DOCSIS Checklists
Headend (Downstream) CMTS or Upconverter Output
Test performed ü
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Parameter
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Parameter value
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Measured value/comments
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CMTS downstream IF output
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+42 dBmV1
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Digitally modulated carrier amplitude at upconverter input
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+25 to +35 dBmV2
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Digitally modulated carrier amplitude at upconverter output
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+50 to +61 dBmV
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Digitally modulated carrier center frequency
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91-857 MHz
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Carrier-to-noise ratio
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Not less than 35 dB
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Modulation error ratio (MER)3
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64-QAM: 27 dB minimum
256-QAM: 31 dB minimum
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Pre-FEC BER4
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N/A
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Post FEC BER5
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Less than or equal to 10-8
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Amplitude ripple (in-channel flatness)
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3 dB6
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Group delay ripple
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75 ns maximum
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Constellation evaluation
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Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping
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-
Check with your CMTS manufacturer to confirm correct downstream IF output level, typically specified at a value similar to that shown here (+/- 2 dB).
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Nominal average power level input range to most external upconverters. Check upconverter manufacturer’s specifications to confirm recommended input level. Note that an in-line attenuator (pad) between the CMTS IF output and upconverter input may be required.
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Downstream MER is not a DOCSIS parameter. The values shown are minimum values that represent good engineering practice. MER measured in the headend generally will be in the 34 to 36 dB or higher range. The upconverter’s input and output MER should be at or near the same value. A change of more than 1~2 dB indicates a possible upconverter problem.
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DOCSIS does not specify a minimum pre-FEC BER. Ideally there should be no pre-FEC bit errors at the CMTS or upconverter output.
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Ideally there should be no post-FEC bit errors at the CMTS or upconverter output.
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DOCSIS 1.0 specifies 0.5 dB for this parameter, however, it was relaxed to 3 dB in DOCSIS 1.1.
DOCSIS Checklists
Headend (Downstream) Laser Transmitter or First Amplifier Input, Node Output
Test performed ü
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Parameter
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Parameter value
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Measured value/comments
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Digitally modulated carrier average power level relative to analog TV channel visual carrier amplitude
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-10 to -6 dBc
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Digitally modulated carrier center frequency
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91-857 MHz
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Carrier-to-noise ratio
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Not less than 35 dB
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Modulation error ratio (MER)1
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64-QAM: 27 dB minimum
256-QAM: 31 dB minimum
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Pre-FEC BER2
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N/A
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Post FEC BER3
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Less than or equal to 10-8
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Amplitude ripple (in-channel flatness)
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3 dB4
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Group delay ripple
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75 ns maximum
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Constellation evaluation
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Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping
|
|
-
Downstream MER is not a DOCSIS parameter. The values shown are minimum values that represent good engineering practice. MER measured in the headend and node output generally will be in the 34 to 36 dB or higher range.
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DOCSIS does not specify a minimum pre-FEC BER. Ideally there should be no pre-FEC bit errors at the downstream laser or first amplifier input.
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Ideally there should be no post-FEC bit errors at the downstream laser or first amplifier input.
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DOCSIS 1.0 specifies 0.5 dB for this parameter, however, it was relaxed to 3 dB in DOCSIS 1.1.
DOCSIS Checklists
Input to Cable Modem
Test performed ü
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Parameter
|
Parameter value
|
Measured value/comments
|
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Digitally modulated carrier center frequency
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91-857 MHz
|
|
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Digitally modulated carrier average power level relative to analog TV channel visual carrier amplitude
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-10 to -6 dBc
|
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Digitally modulated carrier average power level1
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-15 to +15 dBmV
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Carrier-to-noise ratio
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Not less than 35 dB
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Total downstream RF input power2
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<+30 dBmV
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Modulation error ratio (MER)3
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64-QAM: 27 dB minimum
256-QAM: 31 dB minimum
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Pre-FEC BER4
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N/A
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Post FEC BER
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Less than or equal to 10-8
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Constellation evaluation
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Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping
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Amplitude ripple (in-channel flatness)
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3 dB5
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Hum modulation
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5% (-26 dBc)
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Maximum analog TV channel visual carrier level
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+17 dBmV
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Minimum analog TV channel visual carrier level
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-5 dBmV
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Transit delay from CMTS to most distant cable modem6
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<=0.800 millisecond
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Signal level slope, 50-750 MHz
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16 dB
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Group delay ripple7
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75 ns
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-
While DOCSIS specifies –15 to +15 dBmV, good engineering practice suggests that maintaining digitally modulated carrier levels in the –5 to +5 dBmV range will be less likely to overload the modem.
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Total power of all downstream signals in the 40-900 MHz frequency range.
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Downstream MER is not a DOCSIS parameter. The values shown are minimum values that represent good engineering practice, although in a properly operating cable network the MER should be in the low- to mid-30s at the cable modem input.
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DOCSIS does not specify a value for pre-FEC bit error rate.
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DOCSIS 1.0 specifies 0.5 dB for this parameter, however, it was relaxed to 3 dB in DOCSIS 1.1 and 2.0.
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Transit delay may be estimated.
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In-channel group delay may be measured using Sunrise Telecom’s Avantron AT2000RQ (must have latest firmware/software) or AT2500RQv. See http://www.sunrisetelecom.com/broadband.
DOCSIS Checklists
CMTS Upstream Input
Test performed ü
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Parameter
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Parameter value
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Measured value/comments
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Digitally modulated carrier bandwidth
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200, 400, 800, 1,600 or 3,200 kHz
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Digitally modulated carrier symbol rate
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160, 320, 640, 1,280 or 2,560 ksym/sec
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Digitally modulated carrier center frequency
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Must be within 5-42 MHz spectrum
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Digitally modulated carrier amplitude1
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-16 to +26 dBmV depending on symbol rate
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Total 5-42 MHz RF spectrum power
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Must not exceed +35 dBmV
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Carrier-to-noise ratio
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Not less than 25 dB2
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Carrier-to-interference ratio
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Not less than 25 dB2
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Carrier-to-ingress power ratio
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Not less than 25 dB2
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Hum modulation
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7% (-23 dBc)
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Amplitude ripple
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0.5 dB/MHz
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Group delay ripple3
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200 ns/MHz
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Transit delay from most distant cable modem to CMTS4
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<=0.800 millisecond
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-
Default value for most CMTSs is 0 dBmV, although this may be set to other values.
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Measured at CMTS upstream input port. The value shown is an in-channel value.
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Upstream group delay may be measured with an instrument such as Holtzman, Inc.’s Cable Scope™. See http://www.holtzmaninc.com/cscope.htm.
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Transit delay may be estimated.
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