Troubleshooting guide

5-4
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Understanding Initialization States
To troubleshoot, use a signal or spectrum analyzer off a drop port (the final splitter to which modems are
attached) to measure the digital channel power coming from the splitter to the modem. The optimal input
power level at the modem is 0 dBmV. The receiver has a range of -15dBmV to +15dBmV.
• If the power is too low, configure the upconverter as per Chapter 4 of the Cisco uBR7246 Hardware
Installation Guide. This is preferred to removing attenuation because the carrier to noise ratio must
remain sufficiently high to reduce the chance of bit errors due to noise.
• If the signal is too strong, add attenuation at the high frequency port on the diplexer. This is preferred
to reducing the power output of the upconverter, in order to maintain the carrier to noise ratio. If the
frequency is off, adjust the frequency of the upconverter.
A modem that resets back to this state frequently is one that is not maintaining sync or is losing sync
with the CMTS. The cause is similar to not acquiring sync (discussed above), and the same
troubleshooting methods should apply.
Wait UCD State
Found DS frequency (64 QAM): 855000000 Hz
MAC State --->>> 'wait_ucd_state'
Found US channel: 1
After a downstream channel has been acquired and latency has been calculated, the next task is to locate
a suitable upstream channel. The modem listens for an upstream channel descriptor (UCD), which
contains the physical properties of the upstream channel, such as, frequency, modulation, channel width,
and other parameters defined in the burst descriptors discussed in Section 4 of [DOCSIS].
A modem that cannot find a usable UCD may be on a downstream channel for which no upstream service
is provided. This is likely to be a headend misconfiguration. The show controller interface command is
a good place to start.
Another possible reason a modem may not find a usable UCD is that its hardware or MAC may not
support the parameters in the burst descriptors. This is likely to be either a headend misconfiguration, or
a modem that is not DOCSIS compliant.
Wait MAP State
Once a usable UCD is found, the modem begins to listen to MAP messages which contain the upstream
bandwidth allocation map of time. A section of time is mapped out into mini-slots, and assigned to
individual modems. There are also regions in the MAP for broadcast, contention-based initial
maintenance (or broadcast) ranging. It is to these regions of the MAP that the modem must send its initial
ranging requests until the CMTS responds with a ranging response (RNG-RSP).
If a modem cannot find an initial maintenance region before a T2 timer expiry, there is likely a headend
misconfiguration. You should check the insertion-interval for the cable interface.
Ranging Broadcast State
Trying Upstream Channel 1 (41200000 Hz)
MAC State --->>> 'ranging_broadcast_state'
At this stage, the modem begins a ranging process to calculate the necessary transmit power level to
reach the CMTS at its desired input power level. The MAC sends a ranging request (RNG-REQ) message
to the CMTS and waits for a ranging response (RNG-RSP) message, or a T3 timer expiry. If a T3 timeout
occurs, the retry count increments. If the retry count is less than the maximum number of retries, the
modem transmits another RNG-REQ at a higher power level.