISDN (and similar techniques) a T1 would have to be laid from the PBX to the site, where it would be terminated into a multiport NT

  • With VoIP the one line would have to be repurposed into xDSL to provide connectivity to a suitable branch PBX on site, that could easily supply 100 lines (each via VoIP oder with an ATA to a classic phone plug)

Eugen Rieck

Posted 2020-01-05T02:49:11.077

Reputation: 15 128

PBX means Private Branch Exchange, as in a customer-owned phone switch, so it would be on-site. The switching equipment at the LEC's CO wouldn't be called a PBX. – Spiff – 2020-01-05T04:16:46.690

In the spirit of inquiry, I further researched my own question, and it turns out that 1) My view of a PBX was too narrow. It is also a physical device which can route calls rather than merely a private telephone network. 2) a PBX device can have analog and digital ports, which eliminates the need for a modem. I appreciate your answer and I will look into ISDN / VoIP as time permits. – Harkut – 2020-01-05T04:38:17.467

Although in US (at least) in copper-pair days they would never lay a single pair to anything but a standalone house. For a commercial or shared building (like apartments) they would start with at least a 25-pair cable, and very often 100-pair or more depending on circumstances, terminated on a block usually in a basement utility closet along with the electric meter and fuse/breaker panel, water meter and shutoff, and maybe (heating) gas ditto, and from there pull a single pair or 'quad' (two pairs in plastic cover, colorcoded red, green, yellow, black) to an individual room. – dave_thompson_085 – 2020-01-05T09:31:24.850

@Spiff This might be a simple matter of language: Here in Europe the "last" telephone switch (the one that connects customers, not other exchanges) in the analog world was also called "PBX", with "private" expressing that the down side coinnections go to private sites. – Eugen Rieck – 2020-01-05T11:29:37.853

Asked: 2020-01-05T02:49:11.077

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Active: 2020-01-05T03:03:41.533