 
Impedance
Characteristic impedance is the impedance that a cable would have if the cable were
infinitely long. Impedance is a type of resistance that opposes the flow of
alternating current (ac). A cable's characteristic impedance is a complex property
resulting from the combined effects of the cable's inductive, capacitive, and resistive
results. These values are determined by physical parameters such as the size of the
conductors, distance between conductors, and the properties of the cable's insulation
material.
Proper network operation depends on a constant characteristic impedance throughout the
system's cables and connectors. Abrupt changes in characteristic impedance, called
impedance discontinuities or impedance anomalies, cause signals transmitted through LAN
cables and cause network faults.
Minimizing Impedance
Characteristic impedance is usually altered slightly by cable connections and
terminations. Sharp binds or kinks in LAN cable can also alter the cable's
characteristic impedance. Networks can operate with small discontinuities because
the resulting signal reflections are small and are attenuated in the cable. Larger
impedance discontinuities can interfere with data transmission. Such discontinuities
are caused by poor electrical contacts, improper cable terminations, mismatched cable or
connector types, and by disturbances in the twisting pattern of twisted pair cable.
You can avoid problems with impedance discontinuities by observing the following
precautions during installation:
- Never mix cables with different characteristic impedance's (unless you use special
impedance-matching circuitry.)
- Always terminate coaxial cables with a resistance equal to the cable's characteristic
impedance. The terminating resistance prevents signal reflections by absorbing the
signal's energy.
- When untwisting cable pairs to install connectors or make connections at punch-down
blocks, make the untwisted sections as short as possible.
- Do not make sharp bends or kinks in the cable. The radius of bends in a cable
should be larger than 1 inch (2.54 cm).
- Handle LAN cable carefully during installation. Do not step on the cable or pinch
it with tight cable ties.
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