Computer Networks: The Physical Layer
Formulae
2.1 The Theoretical Basis for Data Communication |
baud rate
baud = number of signal changes / second
The baud rate is NOT the same as the data rate. Baud tells how many
signals are sent per second, but there are ways of encoding more than one bit
per signal change. AVIOD getting this confused.
maximum data rate of a channel (Nyquist's Theorem)
max data rate = 2 H log2 V (measured in bits/second)
where . . .
H = bandwidth (in Hz)
V = discrete levels (i.e., bits per signal change)
This formula shows the maximum number of bits that can be sent per second on a
data line with a bandwidth of H, is V
bits are sent per signal. The max data rate should be in bits per second (bps).
signal-to-noise ratio
The signal-to-noise ration is used to quantify the quality of a line. It is not
usually presented as a ratio, but instead it is given in the units decibels (dB).
maximum number of bits per second (Shannon's Result)
max number of bits per second = H log2 (1 + (S/N))
This formula shows the maximum achievable data rate on a noisy line. The difference
between Shannon's result and Nyquist's theorem is that Shannon's result takes
the noise on a line into consideration. The noise can drastically reduce a
line's capacity to send data. It is measured in bits per second.
attenuation
10 log10 (transmitted power / received power) (measured in decibels)
Attenuation is a way to measure the amount of power lost in a signal's strength
from when it was sent to when it was received. Attenuation is measured in decibels (dB).
2.3 Wireless Transmission |
relationship between frequency, wavelength, and the speed of light
bits per second
bits per second =
number of bits per signal change * baud
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f = c