Specifications

Comparing the Broadcom's upstream SNR estimate to what one would measure with a spectrum analyzer will
often yield quite different results. The Broadcom chips upstream SNR estimating process is most reliable in
the 25 to 32 dB range. If the upstream SNR estimate reaches 35 dB or greater consider the result to be
unreliable and gain a true upstream CNR measurement using a spectrum analyzer.
The optimal period to collect the 10,000 symbols is 10−20 msec of 100% utilization upstream for a 3.2 or 1.6
MHz channel width. It would not be usual to have this quantity of traffic being passed and at the same time to
be experiencing a low upstream SNR. The lower the upstream SNR the greater the degradation of traffic
passed. This degradation causes the Broadcom chip to take too long to collect the 10,000 symbols and for the
resulting upstream SNR estimate to be inaccurate. If the upstream SNR estimate falls below 25 dB consider it
to be unreliable. At this low upstream SNR level the system is experiencing lots of errors and too little traffic.
Expect a lot of flap list entries and SID connectivity numbers to be low. The command show cable hop
should indicate lots of FEC correctable and uncorrectable errors.
After mentioning the above limitations however, if the upstream SNR level is between 25 and 32 dB as shown
by show controller cable x/x , use this command to determine whether there is an apparent RF issue.
The upstream CNR estimate is generally 3−4dB lower than the upstream SNR estimate. In a typical cable
plant you should expect to see an upstream SNR of 32−33dB and an upstream CNR of 28−29dB.
Cable Show Commands for RF Problems
The following show commands are used on the CMTS to help diagnose RF issues:
show controllers cable x/x downstream•
show controllers cable x/x upstream•
show cable modem detail•
show interface cable x/x upstream y•
show cable hop•
ping docsis•
show cable flap−list•
The following show commands are used on the CM to help diagnose RF issues:
show controllers cable−modem 0 | include snr•
For reference also refer to the Understanding Show Command Responses document.
The show controllers cable x/x downstream and show controllers cable x/x upstream commands can be
used to show the layer two status of the cable card on the CMTS when diagnosing suspected RF problems.
Use these commands to check the frequency settings being used and the upstream SNR value. The show
controllers cable x/x upstream command should be entered several times to see if the SNR value fluctuates
rapidly. Even with good upstream SNR values a very rapid fluctuation also means RF problems.
Use the show interface cable x/x upstream y command to check for noise within the RF plant. If the
uncorrectable errors, noise and microreflection counters are high and increasing quickly, then this typically
indicates there is noise present within the RF plant. You can also use the ping docsis command to verify the
layer two connectivity to the cable modem.
Use these commands to check:
The configuration parameters•
Cisco − Determining RF or Configuration Issues On the CMTS