Troubleshooting guide

7-8
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 7 Troubleshooting the CMTS
Troubleshooting the Cisco uBR7246VXR
Tuner: status=0x00
Rx: tuner_freq 529776400, symbol_rate 5361000, local_freq 11520000
snr_estimate 166(TenthdB), ber_estimate 0, lock_threshold 26000
QAM not in lock, FEC not in lock, qam_mode QAM_64 (Annex B)
Tx: tx_freq 27984000, symbol rate 8 (1280000 sym/sec)
power_level: 6.0 dBmV (commanded)
7 (gain in US AMP units)
63 (BCM3300 attenuation in .4 dB units)
...[Rest of the displayed output is omitted]
Note in the above output that the Signal to Noise ratio (SNR) estimate is 16.6 dB. Ideally this should be
at least 30dB in order for the MTA to operate properly for 64 QAM. See the RF specifications for
DOCSIS Downstream and Upstream signals. In some cases you may have a good SNR (of say 34dB) but
still have noise present, such as impulse noise. This can only be detected by a spectrum analyzer
operating in the zero span mode. One indication of impulse noise is the uncorrectable errors seen in the
output of show interfaces cable 2/0 upstream 0 as shown in the following output:
#show interfaces cable 2/0 upstream 0
Cable2/0: Upstream 0 is up
Received 46942 broadcasts, 0 multicasts, 205903 unicasts
0 discards, 12874 errors, 0 unknown protocol
Note If a value is greater than 1 in 10,000, most likley impulse noise is present.
252845 packets input, 1 uncorrectable
12871 noise, 0 microreflections Total Modems On This Upstream Channel : 3 (3 active)
Default MAC scheduler
Queue[Rng Polls] 0/64, fifo queueing, 0 drops
Queue[Cont Mslots] 0/104, fifo queueing, 0 drops
Queue[CIR Grants] 0/64, fair queueing, 0 drops
Queue[BE Grants] 0/64, fair queueing, 0 drops
Queue[Grant Shpr] 0/64, calendar queueing, 0 drops
Reserved slot table currently has 0 CBR entries
Req IEs 77057520, Req/Data IEs 0
Init Mtn IEs 1194343, Stn Mtn IEs 117174
Long Grant IEs 46953, Short Grant IEs 70448
Avg upstream channel utilization : 1%
Avg percent contention slots : 96%
Avg percent initial ranging slots : 4%
Avg percent minislots lost on late MAPs : 0%
Total channel bw reserved 0 bps
CIR admission control not enforced
Current minislot count : 7192093 Flag: 0
Scheduled minislot count : 7192182 Flag: 0
The optimal input power level at the MTA is 0dBmV; the receiver has a range of -15dBmv to +15dBmV.
This can be measured by the spectrum analyzer. If the power is too low you may need to configure the
upconverter per the Cisco uBR 7246 Hardware Installation Guide. If the signal is too strong, then you
may need to add more attenuation at the high frequency port connection. If a particular frequency has
too much noise present, you may need to select another frequency in the spectrum.
To confirm that the MTA has not been able to achieve QAM lock, turn on debug cable-modem mac log
verbose. You should see output similar to the following:
2d18h: 239198.516 CMAC_LOG_LINK_DOWN
2d18h: 239198.516 CMAC_LOG_LINK_UP
2d18h: 239198.516 CMAC_LOG_STATE_CHANGE ds_channel_scanning_state
2d18h: 239198.520 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 99/805790200/99770
2d18h: 239198.520 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 98/601780000/79970
2d18h: 239198.520 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 97/403770100/59570
2d18h: 239198.524 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 96/73753600/115750