E1 testing
5
transmission and modify the original characteristics of the signals, which may end
up being degraded by the time they reach their destination.
The most significant impairments are attenuation, noise, and distortion. We
look at these below in respect to a communications channel, which is defined as a
means of unidirectional transmission of signals between two points.
1.2.1 Attenuation
Attenuation
weakens the power of the signal proportionally to the transmission me-
dium length. It is expressed in decibels (A
dB
) through the logarithmic ratio of the
transmitted power (P
Tx
) and received power (P
Rx
), measured at both ends of the
distance (d) being examined (see Figure 3). Transmission media can usually be
characterized by their attenuation per unit of length (A
dB / Km
):
Example:
Thus for a transmission medium with A=0.2 dB/Km, after 15 Km, the at-
tenuation is A
dB
=3 dB. If the transmitted power is P
Tx
=1W. After 10 Km received
power is P
Rx
= 0.5W, because 10 log (1/P
Rx
) = 3 dB (see Figure 4).
At the far end the received signal must have enough power (P
Rx
) to be interpret-
ed, otherwise amplifiers (also known as repeaters or regenerators in digital transmis-
sion) must be inserted along the transmission medium to improve the power of the
received signal.
10
P
Tx
P
Rx
⁄
(
)
log
d A
dB Km
⁄
⋅
=
A
dB
d A
⋅
dB Km
⁄
=
0.0
0.5
1.0
2.5
3.0
10
1000
1310 1400
1550
Wavelength (nm)
At
tenua
tio
n
(
d
Bm/
K
m)
2.5
1.5
Figure 4
Typical attenuation values for single mode optical fiber and coaxial cable.
1
100
Att
e
nuat
ion (dB
m
/K
m
)
10
1200
1KHz
100
1MHz
10
Frequency
800
Optical fiber
Coaxial cable