Basic Cable Testing
Nominal Velocity of Propagation (NVP)
7
7-19
Nominal Velocity of Propagation (NVP)
NVP is the speed of a signal through a cable relative to the speed of light. In a
vacuum, electrical signals travel at the speed of light. In a cable, signals travel
slower than the speed of light. Typically, the speed of an electrical signal in a cable
is between 60 % and 80 % of the speed of light. Figure 7-11 shows how the NVP
percentage is calculated.
Signal Speed
in Cable
300,000,000 m/s
Speed Pulse Travels in Cable
Speed of Light
=
X 100%
300,000,000 NVP m/s
OFF
PRINT
MONITOR
SETUP
SPECIAL
FUNCTIONS
SINGLE
TEST
AUTO
TEST
DSP-658
CABLE ANALYZER
DSP-601
CABLE ANALYZER ADAPTER
TALK
EXIT
ENTER
TEST
SAVE
1
23
4
INFO
oy34f.eps
Figure 7-11. How NVP is Calculated
NVP values affect the limits on cable length for Ethernet systems because Ethernet
operation depends on the system’s ability to detect collisions in a specified amount
of time. If a cable’s NVP is too low or the cable is too long, signals are delayed
and the system cannot detect collisions soon enough to prevent serious problems in
the network.
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