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Docsis checklists Headend (Downstream) cmts or Upconverter Output


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DOCSIS Checklists

Headend (Downstream) CMTS or Upconverter Output



Test performed ü

Parameter

Parameter value

Measured value/comments

 

CMTS downstream IF output

+42 dBmV1

 

 

Digitally modulated carrier amplitude at upconverter input

+25 to +35 dBmV2

 

 

Digitally modulated carrier amplitude at upconverter output

+50 to +61 dBmV

 

 

Digitally modulated carrier center frequency

91-857 MHz

 

 

Carrier-to-noise ratio

Not less than 35 dB

 

 

Modulation error ratio (MER)3

64-QAM: 27 dB minimum

256-QAM: 31 dB minimum



 

 

Pre-FEC BER4

N/A

 

 

Post FEC BER5

Less than or equal to 10-8

 

 

Amplitude ripple (in-channel flatness)

3 dB6

 

 

Group delay ripple

75 ns maximum

 

 

Constellation evaluation

Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping

 




  1. Check with your CMTS manufacturer to confirm correct downstream IF output level, typically specified at a value similar to that shown here (+/- 2 dB).

  2. 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.

  3. 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.

  4. DOCSIS does not specify a minimum pre-FEC BER. Ideally there should be no pre-FEC bit errors at the CMTS or upconverter output.

  5. Ideally there should be no post-FEC bit errors at the CMTS or upconverter output.

  6. 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

Parameter

Parameter value

Measured value/comments

 

Digitally modulated carrier average power level relative to analog TV channel visual carrier amplitude

-10 to -6 dBc

 

 

Digitally modulated carrier center frequency

91-857 MHz

 

 

Carrier-to-noise ratio

Not less than 35 dB

 

 

Modulation error ratio (MER)1

64-QAM: 27 dB minimum

256-QAM: 31 dB minimum



 

 

Pre-FEC BER2

N/A

 

 

Post FEC BER3

Less than or equal to 10-8

 

 

Amplitude ripple (in-channel flatness)

3 dB4

 

 

Group delay ripple

75 ns maximum

 

 

Constellation evaluation

Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping

 




  1. 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.

  2. 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.

  3. Ideally there should be no post-FEC bit errors at the downstream laser or first amplifier input.

  4. 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

Parameter

Parameter value

Measured value/comments




Digitally modulated carrier center frequency

91-857 MHz







Digitally modulated carrier average power level relative to analog TV channel visual carrier amplitude

-10 to -6 dBc







Digitally modulated carrier average power level1

-15 to +15 dBmV







Carrier-to-noise ratio

Not less than 35 dB







Total downstream RF input power2

<+30 dBmV







Modulation error ratio (MER)3

64-QAM: 27 dB minimum

256-QAM: 31 dB minimum









Pre-FEC BER4

N/A







Post FEC BER

Less than or equal to 10-8







Constellation evaluation

Look for evidence of gain compression; phase noise; I-Q imbalance; coherent interference; excessive noise; and clipping







Amplitude ripple (in-channel flatness)

3 dB5







Hum modulation

5% (-26 dBc)







Maximum analog TV channel visual carrier level

+17 dBmV







Minimum analog TV channel visual carrier level

-5 dBmV







Transit delay from CMTS to most distant cable modem6

<=0.800 millisecond







Signal level slope, 50-750 MHz

16 dB







Group delay ripple7

75 ns






  1. 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.

  2. Total power of all downstream signals in the 40-900 MHz frequency range.

  3. 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.

  4. DOCSIS does not specify a value for pre-FEC bit error rate.

  5. DOCSIS 1.0 specifies 0.5 dB for this parameter, however, it was relaxed to 3 dB in DOCSIS 1.1 and 2.0.

  6. Transit delay may be estimated.

  7. 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

Parameter

Parameter value

Measured value/comments




Digitally modulated carrier bandwidth

200, 400, 800, 1,600 or 3,200 kHz







Digitally modulated carrier symbol rate

160, 320, 640, 1,280 or 2,560 ksym/sec







Digitally modulated carrier center frequency

Must be within 5-42 MHz spectrum







Digitally modulated carrier amplitude1

-16 to +26 dBmV depending on symbol rate







Total 5-42 MHz RF spectrum power

Must not exceed +35 dBmV







Carrier-to-noise ratio

Not less than 25 dB2







Carrier-to-interference ratio

Not less than 25 dB2







Carrier-to-ingress power ratio

Not less than 25 dB2







Hum modulation

7% (-23 dBc)







Amplitude ripple

0.5 dB/MHz







Group delay ripple3

200 ns/MHz







Transit delay from most distant cable modem to CMTS4

<=0.800 millisecond







  1. Default value for most CMTSs is 0 dBmV, although this may be set to other values.

  2. Measured at CMTS upstream input port. The value shown is an in-channel value.

  3. Upstream group delay may be measured with an instrument such as Holtzman, Inc.’s Cable Scope™. See http://www.holtzmaninc.com/cscope.htm.

  4. Transit delay may be estimated.


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