Specifications

22
Schneider Electric
Choice of the Physical
Communication Medium (continued)
Attenuation in dB/km:
b This is the loss of signal quality in terms of amplitude.
b It depends on cable geometry.
b It varies according to frequency and length.
Resistance per unit length in Ω /m:
b This is the resistance to the passage of a direct current (or a low frequency).
b It depends on the cross-section of the wire and the material from which it is made.
b It varies according to length and temperature.
Capacitance between two conductors in pF/m:
b This is the capacitance measured between two wires in the same pair.
b It depends on the type of insulation used and its thickness.
Transmission speed in m/s:
b This is the time it takes the signal to travel from one point to another on the wire.
b For low frequencies (for which voltage variations are much slower than signal
transmission) transmission speed is considered infinite, i.e., transmission is
instantaneous.
b This approximation is not valid for high frequencies.
Crosstalk between pairs in dB:
Temperature of use in °C
b Temperature of use should take into account:
v ambient temperature,
v how the cable was laid,
v the cable’s own heating,
v the proximity of power devices, etc.
E52293
Near-end crosstalk
Far-end crosstalk
Cabling Guidelines