In the four-level scheme, groups of two data bits are mapped to one of four symbols. Only one symbol need be transmitted for each pair of data bits, so the symbol rate is half the bit rate. The drawback of the multilevel scheme is that symbols
are separated by a smaller voltage than in the binary scheme. This means that
when noise is added to the data signal (cross talk or impulse), the probability of
the noise changing one symbol to another is increased. The symbol separation
could be increased to that of the binary scheme by increasing the peak-to-peak
transmitted voltage by a factor of (m – 1) for an m-level scheme, but this is generally not possible given fixed power supply voltages, and in any case it increases
the power required for a transmitter.
0
0
1
0
1
1
0
The susceptibility of a scheme to errors caused by noise is measured by the ratio
of signal separation to noise. Fig. 2 shows the signal-to-NEXT-noise ratio plotted
against the transmission bandwidth for several multilevel schemes and for multipair schemes for a bit rate of 100 Mbits/s. A 16-level scheme reduces the bandwidth to 25% of the bit rate, but the S/NEXT ratio is 13 dB (a factor of 4.5) worse
than for a four-pair scheme with 25 Mbits/s per pair, which is the scheme used in
the 100VG-AnyLAN standard.
20.0
n=4
n=3
15.0
S/NEXT (Normalized) (dB)
Multilevel signaling is often used as a means of compressing the bandwidth required to transmit data at a given bit rate. In a simple binary scheme, two single
symbols, usually two voltage levels, are used to represent a 1 and a 0. The symbol rate is therefore equal to the bit rate. The principle of multilevel signaling is to
use a larger alphabet of m symbols to represent data, so that each symbol can
represent more than one bit of data. As a result, the number of symbols that needs
to be transmitted is less than the number of bits (that is, the symbol rate is less
than the bit rate), and hence the bandwidth is compressed. The alphabet of symbols may be constructed from a number of different voltage levels. Fig. 1 shows an
example for a four-level scheme.
n=2
10.0
Multipair Signaling
1
m=5
m=8
m=4
m = 12
5.0
m=2
m=6
m=3
m = 16
V
Multilevel Signaling
00
10
m=4
11
01
V/3
0.0
20
40
60
80
Transmission Bandwidth (MHz)
m=2
n=1
100
Fig. 1. Two-level and four-level signaling.
Fig. 2. Susceptibility to errors from noise.
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