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cable spectrum-group hop threshold
For example, if the hop threshold is at its default of 20 percent and an upstream has 100 active CMs, a
power outage that affected 20 CMs would usually cause a frequency hop since this is a 20 percent loss
of CMs, which in turn would be responsible for at least 20 percent loss of station maintenance messages.
But in this situation, the frequency hop would be unneeded because changing the upstream frequency
could not correct the original problem (the power outage). If this were a common situation on this
upstream, the network administrator might increase the hop threshold so that the repeated power outages
would not generate unneeded frequency hops.
If, on the other hand, the power outage affected only 10 CMs, a frequency hop would not occur unless
another factor, such as ingress noise, created a sufficient loss of station maintenance messages to reach
the 20 percent threshold. In this situation, the default threshold of 20 percent might be sufficient.
Downstream problems can also generate frequency hops. For example, if 20 CMs were on a particularly
noisy downstream, over time they could miss a sufficient number of station maintenance messages to
generate a frequency hop. The network administrator could increase the hop threshold to limit the
possibility of frequency hops due to downstream impairments.
Also, faulty CMs could generate a frequency hop under certain conditions. For example, if a number of
faulty CMs generated a large number of uncorrectable forward error correction (FEC) errors or
otherwise missed 50 to 60 percent of their station maintenance messages, without actually going offline,
over time they could miss a sufficient number of station maintenance messages to cause a frequency hop
or modulation change. The network administrator could increase the hop threshold to prevent the CMTS
from generating a frequency hop or modulation change for problems such as these, which are unrelated
to actual noise on the upstream.
If a previous frequency hop had already occurred within the user-configurable hop period, the CMTS
Note
will not immediately frequency hop. Instead, the CMTS would wait until the hop period expires, and if
the percentage of station maintenance messages still exceeds the hop threshold, the CMTS would
perform another frequency hop.
Tip
When an upstream has 25 or fewer CMs (which is typical with lab and test environments), the CMTS
increases the rate at which it sends station maintenance messages to the CMs. This higher polling rate,
along with the small number of CMs, means that frequency hopping can occur more quickly than with
a normally loaded upstream, especially when a small number of CMs are powered down or generate
noisy traffic.
The DOCSIS specification states that when a CM misses 16 sequential station maintenance messages,
Note
the CMTS should consider the CM offline and should stop sending station maintenance messages to that
CM. The CM must then reregister with the CMTS to resume connectivity.
Examples
The following example shows how to set the threshold that triggers frequency hop to 25 percent of
station maintenance messages on the upstream that is assigned to spectrum-group 4:
router# configure terminal
router(config)# cable spectrum-group 4 hop threshold 25
router(config)#
Cisco Broadband Cable Command Reference Guide
2-240
Chapter 2
Cisco CMTS Configuration Commands
OL-1581-08
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