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Troubleshooting
Ideal HFC Plant Configuration Issues
155
! Congestion exists in the downstream.
You can compute the approximate downstream channel utilization by monitoring the
ifOutOctets object in the DOCS-IF-MIB (RFC 2670). The ifOutOctets object contains the
total number of octets transmitted on an interface, including data packets as well as MAC
layer packets, and includes the length of the MAC header. However, this object does not
account for overhead—such as FEC, MPEG, and DOCSIS MAC—which consumes a
certain percent of the available raw channel bandwidth. The following procedure
explains how to compute the approximate downstream channel utilization using an
SNMP MIB browser:
1. Set the SNMP polling time to a value large enough to capture a statistically
significant amount of upstream traffic. In this example, assume the polling time is
60 seconds.
2. Browse the ifOutOctets object for the interface that corresponds to the downstream
interface you are measuring. Wait for the value of the object to change and record
this value. Assume the value is 383,456,157 octets.
3. Wait 60 seconds for the value of the object to change and record this value. Assume
the value is 563,344,189 octets.
4. Subtract the value of the object measured in step 2 from the value measured in
step 3 to obtain the number of octets transmitted by the CMTS on this downstream
interface over the polling time: (563,344,189–383,456,157=179,888,032 octets).
5. Multiply the value computed in step 4 by eight (to convert to bits), then divide by
the polling time to compute the downstream channel bandwidth:
[(179,888,032 octets * 8) / 60 sec]=23,985,071 bps.
6. Compute the maximum available raw bandwidth by multiplying the symbol rate of
the channel by the number of bits/symbol. Assume the symbol rate is
5.056941 Msym/sec, and the modulation is 64QAM (6 bits/symbol), which yields a
bandwidth of 30,341,646 bps.
7. Assuming a channel efficiency of 85 percent (due to overhead), derating the
maximum available raw bandwidth of the channel yields 25,790,399 bps
(30,341,646 bps*0.85).
8. You can compute the approximate downstream channel utilization by dividing the
measured bandwidth calculated in step 5 by the derated maximum bandwidth
calculated in step 7: (23,985,071 / 25,790,399)=93 percent. This represents a
highly utilized interface.
Congestion in the downstream might caused by an excessive number of cable modems
attached to a DOCSIS Module in the forward path of the HFC plant. Review your
corporate guidelines to ensure you have not exceeded the maximum number of cable
modems per DOCSIS Module. If necessary, install additional DOCSIS Modules.
! The CMTS is transmitting using 64QAM modulation. If the HFC plant can support
reliable downstream transmissions using 256QAM modulation, change the modulation
to 256QAM.
! The performance on the network-side interface (NSI) is slow. Find the NSI bottleneck
and address the performance issue appropriately.