Juniper Networks G10 CMTS manual Ideal HFC Plant Configuration Issues

Ideal HFC Plant Configuration Issues

! Congestion exists in the downstream.	•
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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:	•
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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.	•
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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.	•
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3. Wait 60 seconds for the value of the object to change and record this value. Assume	•
the value is 563,344,189 octets.	•
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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).	•
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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.	•
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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.	•
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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).	•
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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.	•
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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.	•
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! The CMTS is transmitting using 64QAM modulation. If the HFC plant can support	•
reliable downstream transmissions using 256QAM modulation, change the modulation	•
to 256QAM.	•
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! The performance on the network-side interface (NSI) is slow. Find the NSI bottleneck	•
and address the performance issue appropriately.	•
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Troubleshooting 155