Installation Guide

4002804 Rev C QPSK (Release A63) Software Upgrade and Installation Instructions 35
What Is the QPSK Range Extension Feature?, Continued
Distance and Delay
The distance that appears on the front panel of the QPSK modulator is approximate
and based on fiber with a propagation velocity of 68 percent of the speed of light in a
vacuum.
Note: The modulator has no way of calculating how much coax or fiber is deployed
between and to the DHCTs.
If you anticipate that DHCTs will be deployed near the limits of the distance ranges,
Cisco recommends that you use delay numbers and translate the numbers to the
actual length of fiber and/or coaxial cable deployed.
The delay numbers used in the modulator are as follows:
 With no extension, the modulator can range DHCTs to a distance corresponding
to 628 microseconds round trip.
 Each front-panel step of 31 km (one way) corresponds to a round-trip delay
extension of 300 microseconds.
Distance Between DHCTs
When the range extension mode is activated, the QPSK modulator expects a
transmission delay that is equal to or greater than the distance value programmed
into the QPSK modulator.
The distance between the closest and the farthest DHCT is still a maximum of 64 km.
Thus, the closest a DHCT can be located is the distance programmed on the front
panel of the QPSK modulator. The farthest a DHCT can be located from the headend
is the sum of the programmed distance plus the 64 km ranging distance.
DHCTs operated outside these bounds may not be able to connect to the system and
may also potentially interfere with the transmissions of other DHCTs. The
programmed distance applies to all of the demodulators connected to that
modulator.