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"No Spectrum available for this Modem"

What cable modems do support measuring the RF spectrum or what criteria do they have to met?



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7 Comments

  1. Criteria

    In general you will find the feature advertised in the vendors datasheet for the cable modem - like: 'Built-in RF spectrum analyzer'. These modems have to support to start the process and get the values via SNMP. If you don't have the datasheet you can check it with the following commands via CLI:

    SNMP queries
    # Replace <community> by your community string and <ip or hostname> by the corresponding IP or hostname of the modem to query
    
    # Configure the measurement
    snmpset -v2c -c<community> <ip or hostname> .1.3.6.1.4.1.4491.2.1.20.1.34.3.0 u 154000000
    snmpset -v2c -c<community> <ip or hostname> .1.3.6.1.4.1.4491.2.1.20.1.34.4.0 u 860000000
    snmpset -v2c -c<community> <ip or hostname> .1.3.6.1.4.1.4491.2.1.20.1.34.5.0 u 8000000
    
    # Start the process
    snmpset -v2c -c<community> <ip or hostname> .1.3.6.1.4.1.4491.2.1.20.1.34.1.0 i 1
    
    # wait some seconds and get the values
    snmpwalk -v2c -c<community> <ip or hostname> .1.3.6.1.4.1.4491.2.1.20.1.35.1.3

    If the last command returns 'No such instance at oid ...' the spectrum measurement is not supported!

    Failures during measurement

    Until NMSPrime version 2.5.1 many modems react very unpredictable when starting the measurement process as they somehow seem to have problems with PHPs implementation of SNMP (snmpwalk v1 and v2). That means that the modems SNMP server often stops to respond and resets itself after a while resulting in the shown message "No spectrum available ...". Sometimes it works, sometimes not.

    From version 2.5.1 on we switched from calling snmpwalk via PHP to the CLI way which delivers results way more reliably.

    NOTE: We stop querying the modem for the spectrum values after approximately 20 seconds. If the modem needs more time for measuring you just have to click 'Create Spectrum' again after some more seconds.

    Known working modems

    We currently tested the feature successfully with the following modems:

    vendormodeltime to measure in secnotes
    AVMFritzBox 649010
    Thomson/TechnicolorCGA21212
    ArrisTG249240 to 60Start measurement by initially clicking 'Create spectrum'. Get the spectrum by another click to the button after some more seconds.
  2. We have tested NMSPrime Spectrum measurement feature on Askey TCG310 and it works fine.

    Would you possibly know, if we can measure the spectrum via CLI with snmp commands you listed from local interface (e.g. 192.168.0.1 for Askey, or 192.169.100.1 for Arris, ...)

  3. It depends on the DOCSIS configuration file. You need to add the IP of your server to the SNMP access list if one exists (look for 'docsDevNmAccessIp' in your base configfile). Then yes, if the IP connection persists you will be able to query the modem via SNMP.

  4. But we need to analyze spectrum even before the cable modem downloads its DOCSIS configuration file. We need to know, if there is an interference in the air, if the cable modem is not able to connect.

    Is there a way how to enable SNMP on the cable modem in "Offline" mode?

  5. No this is absolutely not possible. SNMP works over IP and therefore the device must be reachable in the network. In Offline mode you can just try to connect directly to the modem and start a measurement via the webGUI if this is implemented in the modems firmware.

  6. So the SNMP absolutely does not work over cable modem LAN IP 192.168.0.1 or 192.168.100.1 ?

  7. As far as I know this doesn't work via SNMP on LAN ports. However, some modems provide a spectrum analyzer via the WebGUI on port 8080, i.e. 192.168.100.1:8080. If I remember correctly for example the TC7200 does provide such a functionality.