Installation guide
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Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide
OL-2238-03
Chapter 3 Configuring the Cisco Cable Interface
Configuring the Upstream Cable Interface
Caution Higher symbol rates are more susceptible to RF noise and interference. If you use a symbol rate or
modulation format beyond the capabilities of your HFC network, you might experience packet loss or
loss of cable interface connectivity.
Note For QAM-16 channel widths of 400 kHz (320 ksps) or greater, Cisco recommends that you use QAM-16
modulation for long and short data, and that you use QPSK for request, initial, and station
communications. For QAM-16 channel widths of 200 kHz (160 ksps), all communication must be able
to use QAM-16. That is, 160 ksps with QAM-16 requires an exceptional signal-to-noise ratio (SNR) in
your upstream channel(s). When you use QAM-16 for request, initial, and station maintenance messages
with channel widths greater than 400 kHz, the QAM-16 preamble and message data take longer to
transmit than the QPSK format.
Note To set the upstream channel width, use the following commands in cable interface configuration mode.
Verifying Upstream Channel Width
To verify the current value of the upstream channel width, enter the show controllers cable command
for the upstream port you just configured. A sample appears below:
CMTS01# show controllers cable1/0 u0
Cable1/0 Upstream 0 is up
Frequency 24.016 MHz, Channel Width 0.800 MHz, QPSK Symbol Rate 0.640 Msps
Spectrum Group is overridden
SNR 33.2560 dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2288
Ranging Backoff automatic (Start 0, End 3)
Ranging Insertion Interval automatic (60 ms)
Tx Backoff Start 0, Tx Backoff End 4
Modulation Profile Group 1
Perform these steps if you are having difficulty with verification:
1. Use a valid combination of modulation format (QPSK/QAM-16), minislot size, frequency, and
no shutdown.
2. Use a recommended or previously tested modulation profile. It is not uncommon to create a
modulation profile that does not allow cable interface-to-headend communication. Because each
message type is individually specified, some messages might not work.
3. Verify using IP ping packets of varying lengths (64 to 1500 bytes). Ping from the headend to the
cable interface.
4. Verify with your cable interface vendor that CM software is fully certified or compatible with
DOCSIS 1.0 or DOCSIS 1.0 extensions as appropriate.
Command Purpose
CMTS01(config-if)# cable upstream
usport channel-width width
Enter the channel width for your upstream RF carrier
in Hz.
CMTS01(config-if)# no cable upstream
usport channel-width
Return the channel width to its default setting of
1600000 Hz.