Ungerboeck published a paper describing the principles of trellis modulation.
A flurry of research activity ensued, and by 1990 the International Telecommunication Union had published modem standards
for the first trellis-modulated modem at 14.4 kbit/s (2,400 baud and 6 bits per symbol). Over the next several years further
advances in encoding, plus a corresponding symbol rate increase from 2,400 to 3,429 baud, allowed modems to achieve rates
up to 34.3 kbit/s (limited by maximum power regulations to 33.8 kbit/s). Today, the most common trellis-modulated V.34
modems use a 4-dimensional set partition which is achieved by treating two 2-dimensional symbols as a single lattice. This set
uses 8, 16, or 32 state convolutional codes to squeeze the equivalent of 6 to 10 bits into each symbol sent by the modem (for
example, 2,400 baud × 8 bits/symbol = 19,200 bit/s).
Once manufacturers introduced modems with trellis modulation, transmission rates increased to the point where interactive
transfer of multimedia over the telephone became feasible (a 200 kilobyte image and a 5 megabyte song could be downloaded
in less than 1 minute and 30 minutes, respectively). Sharing a floppy disk via a BBS could be done in just a few minutes,
instead of an hour. Thus Ungerboeck's
Echo cancellation : The term echo cancellation is used in telephony to describe the process of removing echo from a voice
communication in order to improve voice quality on a telephone call. In addition to improving subjective quality, this process
increases the capacity achieved through silence suppression by preventing echo from traveling across a network.
Two sources of echo have primary relevance in telephony: acoustic echo and hybrid echo.
Echo cancellation involves first recognizing the originally transmitted signal that re-appears, with some delay, in the transmitted
or received signal. Once the echo is recognized, it can be removed by 'subtracting' it from the transmitted or received signal.
This technique is generally implemented using a digital signal processor (DSP), but can also be implemented in software. Echo
cancellation is done using either echo suppressors or echo cancellers, or in some cases both.
INP(Impulse Noise Protection): Impulse noise in multicarrier communication systems behaves effectively as a modulating
