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Troubleshooting Tips for the Cisco uBR904 Cable Modem 11
Event 2—Scan for a Downstream Channel then Synchronize
Different geographical regions and different cable plants use different frequency bands. The
Cisco uBR904 cable modem uses a built-in default frequency scanning feature to address this issue.
After the cable modem finds a successful downstream frequency channel, it saves the channel to
NVRAM. The cable modem recalls this value the next time it needs to synchronize its frequency.
The field
CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND tells you what frequency the cable modem
will scan for. The field
CMAC_LOG_WILL_SEARCH_SAVED_DS_FREQUENCY tells you the frequency the
cable modem locked onto and saved to NVRAM (for future recall). The field
CMAC_LOG_DS_64QAM_LOCK_ACQUIRED communicates the same information. The field
CMAC_LOG_DS_CHANNEL_SCAN_COMPLETED indicates that the scanning and synchronization was
successful.
508144.348 CMAC_LOG_STATE_CHANGE ds_channel_scanning_state
508144.350 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 88/453000000/855000000/6000000
508144.354 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 89/93000000/105000000/6000000
508144.356 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 90/111250000/117250000/6000000
508144.360 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 91/231012500/327012500/6000000
508144.362 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 92/333015000/333015000/6000000
508144.366 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 93/339012500/399012500/6000000
508144.370 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 94/405000000/447000000/6000000
508144.372 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 95/123015000/129015000/6000000
508144.376 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 96/135012500/135012500/6000000
508144.380 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 97/141000000/171000000/6000000
508144.382 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 98/219000000/225000000/6000000
508144.386 CMAC_LOG_WILL_SEARCH_DS_FREQUENCY_BAND 99/177000000/213000000/6000000
508144.390 CMAC_LOG_WILL_SEARCH_SAVED_DS_FREQUENCY 699000000
508145.540 CMAC_LOG_UCD_MSG_RCVD 3
508146.120 CMAC_LOG_DS_64QAM_LOCK_ACQUIRED 699000000
508146.122 CMAC_LOG_DS_CHANNEL_SCAN_COMPLETED
A frequency band is a group of adjacent channels. These bands are numbered from 88 to 99. Each
band has starting and ending digital carrier frequencies and a 6 MHz step size. For example, a search
of EIA channels 95-97 is specified using band 89. The starting frequency is 93 Mhz, the ending
frequency is 105 Mhz.
The cable modem’s default frequency bands correspond to the North American EIA CATV channel
plan for 6 MHz channel slots between 90 MHz and 858 MHz. For example, EIA channel 95 occupies
the slot 90-96 MHz. The digital carrier frequency is specified as the center frequency of 93 MHz.
Channel 95 is usually specified using the analog video carrier frequency of 91.25 Mhz, which lies
1.75 Mhz below the center of the slot.
The search table is arranged so that the first frequencies tried are above 450 Mhz. Because many
CATV systems have been upgraded from 450 MHz to 750 MHz coaxial cable, digital channels have
a high chance of being assigned in the new spectrum. The search table omits channels below 90 MHz
and above 860 MHz since the DOCSIS specification does not mandate their coverage.
Some CATV systems use alternative frequency plans such as the IRC (Incrementally Related
Carrier) and HRC (Harmonically Related Carrier) plans. Most of the IRC channel slots overlap the
EIA plan. The HRC plan is not supported by Cisco’s cable modems since so few cable plants are
using this plan.