Cable RF Specifications & Troubleshooting

 

Name Thresholds Description
Frequency DOCSIS 2.0
88-860Mhz Downstream
5-42Mhz Upstream
DOCSIS 3.0
108 to 10002Mhz Downstream
5-88Mhz Upstream
Frequency at which each Channel is connected to
Down/Forward Power (dBmV) -11 to 11 (Rogers)
-15 to 15 (Cogeco)
-10 to 10 (Preferred)
-15 to 15 (DOCSIS)
Perceived Signal Volume coming into modem from node
SNR (MER) 32dB+ (256QAM)
25dB+ (64QAM)
Clarity of incoming signal vs noise on line as perceived by modem
BER 0% Ratio of transmission errors vs good transmissions
Correctables 0 to Infinity # of recoverable transmission failures
Uncorrectables 0 # of unrecoverable transmission failures
Seeing a value here is fine, as long as the value doesn’t increase during operation
Up/Return Power (dBmV)
# Channels Max Power
1 57dBmV
2 54dBmV
3-4 51dBmV

This varies by modem – see the modem’s tech specs for details.

Volume of signal being sent by modem

 

Each Downstream/Forward path channel supports a data rate of 42.88Mbps
Each Upstream channel supports a data rate of 30.72Mbps

Total downstream power should not exceed 30dBmV this is across all channels being broadcast.

In any case check the following:

Follow the Coax cable from the modem to outside.

Is it

  • Connected directly to the cable drop?
  • Connected to Wall Jack?
    • If house, then most likely is split off in basement
    • If Apartment building, then this is the cable drop.
  • In good condition
    • No cracks/breaks
    • Cable securely connected to crimp on connector
    • No staples
  • RG6
    • RG6 cable is for digital signal
    • Older houses/installations might use RG59 which will work ok but if getting low SNR then try replacing with RG6

Are there

  • Splitters
    • Need to be for Digital Cable 5Mhz-1000Mhz
    • Should only be used if service needs to be split for Cable TV or Rogers/Cogeco digital phone
    • Replace with couplers (F81 connectors) or remove where possible
  • Couplers (F81)
  • Filters
    • Used to block certain frequencies (Normally cable TV)
  • Attenuators
    • Increases resistance/attenuation on line
      • Strength of attenuator will be printed on it. 3dB/6dB etc
    • Reduces forward power coming into modem
    • Increases return power.
  • Amplifiers
    • Needs to be 2 way, have return gain, or say designed for cable internet
    • Passive return will only amplify Forward Path Power and the modem’s return power will then max out.
    • Amplifiers should only be used if installed by cable tech.

Check modem power supply

Modem Power Supply Name Power Specification
Thompson DCM475/476 Leader Electronics
OEM
12v 1.0a
TC4300 Leader Electronics 12v 1.0a
Motorola SB6121 Leader Electronics 12v 750ma
Hitron CDA3 Asian Power Devices 12v 1.5a
SmartRG SR804 Switching Adapter
Shenzhen GongJin
12v 2.0a
Others See input on modem

 

See Also:

General Cable internet FAQ:
https://www.dslreports.com/faq/cabletech

Information on SNR / MER / BER:
Ron.Hranac_Presentation-BER + MER Fun

CableLabs DOCSIS 3 physical specification:
https://apps.cablelabs.com/specification/CM-SP-PHYv3.0 

 

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