digital telephony systems.

In the early days of fiber optics, every telephone company had its own proprietary optical TDM system. After AT&T was broken up in 1984, local tele phone companies had to connect to multiple long-distance carriers, all with dif ferent optical TDM systems, so the need for standardization became obvious. In 1985, Bellcore, the RBOCS research …

The initial ADSL offering was from AT&T and worked by dividing the spec trum available on the local loop, which is about 1.1 MHz, into three frequency bands: POTS (Plain Old Telephone Service) upstream (user to end office) and downstream (end office to user). The technique of having multiple frequency bands is called frequency division …

Diagrams such as those of Fig. 2-25, which show the legal combinations of amplitude and phase, are called constellation diagrams. Each high. dem standard has its own constellation pattern and can talk only to other modems that use the same one (although most modem can emulate all the slower on With many points in the …