IRIS DC1100, Installation And Operational Manual

Page: 28 / 60

Coaxial Networks, Inc. Page 28 of 60
Configuring Downstream RF Settings
DOCSIS protocol utilizes an available channel to send downstream signal for data
communications. Depending upon the frequency plan NTSC or PAL, IRIS automatically provides
you the channel number and the center frequency in a drop down menu.
Figure 15 Downstream RF Settings
Using the “Center frequency with channel number” select the channel that you wish to use to
send downstream signal. This channel should not be used to send any video signal.
Using the “RF Power” setting you can adjust the output level of the downstream signal. If you
wish to adjust output power levels beyond the range you have to use outside attenuators. Digitally
modulated signal levels are usually 10 to 12 dB less than the analog signal levels for the adjacent
carriers.
256 and 64 QAM modulation is used for downstream communication. 256 QAM requires a much
cleaner plant and channel. For noisier cable plants it is recommended to use 64 QAM for
downstream. Using the ‘Modulation scheme’ select the appropriate downstream modulation plan.
After making the selection click the “Save” button to apply these changes. Center frequency and
RF power changes do not require system to be rebooted. Changing the downstream modulation
scheme requires a system restart.