55 or 65 dBmV max?


I've always thought 55 dBmV was the maximum upstream power level cable
modems are capable of. Tonight while speaking to a Time Warner / Road
Runner tech. support guy, he said he'd seen numbers up to 65 and that
his DOCSIS monitoring software showed anything up to 58 as green, up to
62 as yellow, and above 62 as red (I may be off by 1 in those cutoffs,
but roughly that, and all the numbers were >55).
Anyone know what the story is? Was I wrong about 55 dBmV being the limit?
Are newer cable modems capable of higher upstream power levels?
Are numbers >55 some quirk of the DOCSIS monitoring software?
Thanks in advance,
-WBE
Reply to
Winston
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You are correct. As specified by DOCSIS, +55dBmV is the max transmit power at which cable modems must work. That does not mean that they won't work above 55. Most will keep on ticking at a bit higher than that, but anything above +55 is pretty much just a safety margin.
CIAO!
Ed N.
W> I've always thought 55 dBmV was the maximum upstream power level cable
Reply to
Ed Nielsen
I posted: # I've always thought 55 dBmV was the maximum upstream power level cable # modems are capable of. Tonight while speaking to a Time Warner / Road # Runner tech. support guy, he said he'd seen numbers up to 65 and that # his DOCSIS monitoring software showed anything up to 58 as green, up to # 62 as yellow, and above 62 as red (I may be off by 1 in those cutoffs, # but roughly that, and all the numbers were >55). # # Anyone know what the story is? Was I wrong about 55 dBmV being the # limit? Are newer cable modems capable of higher upstream power levels? # Are numbers >55 some quirk of the DOCSIS monitoring software?
Ed Nielsen kindly replied:
So then you're saying that there do exist cable modems that can send above +55dBmV? In that case I was wrong, since my impression was that +55dBmV was the limit of what cable modems *can* send. For example, I never saw my Motorola Surfboard 4100 go above +55. Maybe that's just a 4100 limitation?
Thanks in advance (again), -WBE
Reply to
Winston
Well, the DOCSIS 1.0 spec stated that a modem must transmit from +8dB to +58dB. I haven't really checked transmit levels in the later specs, so I can't address any changes they may have made. Manufacturers were allowed to have their modems transmit higher levels if they wanted, but the spec says 8-58dB. If the upstream bandwidth is 1.6MHz, QPSK modulation, modems will be able to transmit at a slightly higher level. Some modems show this number, some just show the same level no matter what the modulation. Moving the 3.2MHz QPSK will cost you 3dB, and 16QAM will cost another 3dB. Again, depending on the modem, it may or may not show a change, and the CMTS may take the power level change into account and not need to make the modems transmit higher (Cisco uBRs know that the levels are different, Arris Pizza boxes did not, IIRC).
At any rate, your cable tech needs to go through the class again. Any modem transmitting higher than about 55dB should be investigated. Most system designs plan for modem (and STB) transmit levels to be in the mid to upper 40's range.
Reply to
Eric
When I was having problems once, I saw my SB5120 trying to transmit at 61dB (my normal is around 45-51). May cable box (Motorola DCT-6412) is transmitting at 60dB, but OnDemand works, so it is working.
Reply to
Andrew Rossmann
I originally posted:
"Eric" kindly replied:
And indeed, I've now seen +58dB from a Motorola 4200, indicating that at least some cable modems do go higher than +55.
Thanks for confirming that.
...
In my conversation with him, it's RR's DOCSIS monitoring software that shows green for anything up to 58, yellow up to 62, or red if higher, so I'm not going to fault him for using the tool he's given. However, if those cutoffs are unadjusted copies of the upstream power level, it does sound like the monitoring software's cutoffs are too high.
Thanks, -WBE
Reply to
Winston
Specs are still the same -- 8-55 for 16QAM and 8-58 for QPSK
CIAO!
Ed N.
Eric wrote:
Reply to
Ed Nielsen

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