E1 testing
7
Example:
For a modem using 16 signal elements and a channel bandwidth (B) of
4,000 hertz (Hz), the maximum data transfer rate (C) is 32,000 bits per second
(bit/s).
1.2.3 Noise
Noise refers to any undesired and spurious signal that is added to an information
signal. It is usually divided into five categories:
1.
Thermal noise:
This is caused by the agitation of electrons in any conductor in
a temperature different than absolute zero. The noise (N) is independent of the
frequency and proportional to the bandwidth (B) and the temperature (T) in
degrees Kelvin:
N
k T B
⋅ ⋅
=
(k is the Boltzmann’s constant in joules/kelvin, k = 1.3803 x 10
-23
)
2.
Intermodulation noise:
This is caused when two or more signals of frequencies
f
1
and f
2
, transmitted in the same medium, produce a spurious signal at fre-
quencies that are a linear combination of the previous ones.
3.
Atmospheric noise:
This is caused by the static discharge of clouds, or ionized
gas from the sun, or high frequency signals radiated by the stars.
4.
Impulse noise:
Of short duration but high amplitude, these energy bursts are
caused by sources such as electrical machinery, a drop in voltage, atmospheric
interference, and so on. These do not tend to be a problem for analog signals,
but are a prime cause of errors in digital transmission.
5.
Crosstalk:
Whenever a current flows through a conductor a magnetic field is
set up around it that can induct a current into a second conductor collocated in
a short distance.
Noise is always present in transmission channels, even when no signal is being
transmitted. A key parameter at the receiver end to distinguish between information
and spurious power is the signal-to-noise ratio (S/N):
Claude Shannon proved that the signal-to-noise ratio (S/N) determines the the-
oretical maximum transmission capacity (C) in bits per second of channel with a lim-
ited bandwidth (B):
S N
⁄
(
)
dB
10
Power
Signal
Power
Noise
⁄
(
)
log
=
C
bps
Blog
2
1 S N
⁄
+
(
)
=