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Any fixed, that is permanent, point to point data communications link, which is leased from a telco or similar organisation. The leased line involves cables, such as twisted pair, coax or fiber optic, and may involve all sorts of other hardware such as (pupin) coils, transformers, amplifiers and regenerators.
Configuring your modem and pppd to use a 2 wire twisted pair leased line.
SLIP, getting or installing pppd, synchronous data communication, baseband modems, xDSL.