84
Chapter 11 - DOCSIS Analyzer
10713 Vxx.11
The offset is note qual to zero, if, for instance, you set a return channel amplifier and adjust the
amplification during measurement. The modem adjusts its transmission power so that the offset is
equal to zero again the next time ranging messages are exchanged.
If the level offset is constantly not equal to zero then there is an error in the return channel path
(example: the modem is already transmitting at 114 dBμV and the level offset is +9 dB. The modem
would therefore have to transmit at 123 dBμV so that the head end can identify the transmission
power as being correct, but the modem cannot achieve this transmission power. A possible cause
of this error is an incorrectly set or faulty return amplifier).
11.4.3.3
More advanced upstream time slice analysis
As soon as ranging has started, you can select the
ADVANCED
menu item by pressing
MODE
and
ENTER
.
The following figure shows the DOCSIS 3.0 analyzer.
Figure 11-8 DOCSIS Upstream Details
Use the
←/→
keys to select the relevant upstream channel in the graphic.
Press
MODE
and
ENTER
to leave the
ADVANCED
menu.
11.4.3.4
Upstream frequency response analysis
As soon as ranging is complete, you can select the
FRQRESP
menu item by pressing
MODE
and
ENTER
.
The various DOCSIS specifications provide an equalizer for the upstream sender of the modem.
This equalizer filter can be configured with the filter parameters that the CMTS sends to each
modem individually.
The purpose of this filter is to equalize the frequency response between the modem and head end
that arises through the upstream transmission link between the modem and head end (for example,
through micro-reflections where the cable is damaged). This means that equalizer parameters can
be used to find which frequency response is prevalent within the transmission bandwidth of the
modem (e.g. 3.2 MHz at a symbol rate of 2,560 kSymb/s). This frequency response can be
determined in the
FRQRESP
submenu. If the submenu cannot be entered, the head end is not
sending any equalizer data to the connected modems (this feature is optional in the DOCSIS
specifications).