6-12
Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
Chapter6 Troubleshooting the System
Using a Headend CM to Verify Downstream Signals
Using a Headend CM to Verify Downstream Signals
You can use a Cisco uBR924 cable access modem to verify the downstream signal originating from a
CiscouBR10012router. Be sure that you configure the CiscouBR924 according to DOCSIS CM
practices.
To verify the downstream signal from a CiscouBR10012router using a CiscouBR924, follow the
procedure below:
Step1 After the CiscouBR924 is operational and you have an input signal between 0 and +5 dBmV, use the
show controller c0 tuner command.
Step2 Scan the output for the value corresponding to the signal-to-noise (SNR) estimate variable. If this value
is at least 35dB, you have an optimized signal. If the value is less than 34dB, adjust the upconverter at
the cable headend.
Tip The SNR estimate for a CM installed at a headend should be between 35 and 39dB. Although the
exact value displayed varies from CM to CM, values collected on the same CM from measurement
to measurement will be consistent. Maximizing SNR optimizes CM reliability and service quality.
Performing Amplitude Averaging
The CiscouBR10012router uses an averaging algorithm to determine the optimum power level for a CM
with low carrier-to-noise ratio that is making excessive power adjustments—known as flapping. To avoid
dropping flapping CMs, the CiscouBR10012router averages a configurable number of RNG-REQ
messages before it makes power adjustments. By compensating for a potentially unstable return path,
the CiscouBR10012router maintains connectivity with affected CMs. You can interpret these power
adjustments, however, as indicating unstable return path connections.
The show cable flap-list and show cable modem commands are expanded to indicate the paths on which
the CiscouBR10012router is making power adjustments and the modems that have reached maximum
transmit power settings. These conditions indicate unstable paths that should be serviced.
The following example shows the output of the show cable flap-list command:
Router# show cable flap-list
MAC Address Upstream Ins Hit Miss CRC P-Adj Flap Time
0010.7bb3.fd19 Cable5/0/0/U1 0 2792 281 0 *45 58 Jul 27 16:54:50
0010.7bb3.fcfc Cable5/0/0/U1 0 19 4 0 !43 43 Jul 27 16:55:01
0010.7bb3.fcdd Cable5/0/0/U1 0 19 4 0 *3 3 Jul 27 16:55:01
The asterisk (*) indicates that the CMTS is using the power-adjustment method on this modem. An
exclamation point (!) indicates that the modem has reached maximum transmit power.