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pip08456 · 01-06-2011, 21:37

You didn't carry on reading though did you?

"At this point, more needs to be said about the downstream. As most of you know, the downstream RF signal is is QAM modulated (which you can read about here). High power levels can cause QAMs around the tuned one to break through the tuner's isolation and interfere with the intended signal.

RECOMMENDED POWER LEVELS

Bearing in mind the above explanations, it should be clear why the following levels are recommended. They take into account

the modem's specifications
the technical explanations provided above
the anecdote on the forums as to where problems arise
the values based on geography (i.e. network build) recommended by Virgin Media staff

DOWNSTREAM POWER (Optimal)

Rule of thumb: -3 dBmv to +6 dBmv

The above advice compares with the VM stated optimum values by region (according to implemented infrastructure standards) and modulation:

QAM64 (Knowsley networks) -7 dBmv to +3 dBmv

QAM64 (Langley/Bromley) -10 dBmv to 0 dBmv

QAM256 -3 dBmv to +7 dBmv

This is not cast in stone. A good modem (Cisco 2100 comes to mind) can handle nearer the DOCSIS spec if the level is steady and the experience good. If the experience is bad, then look for high downstream power, particularly above the 9 dBmv mark."

01-06-2011, 21:37	#25
pip08456 Sad Doig Fan! Join Date: Aug 2007 Location: Barry South Wales Age: 69 Services: With VM for BB 250Mb service.(Deal) Posts: 11,845	Re: Speed and ping way low today DN35 You didn't carry on reading though did you? "At this point, more needs to be said about the downstream. As most of you know, the downstream RF signal is is QAM modulated (which you can read about here). High power levels can cause QAMs around the tuned one to break through the tuner's isolation and interfere with the intended signal. RECOMMENDED POWER LEVELS Bearing in mind the above explanations, it should be clear why the following levels are recommended. They take into account the modem's specifications the technical explanations provided above the anecdote on the forums as to where problems arise the values based on geography (i.e. network build) recommended by Virgin Media staff DOWNSTREAM POWER (Optimal) Rule of thumb: -3 dBmv to +6 dBmv The above advice compares with the VM stated optimum values by region (according to implemented infrastructure standards) and modulation: QAM64 (Knowsley networks) -7 dBmv to +3 dBmv QAM64 (Langley/Bromley) -10 dBmv to 0 dBmv QAM256 -3 dBmv to +7 dBmv This is not cast in stone. A good modem (Cisco 2100 comes to mind) can handle nearer the DOCSIS spec if the level is steady and the experience good. If the experience is bad, then look for high downstream power, particularly above the 9 dBmv mark."