Wait – credit where credit is due. I am not the author of this. However, there are still dial-up users and they often ask "why can't I get 56K". I've found this post to be timeless.
Repost of an article by "Ed" from Lucent Forum: comp.dcom.modems Posted on: 1998/03/12 Message-ID: <email@example.com> Newsgroups: comp.dcom.modems Organization: Lucent Technologies
SLC = Subscriber Loop Carrier, aka digital loop carrier system. Instead of 1 call on 1 pair of wires, you put a SLC terminal on each end and carry 24 calls on 2 pairs. That's why the telcos also call it "pair gain," which is also the name of one company who makes this equipment.
Although the digital side of a SLC can now use fiber-optic technology, traditionally it's used 1.5 Mbit/s T1 on copper. Western Electric, then AT&T, now Lucent Technologies has had SLC-96 (96 channels on 5 T1 lines),
SLC Series 5, and now SLC-2000. Successive versions have had higher density, more types of plug-in line cards, more flexible port concentration, better remote maintenance, etc.
When the central office has a digital switch, you can omit the SLC central-office terminal and run the T1 lines directly to the switch. This is called "integrated" SLC. In this configuration, there's only the single analog/digital conversion at the SLC remote terminal in your neighborhood. It's digital all the way from the digital carrier to your digitally-connected Internet service provider. Integrated SLC allows V.90 modems to work.
A non-integrated, or "universal" SLC has an additional digital/analog conversion in the central office. Your line hits the SLC in your neighborhood, is converted to a digital signal, goes to the central- office SLC terminal, is converted back to a voiceband signal, then connects to an analog line card on the switch, where it's probably digitized again. V.90 modem operation is impossible in this configuration, so you are limited to V.34. The multiple conversions, plus the robbed-bit signaling that's typically used in SLC, limits V.34 rates to around 26.4 kbit/s, or 28.8 kbit/s if you're lucky (as I happen to be on this call).