Juniper G10 CMTS, Hardware Manual

Page: 174 / 210

HFC Plant Related Issues
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JUNOSg 3.0 G10 CMTS Hardware Guide 158
Cable Modem Throughput is Slow
If the throughput of a cable modem seems slow, the cause might be one or more of the
following:
!
HFC plant issues, such as impulse noise or ingress, that corrupt upstream burst
transmissions from the cable modem. A high
CERavg value or a low MERavg value in the
flap list is indicative of this. Uncorrectable codewords cause packets to be dropped by
the CMTS, which reduces the cable modem throughput.
If the CER value is high, but the CERavg value is low, this suggests that burst noise is
occurring, but its duration is too short to render a codeword uncorrectable. However, you
should investigate the source of the noise as part of your preventive HFC plant
maintenance routine.
!
HFC plant issues, such as impulse noise, that corrupt downstream transmissions to the
cable modem. Increasing the depth of the interleaver can increase the amount of burst
protection in the downstream. For example, the default interleaver depth using 64QAM
modulation provides 5.9 microseconds of burst protection. You can increase the burst
protection to 12, 24, 47, or 95 microseconds. Be aware that increasing the interleaver
depth increases the latency of the transmission.
In general, a number of HFC-related issues can be responsible for the receipt of uncorrectable
codewords at the CMTS. Table 45 on page 143 describes how to associate flap list statistics to
the presence of these issues.
Issuing one or more of the following commands can provide you with additional insight into
HFC-related issues that affect cable modem performance:
!
show cable modem errors
!
show cable modem flap
!
show cable modem physical-statistics
!
show cable modem remote-query