Multi-level signalling using a single cableThere is no reason why data transmission should be limited to a binary (two symbol
state) format over a channel, and in theory, it should be possible to use any number of
voltage levels or symbol types.
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For example, using four voltage levels means that we can
uniquely encode two bits into each of the four levels (00 = level A, 01 = level B, 10 = level C,
11 = level D). This means that every time we change the symbol state, two bits of
information are conveyed compared with only one for the binary system. Hence, we can send
information twice as fast for a given bandwidth of link, or
use a link with half the bandwidth and maintain equivalent
transmission rate.
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Multi-level signalling using multiple cablesIt is of course possible to use multi-level signalling (often termed M-ary signalling) over parallel channels if so desired, with the
consequent increase in throughput or opportunity to reduce the bandwidth on each channel
as required.
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