55 or 65 dBmV max?

55 or 65 dBmV max?
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55 or 65 dBmV max? Winston 09-05-2006
Posted by Winston on September 5, 2006, 8:58 pm
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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

Posted by Ed Nielsen on September 6, 2006, 10:43 am
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.

Winston wrote:
> 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

Posted by Winston on September 8, 2006, 8:46 pm
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?


> 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.

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

Posted by Andrew Rossmann on September 9, 2006, 10:47 am
@ubeblock.psr.com.invalid (Winston) says...
> 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?

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.

--
If there is a no_junk in my address, please REMOVE it before replying!
All junk mail senders will be prosecuted to the fullest extent of the
law!!
http://home.att.net/~andyross

Posted by Eric on September 9, 2006, 10:10 am
Ed Nielsen wrote:
> 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.
>
> Winston wrote:
> > 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

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.



other useful resources:
The Federal Communications Commission (FCC)
Telecommunications Industry Association
Electronic and Software Security Products and Services
International Telecommunication Union

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