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Posted by Timothy Daniels on December 23, 2006, 7:53 pm
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is receiving and transmitting the right signal levels. But why would
the packet loss increase when the barrel connector is substituted
for the splitter, thus causing the upstream signal level to rise from
+47dBmV to +49 dBmV as a result of the downstream signal level
dropping from +10 dBmv to +7 dBmV? I'd think that the louder
outgoing signal would better the signal-to-noise ratio. as seen by
the network.
*TimDaniels*
"Ed Nielsen" wrote:
> That "reference level" (as you call it), like most numbers that have a
> (+) or a (-) before them is 0. DOCSIS says that cable modems have an
> input level window of -15 - +15dBmV, which simply means that as long as
> the input signal level is within that window, it will work. The output
> level window, as dictated by DOCSIS, is +8 - +58dBmV. Again, as long as
> the output level of the cable modem is within that window, it will work.
>
> The input level, of course, is determined by the plant/drop system. For
> example: There is a line extender (amplifier) at the pedestal that
> feeds your house. [Single family home with a white picket fence and a
> couple of elm trees in the front yard out in the suburbs. Can't forget
> the 2 dogs and a cat, a swing on the front porch, a vegetable garden,
> and a few fruit trees out back, etc.] The tap on the output of that
> L.E. is a 26 value tap, and the length of your drop is 100', and the
> DOCSIS frequency is 600MHz, and it is a QAM256 system. The L.E is
> running out at 44dBmV (no slope, the output is flat), which equates to
> 18dBmV coming out of the tap. 100 feet of RG 6 loses ~5dB@600MHz, so
> you a level of 13dBmV at the groundblock. In a QAM256 system, the data
> carrier runs at 6dB below the nearest analog channel, so that 13dBmV
> becomes 7dBmV. Place a cable modem right at the groundblock, and it
> will see an input level of +7dBmV. Say you have a TV set in the family
> room and your computer is in a room 25' from the groundblock. A 2-way
> splitter loses ~3.9dB at 600MHz and the 25' drop to the cable modem
> loses 1.25dB. That 7dBmV at the groundblock then becomes (+7-3.9-1.25=)
> 1.85dBmV at the input of the cable modem. Suppose you have 4 TV sets.
> The input level at your cable modem would then be (7-7.9-1.25=)
> -2.15dBmV. Change the attenuation and the input level changes by the
> same amount. A 30dB wide input level, cable modems have.
>
> On the output side in the same scenario, the return amp in the L.E.
> wants to see ~10dBmV. The pathway from cable modem is the drop and the
> tap. Let's say that the upstream frequency is 22MHz. RG 6 loses
> ~1.25dB/100' at 22MHz. The pathway to that amp has the signal losing
> 1.25dB (drop) and 26dB (tap). 10 (required input level to the amp) + 26
> (tap) + 1.25 (drop) would have the cable modem pumping out 37.25dBmV.
> One TV (2-way splitter) and the cable modem 25' from the groundblock
> would have it pumping out (10+26+1.253.6+.31=) 41.16. Four TVs would
> necessitate that the modem transmit at 44.56dBmV.
>
> The CMTS (Cable Modem Termination System (in the headend)) wants a
> signal level of 0dBmV at its input. No wide window like cable modems
> have -- it wants 0dBmV. If the attenuation at a cable modem changes,
> the CMTS tells the modem to increase or decrease its output level
> accordingly. If a cable modem runs so as to hit the CMTS with too hot
> of a signal, the CMTS tells it to turn itself down. Too little and it
> tells the modem to turn itself up. When you remove the splitter from
> your drop, you reduce the attenuation of your drop by 3.5dB, which means
> that there is 3.5dBmV more signal hitting the CMTS than previously, so
> the CMTS tells the modem to turn itself down. Put it back in and the
> modem is told to crank up the power to compensate for the increase in
> attenuation. That is why the transmit levels vary when you
> remove/insert the splitter.
>
> If noise gets into the line, whether it be in the drop or somewhere in
> the plant, the CMTS can't hear the cable modem as well as it needs to,
> so it tells the modem to "speak louder," same thing as you do when
> conversing with someone on the sidewalk and a loud car drives by. Noise
> can be ingress from a local broadcaster or impulse noise such as from a
> blow dryer or vacuum cleaner.
>
>
> CIAO!
>
> Ed N.
>
>
>
> Timothy Daniels wrote:
>> I think you got that backwards. First, the modem appears to be
>> reporting a positive offset - a level *above* some reference level,
>> not a negative offset from some reference. Thus, with the splitter
>> in the line, the downstream level drops in magnitude from 10 dBmV
>> to 9 dBmV (this morning, and during the day yesterday, the downs-
>> stream level with the splitter in the line is/was 7 dBmV). And the
>> upstream level rose to 49 dBmV from 47 dBmV with the splitter
>> (today it's at 50 dBmV with the splitter.)
>>
>> That's agrees with the model that the Transmit upstream level varies
>> counter to the downstream Received level, and it also suggests that
>> the upstream level is a positive value, not a value below a reference
>> level. Given that, it looks as if the high downstream level was set
>> too high for there not to be a splitter since the packet loss *increases*
>> when I substitute a barrel connector for the splitter.
>>
>> This would make sense if the cable company always assumes that
>> there would be one or two TVs hung off the same line - a likely
>> scenario for the average unit in a condo building, but just not the
>> scenario in my condo unit.
>>
>> *TimDaniels*
>>
>>
>> "Ed Nielsen" wrote:
>>> Lower packet loss with the modem pumping out more suggests that there
>>> may be some noise (impulse, ingress) hopping onto your drop. Where
>>> did they pad your cable? Right at the modem? If you can get to the
>>> main splitter, try padding there. Don't worry about the input -- you
>>> have plenty of room there to do what you want. You can go down to
>>> ~-12dBmV before there would start to be any cause of concern. Heck, I
>>> used to run at ~-12/52 for about 4 years until some plant issues
>>> caused me to alter that. Now, I'm at +7/49, and I hate it. One of
>>> these days soon, I'll get around to swapping out my DC to fix it.
>>>
>>>
>>> CIAO!
>>>
>>> Ed N.
>>>
>>> Timothy Daniels wrote:
>>>> Without the 2:1 splitter, the downstream signal level rises to 10 dBmv
>>>> (from 9 dBmV) and the upstream signal level drops to 47 dBmV (from
>>>> 49 dBmV). The tracerts show a quadrupling (or more) of timeouts with
>>>> the splitter removed. It seems that the lower transmit level hinders
>>>> the
>>>> packets' survival rate, and that the splitter, in reducing the
>>>> receive level,
>>>> causes the modem to drop its transmit level and thus increases the
>>>> timeout incidence. Is that right?
>>>>
>>>> BTW, *I* put the splitter between the modem and the RR network
>>>> for analog cable TV reception of local stations. Whenever the techs
>>>> were here, the splitter had been replaced by a barrel connector
>>>> (don't ask). As far as TW is concerned, my only service from them
>>>> is RoadRunner. But no instruments measured levels within my condo
>>>> unit. The only measurements that they took directly were on the level
>>>> from the amp in the vault at the curb and on the output from the
>>>> building's
>>>> amp that resulted from their padding down the signal coming from the
>>>> curb amp. The only measurements that they took specific to my modem
>>>> were the remote readings reported to them from by cell phone from the
>>>> central office. They said that it gave them more information than what
>>>> I got from the modem's webpage and, presumably, what they could learn
>>>> by tapping into the line between the wall outlet and my modem. So it's
>>>> strange that the timeouts are so much less frequent when I put my 2:1
>>>> splitter in the line - it is not what they thought the usual
>>>> configuration was.
>>>>
>>>> *TimDaniels*
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